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e5 HISTORICAL INTRODUCTION required to operate on real patients, or on soldiers injured at Sebastopol or some other battlefield. The book they planned together was a practi - cal one, designed to encourage youngsters to study anatomy, help them pass exams, and assist them as budding surgeons. It was not simply an anatomy textbook, but a guide to dissecting procedure, and to the major operations. Gray and Carter belonged to a generation of anatomists ready to infuse the study of human anatomy with a new, and respectable, scien - tificity. Disreputable aspects of the profession’s history, acquired during the days of body-snatching, were assiduously being forgotten. The Anatomy Act of 1832 had legalized the requisition of unclaimed bodies from workhouse and hospital mortuaries, and the study of anatomy (now with its own Inspectorate) was rising in respectability in Britain. The private anatomy schools that had flourished in the Regency period were closing their doors, and the major teaching hospitals were erecting new, purpose-built dissection rooms (Richardson 2000). The best-known student works when Gray and Carter had qualified
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were probably Erasmus Wilson’s Anatomist’s Vade Mecum , and Elements of Anatomy by Jones Quain. Both works were small – pocket-sized – but Quain came in two thick volumes. Both Quain’s and Wilson’s works were good books in their way, but their small pages of dense type, and even smaller illustrations, were somewhat daunting, seeming to demand much nose-to-the-grindstone effort from the reader. The planned new textbook’s dimensions and character were serious matters. Pocket manuals were commercially successful because they appealed to students by offering much knowledge in a small compass. But pocket-sized books had button-sized illustrations. Knox’s Manual of Human Anatomy, for example, was a good book, but was only 6 inches by 4 (15 × 10 cm) and few of its illustrations occupied more than one- third of a page. Gray and Carter must have discussed this matter between themselves, and with Gray’s publisher, JW Parker & Son, before deci- sions were taken about the size and girth of the new book, and espe -
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cially the size of its illustrations. While Gray and Carter were working on the book, a new edition of Quain’s was published; this time it was a ‘triple-decker’ – in three volumes – of 1740 pages in all. The two men were earnestly engaged for the following 18 months in work for the new book. Gray wrote the text, and Carter created the illustrations; all the dissections were undertaken jointly. Their working days were long – all the hours of daylight, eight or nine hours at a stretch – right through 1856, and well into 1857. We can infer from the warmth of Gray’s appreciation of Carter in his published acknowledge- ments that their collaboration was a happy one. The Author gratefully acknowledges the great services he has derived in the execution of this work, from the assistance of his friend, Dr. H. V. Carter, late Demonstrator of Anatomy at St George’s Hospital. All the drawings from which the engravings were made, were executed by him. (Gray 1858)
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With all the dissections done, and Carter’s inscribed wood-blocks at the engravers, Gray took six months’ leave from his teaching at St George’s to work as a personal doctor for a wealthy family. It was probably as good a way as any to get a well-earned break from the dissecting room and the dead-house (Nicol 2002). Carter sat the examination for medical officers in the East India Company, and sailed for India in the spring of 1858, when the book was still in its proof stages. Gray had left a trusted colleague, Timothy Holmes, to see it through the press. Holmes’s association with the first edition would later prove vital to its survival. Gray looked over the final galley proofs, just before the book finally went to press. THE FIRST EDITION The book Gray and Carter had created together, Anatomy, Descriptive and Surgical, appeared at the very end of August 1858, to immediate Gray’s Anatomy is now on its way to being 160 years old. The book is a rarity in textbook publishing in having been in continuous publication on both sides of the Atlantic Ocean, since 1858. One and a half centu -
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ries is an exceptionally long era for a textbook. Of course, the volume now is very different from the one Mr Henry Gray first created with his colleague Dr Henry Vandyke Carter, in mid-Victorian London. In this introductory essay, I shall explain the long history of Gray’s, from those Victorian days right up to today. The shortcomings of existing anatomical textbooks probably impressed themselves on Henry Gray when he was still a student at St George’s Hospital Medical School, near London’s Hyde Park Corner, in the early 1840s. He began thinking about creating a new anatomy textbook a decade later, while war was being fought in the Crimea. New legislation was being planned that would establish the General Medical Council (1858) to regulate professional education and standards. Gray was twenty-eight years old, and a teacher himself at St George’s. He was very able, hard-working and highly ambitious, already a Fellow of the Royal Society, and of the Royal College of Surgeons. Although little is known about his personal life, his was a glittering career so far, achieved while he served and taught on the hospital wards and in the
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dissecting room (Fig. 1) (Anon 1908). Gray shared the idea for the new book with a talented colleague on the teaching staff at St George’s, Henry Vandyke Carter, in November 1855. Carter was from a family of Scarborough artists, and was himself a clever artist and microscopist. He had produced fine illustrations for Gray’s scientific publications before, but could see that this idea was a much more complex project. Carter recorded in his diary: Little to record. Gray made proposal to assist by drawings in bringing out a Manual for students: a good idea but did not come to any plan … too exacting, for would not be a simple artist (Carter 1855). Neither of these young men was interested in producing a pretty book, or an expensive one. Their purpose was to supply an affordable, accurate teaching aid for people like their own students, who might soon be Fig. 1 Henry Gray (1827–1861) is shown here in the foreground, seated by the feet of the cadaver. The photograph was taken by a medical student, Joseph Langhorn. The room is the dissecting room of St
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George’s Hospital Medical School in Kinnerton Street, London. Gray is shown surrounded by staff and students. When the photo was taken, on 27 March 1860, Carter had left St George’s, to become Professor of Anatomy and Physiology at Grant Medical College, in Bombay (nowadays Mumbai). The second edition of Gray’s Anatomy was in its proof stages, to appear in December 1860. Gray died just over a year later, in June 1861, at the height of his powers.
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Historical introduction e6Fig. 2 Henry Vandyke Carter (1831–1897). Carter was appointed Honorary Surgeon to Queen Victoria in 1890. acclaim. Reviews in The Lancet and the British Medical Journal were highly complimentary, and students flocked to buy. It is not difficult to understand why it was a runaway success. Gray’s Anatomy knocked its competitors into a cocked hat. It was considerably smaller and more slender than the doorstopper with which modern readers are familiar. The book held well in the hand, it felt substantial, and it contained everything required. To contemporaries, it was small enough to be portable, but large enough for decent illustrations: ‘royal octavo’ – 912 × 6 inches (24 × 15 cm) – about two-thirds of modern A4 size. Its medium-size, single-volume format was far removed from Quain, yet double the size of Knox’s Manual. Simply organized and well designed, the book explains itself confi - dently and well; the clarity and authority of the prose are manifest. But what made it unique for its day was the outstanding size and quality
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of the illustrations. Gray thanked the wood engravers Butterworth and Heath for the ‘great care and fidelity’ they had displayed in the engrav- ings, but it was really to Carter that the book owed its extraordinary success. The beauty of Carter’s illustrations resides in their diagrammatic clarity, quite atypical for their time. The images in contemporary anatomy books were usually ‘proxy-labelled’: dotted with tiny numbers or letters (often hard to find or read) or bristling with a sheaf of num - bered arrows, referring to a key situated elsewhere, usually in a footnote, which was sometimes so lengthy it wrapped round on to the following page. Proxy labels require the reader’s eye to move to and fro: from the structure to the proxy label to the legend and back again. There was plenty of scope for slippage, annoyance and distraction. Carter’s illustra - tions, by contrast, unify name and structure, enabling the eye to assimi - late both at a glance. We are so familiar with Carter’s images that it is hard to appreciate how incredibly modern they must have seemed in 1858. The volume made human anatomy look new, exciting, accessible
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and do-able. The first edition was covered in a brown bookbinder’s cloth embossed all over in a dotted pattern, and with a double picture-frame border. Its spine was lettered in gold blocking: sense of calamity. The grand old medical man Sir Benjamin Brodie, Sergeant-Surgeon to the Queen, and the great supporter of Gray to whom Anatomy had been dedicated, cried forlornly: ‘Who is there to take his place?’ (Anon 1908). But old JW Parker ensured the survival of Gray’s by inviting Timothy Holmes, the doctor who had helped proof-read the first edition, and who had filled Gray’s shoes at the medical school, to serve as Editor for the next edition. Other long-running anatomy works, such as Quain, remained in print in a similar way, co-edited by other hands (Quain 1856). Holmes (1825–1907) was another gifted St George’s man, a scholar - ship boy who had won an exhibition to Cambridge, where his brilliance was recognized. Holmes was a Fellow of the Royal College of Surgeons at 28. John Parker junior had commissioned him to edit A System of
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Surgery (1860–64), an important essay series by distinguished surgeons on subjects of their own choosing. Many of Holmes’s authors remain important figures, even today: John Simon, James Paget, Henry Gray, Ernest Hart, Jonathan Hutchinson, Brown-Séquard and Joseph Lister. Holmes had lost an eye in an operative accident, and he had a gruff manner that terrified students, yet he published a lament for young Parker that reveals him capable of deep feeling (Holmes 1860). John Parker senior’s heart, however, was no longer in publishing. His son’s death had closed down the future for him. The business, with all its stocks and copyrights, was sold to Messrs Longman. Parker retired to the village of Farnham, where he later died. With Holmes as editor, and Longman as publisher, the immediate future of Gray’s Anatomy was assured. The third edition appeared in 1864 with relatively few changes, Gray’s estate receiving the balance of his royalty after Holmes was paid £100 for his work. THE MISSING OBITUARY Why no obituary appeared for Henry Gray in Gray’s Anatomy is curious.
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Gray had referred to Holmes as his ‘friend’ in the preface to the first edition, yet it would also be true to say that they were rivals. Both had just applied for a vacant post at St George’s, as Assistant Surgeon. Had Gray lived, it is thought that Holmes may not have been appointed, despite his seniority in age (Anon 1908). Later commentators have suggested, as though from inside knowl - edge, that Holmes’s ‘proof-reading’ included improving Gray’s writing … with ‘DESCRIPTIVE AND SURGICAL’ in small capitals underneath. Gray’s Anatomy is how it has been referred to ever since. Carter was given credit with Gray on the book’s title page for undertaking all the dissec - tions on which the book was based, and sole credit for all the illustra - tions, though his name appeared in a significantly smaller type, and he was described as the ‘Late Demonstrator in Anatomy at St George’s Hospital’ rather than being given his full current title, which was Profes - sor of Anatomy and Physiology at Grant Medical College, Bombay. Gray was still only a Lecturer at St George’s and he may have been aware that
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his words had been upstaged by the quality of Carter’s anatomical images. He need not have worried: Gray is the famous name on the spine of the book. Gray was paid £150 for every thousand copies sold. Carter never received a royalty payment, just a one-off fee at publication, which may have allowed him to purchase the long-wished-for microscope he took with him to India (Fig. 2). The first edition print-run of 2000 copies sold out swiftly. A parallel edition was published in the United States in 1859, and Gray must have been deeply gratified to have to revise an enlarged new English edition in 1859–60, though he was surely saddened and worried by the death of his publisher, John Parker junior, at the young age of 40, while the book was going through the press. The second edition came out in the December of 1860 and it too sold like hot cakes, as indeed has every subsequent edition. The following summer, in June 1861, at the height of his powers and full of promise, Henry Gray died unexpectedly at the age of only 34.
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Gray had contracted smallpox while nursing his nephew. A new strain of the disease was more virulent than the one with which Gray had been vaccinated as a child; the disease became confluent, and Gray died in a matter of days. Within months, the whole country would be pitched into mourning for the death of Prince Albert. The creative era over which he had pre - sided – especially the decade that had flowered since the Great Exhibi - tion of 1851 – would be history. THE BOOK SURVIVES Anatomy Descriptive and Surgical could have died too. With Carter in India, the death of Gray, so swiftly after that of the younger Parker, might have spelled catastrophe. Certainly, at St George’s there was a GRAY’S ANATOMY
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Historical introduction e7 were not as yet perfected, and in any case could not provide the bold simplicity of line required for a book like Gray’s, which depended so heavily on clear illustration and clear lettering. Recognizing the inferior - ity of half-tone illustrations by comparison with Carter’s wood-engraved originals, Pick and Howden courageously decided to jettison the second-rate half-tones altogether. Most of the next edition’s illustrations were either Carter’s, or old supplementary illustrations inspired by his work, or newly commissioned wood engravings or line drawings, intended ‘to harmonize with Carter’s original figures’ . They successfully emulated Carter’s verve. Having fewer pages and lighter paper, the 1905 (sixteenth edition) weighed less than its predecessor, at 4 lb 1 1 oz/2.1 kg. Typographically, the new edition was superb. Howden took over as sole editor in 1909 (seventeenth edition) and immediately stamped his personality on Gray’s. He excised ‘Surgical’ from the title, changing it to Anatomy Descriptive and Applied, and
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removed Carter’s name altogether. He also instigated the beginnings of an editorial board of experts for Gray’s, by adding to the title page ‘Notes on Applied Anatomy’ by AJ Jex-Blake and W Fedde Fedden, both St George’s men. For the first time, the number of illustrations exceeded one thousand. Howden was responsible for the significant innovation of a short historical note on Henry Gray himself, nearly 60 years after his death, which included a portrait photograph (1918, twentieth edition). THE NOMENCLATURE CONTROVERSY Howden’s era, and that of his successor TB Johnston (of Guy’s), was overshadowed by a cloud of international controversy concerning ana - tomical terminology. European anatomists were endeavouring to stand - ardize anatomical terms, often using Latinate constructions, a move resisted in Britain and the United States. Gray’s became mired in these debates for over 20 years. The attempt to be fair to all sides by using multiple terms doubtless generated much confusion amongst students, until a working compromise was at last arrived at in 1955 (thirty-second edition, 1958).
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Johnston oversaw the second retitling of the book (in 1938, twenty- seventh edition): it was now, officially, Gray’s Anatomy, finally ending the fiction that it had ever been known as anything else. Gray’s suffered from paper shortages and printing difficulties in World War II, but suc - cessive editions nevertheless continued to grow in size and weight, while illustrations were replaced and added as the text was revised. Between Howden’s first sole effort (1909, seventeenth edition) and Johnston’s last edition (1958, thirty-second edition), Gray’s expanded by over 300 pages – from 1296 to 1604 pages, and almost 300 addi - tional illustrations brought the total to over 1300. Johnston also intro - duced X-ray plates (1938) and, in 1958 (thirty-second edition), electron micrographs by AS Fitton-Jackson, one of the first occasions on which a woman was credited with a contribution to Gray’s. Johnston felt com - pelled to mention that she was ‘a blood relative of Henry Gray himself’, perhaps by way of mitigation. AFTER WORLD WAR II
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The editions of Gray’s issued in the decades immediately following the Second World War give the impression of intellectual stagnation. Steady expansion continued in an almost formulaic fashion, with the insertion of additional detail. The central historical importance of innovation in the success of Gray’s seems to have been lost sight of by its publishers and editors – Johnston (1930–1958, twenty-fourth to thirty-second editions), J Whillis (co-editor with Johnston, 1938–1954), DV Davies (1958–1967, thirty-second to thirty-fourth editions) and F Davies (co-editor with DV Davies 1958–1962, thirty-second to thirty-third editions). Gray’s had become so pre-eminent that perhaps complacency crept in, or editors were too daunted or too busy to confront the ‘massive undertaking’ of a root and branch revision (Tansey 1995). The unexpected deaths of three major figures associated with Gray’s in this era, James Whillis, Francis Davies and David Vaughan Davies – each of whom had been ready to take the editorial reins – may have contributed to retarding the process. The work became somewhat dull. KEY EDITION: 1973
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DV Davies had recognized the need for modernization, but his unex - pected death left the work to other hands. Two Professors of Anatomy at Guy’s, Roger Warwick and Peter Williams, the latter of whom had been involved as an indexer for Gray’s for several years, regarded it as an honour to fulfill Davies’s intentions. Their thirty-fifth edition of 1973 was a significant departure from tradition. Over 780 pages (of 1471) were newly written, almost a third style. This could be a reflection of Holmes’s own self-regard, but there may be some truth in it. There can be no doubt that, as Editor of seven subsequent editions of Gray’s Anatomy (third to ninth editions, 1864– 1880), Holmes added new material, and had to correct and compress passages, but it is also possible that, back in 1857, Gray’s original manuscript had been left in a poor state for Holmes to sort out. In other works, Gray’s writing style was lucid, but he always seems to have paid a copyist to transcribe his work prior to submission. The original manu -
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script of Gray’s Anatomy, sadly, has not survived, so it is impossible to be sure how much of the finished version had actually been written by Holmes. It may be that Gray’s glittering career, or perhaps the patronage that unquestionably advanced it, created jealousies among his colleagues, or that there was something in Gray’s manner that precluded affection, or that created resentments among clever social inferiors like Carter and Holmes, especially if they felt their contributions to his brilliant career were not given adequate credit. Whatever the explanation, no reference to Gray’s life or death appeared in Gray’s Anatomy itself until the twen - tieth century (Howden et al 1918). A SUCCESSION OF EDITORS Holmes expanded areas of the book that Gray himself had developed in the second edition (1860), notably in ‘general’ anatomy (histology) and ‘development’ (embryology). In Holmes’s time as Editor, the volume grew from 788 pages in 1864 to 960 in 1880 (ninth edition), with the histological section paginated separately in roman numerals
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at the front of the book. Extra illustrations were added, mainly from other published sources. The connections with Gray and Carter, and with St George’s, were maintained with the appointment of the next editor, T. Pickering Pick, who had been a student at St George’s in Gray’s time. From 1883 (tenth edition) onwards, Pick kept up with current research, rewrote and inte - grated the histology and embryology into the volume, dropped Holmes from the title page, removed Gray’s preface to the first edition, and added bold subheadings, which certainly improved the appearance and accessibility of the text. Pick said he had ‘tried to keep before himself the fact that the work is intended for students of anatomy rather than for the Scientific Anatomist’ (thirteenth edition, 1893). Pick also introduced colour printing (in 1887, eleventh edition) and experimented with the addition of illustrations using the new printing method of half-tone dots: for colour (which worked) and for new black- and-white illustrations (which did not). Half-tone shades of grey com -
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pared poorly with Carter’s wood engravings, still sharp and clear by comparison. What Henry Vandyke Carter made of these changes is a rich topic for speculation. He returned to England in 1888, having retired from the Indian Medical Service, full of honours – Deputy Surgeon General, and in 1890, he was made Honorary Surgeon to Queen Victoria. Carter had continued researching throughout his clinical medical career in India, and became one of India’s foremost bacteriologists/tropical disease specialists before there was really a name for either discipline. Carter made some important discoveries, including the fungal cause of mycetoma, which he described and named. He was also a key figure in confirming scientifically in India some major international discoveries, such as Hansen’s discovery of the cause of leprosy, Koch’s discovery of the organism causing tuberculosis, and Laveran’s discovery of the organ - ism that causes malaria. Carter married late in life, and his wife was left with two young children when he died in Scarborough in 1897, aged 65. Like Gray, he received no obituary in the book. When Pick was joined on the title page by Robert Howden (a profes -
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sional anatomist from the University of Durham) in 1901 (fifteenth edition), the volume was still easily recognizable as the book Gray and Carter had created. Although many of Carter’s illustrations had been revised or replaced, many others still remained. Sadly, though, an entire section (embryology) was again separately paginated, as its revision had taken longer than anticipated. Gray’s had grown, seemingly inexorably, and was now quite thick and heavy: 1244 pages, weighing 5 lb 8 oz/2.5 kg. Both co-editors, and perhaps also its publisher, were dis - satisfied with it. KEY EDITION: 1905 Serious decisions were taken well in advance of the next edition, which turned out to be Pick’s last with Howden. Published 50 years after Gray had first suggested the idea to Carter, the 1905 (sixteenth) edition was a landmark one. The period 1880–1930 was a difficult time for anatomical illustra- tion, because the new techniques of photo-lithography and half-tone
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Historical introduction e8had developed a distinct character of its own in the interval), and sold extremely well there (Williams and Warwick 1973). The influence of the Warwick and Williams edition was forceful and long-lasting, and set a new pattern for the following quarter-century. As has transpired several times before, wittingly or unwittingly, a new editor was being prepared for the future: Dr Susan Standring (of Guy’s), who created the new bibliography for the 1973 edition of Gray’s, went on to serve on the editorial board, and has served as Editor-in-Chief for the last two editions before this one (2005–2008, thirty-ninth and fortieth editions). Both editions are important for dif - ferent reasons. For the thirty-ninth edition, the entire content of Gray’s was reorgan- ized, from systematic to regional anatomy. This great sea-change was not just organizational but historic, because, since its outset, Gray’s had prioritized bodily systems, with subsidiary emphasis on how the systems interweave in the regions of the body. Professor Standring explained that this regional change of emphasis had long been asked
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for by readers and users of Gray’s, and that new imaging techniques in our era have raised the clinical importance of local anatomy (Standring 2005). The change was facilitated by an enormous collective effort on the part of the editorial team and the illustrators. The subsequent and current editions consolidate that momentous change. (See Table 1.) Table 1  Gray’s Anatomy  Editions Edition Date Author/Editor(s) Publisher Title 1st 1858 Henry Gray JW Parker & Son Anatomy Descriptive and Surgical The drawings by Henry Vandyke Carter. The dissections jointly by the author and Dr Carter 2nd 1860 Henry Gray JW Parker & Son 3rd 1864 T Holmes Longman 4th 1866 T Holmes Longman 5th 1869 T Holmes Longman 6th 1872 T Holmes Longman 7th 1875 T Holmes Longman 8th 1877 T Holmes Longman 9th 1880 T Holmes Longman 10th 1883 TP Pick Longman 11th 1887 TP Pick Longman 12th 1890 TP Pick Longman 13th 1893 TP Pick Longman
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Gray’s preface removed 14th 1897 TP Pick Longman 15th 1901 TP Pick & R Howden Longman 16th 1905 TP Pick & R Howden Longman 17th 1909 Robert Howden Longman Anatomy Descriptive and Applied Notes on applied anatomy by AJ Jex-Blake & W Fedde Fedden 18th 1913 Robert Howden & Blake & Fedden Longman 19th 1916 Robert Howden & Blake & Fedden Longman 20th 1918 Robert Howden & Blake & Fedden Longman First edition ever to feature a photograph and obituary of Henry Gray 21st 1920 Robert Howden Longman Notes on applied anatomy by AJ Jex-Blake & John Clay 22nd 1923 Robert Howden Longman Notes on applied anatomy by John Clay & John D Lickley 23rd 1926 Robert Howden Longman 24th 1930 TB Johnston Longman 25th 1932 TB Johnston Longman 26th 1935 TB Johnston Longman 27th 1938 TB Johnston & J Whillis Longman Gray’s Anatomy
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28th 1942 TB Johnston & J Whillis Longman 29th 1946 TB Johnston & J Whillis Longman 30th 1949 TB Johnston & J Whillis Longman 31st 1954 TB Johnston & J Whillis Longman 32nd 1958 TB Johnston & DV Davies & F Davies Longman 33rd 1962 DV Davies & F Davies Longman 34th 1967 DV Davies & RE Coupland Longman 35th 1973 Peter L Williams & Roger Warwick Longman With a separate volume: Functional Neuroanatomy of Man – being the neurology section of Gray’s Anatomy. 35th edition, 1975 36th 1980 Roger Warwick & Peter L Williams Churchill Livingstone 37th 1989 Peter L Williams Churchill Livingstone 38th 1995 Peter L Williams & Editorial Board Churchill Livingstone 39th 2005 Susan Standring & Editorial Board Elsevier The Anatomical Basis of Clinical Practice 40th 2008 Susan Standring & Editorial Board Elsevier The Anatomical Basis of Clinical Practice
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41st 2015 Susan Standring & Editorial Board Elsevier The Anatomical Basis of Clinical Practiceof the illustrations were newly commissioned, and the illustration cap - tions were freshly written throughout. With a complete re-typesetting of the text in larger double-column pages, a new index and the innova - tion of a bibliography, this edition of Gray’s looked and felt quite unlike its 1967 (thirty-fourth edition) predecessor, and much more like its modern incarnation. This 1973 edition departed from earlier volumes in other significant ways. The editors made explicit their intention to try to counter the impetus towards specialization and compartmentalization in twentieth- century medicine, by embracing and attempting to reintegrate the com - plexity of the available knowledge. Warwick and Williams openly renounced the pose of omniscience adopted by many textbooks, believ - ing it important to accept and mention areas of ignorance or uncer - tainty. They shared with the reader the difficulty of keeping abreast in the sea of research, and accepted with a refreshing humility the impos- sibility of fulfilling their own ambitious programme. Warwick and Williams’s 1973 edition had much in common with
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Gray and Carter’s first edition. It was bold and innovative – respectful of its heritage, while also striking out into new territory. It was visually attractive and visually informative. It embodied a sense of a treasury of information laid out for the reader (Williams and Warwick 1973). It was published simultaneously in the United States (the American Gray’s
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Historical introduction e9 Howden R, Jex-Blake AJ, Fedde Fedden W (eds) 1918 Gray’s Anatomy, 20th ed. London: Longman. Lewis H Sinclair 1925 Arrowsmith. New York: Harcourt Brace; p. 4. Nicol KE 2002 Henry Gray of St George’s Hospital: a Chronology. London: published by the author. Quain J 1856 Elements of Anatomy. Ed. by Sharpey W, Ellis GV. London: Walton & Maberly. Richardson R 2000 Death, Dissection and the Destitute. Chicago: Chicago University Press; pp. 193–249, 287, 357. Richardson R 2008 The Making of Mr Gray’s Anatomy. Oxford: Oxford University Press. Standring S (ed.) 2005 Preface. In: Gray’s Anatomy, 39th ed. Elsevier: London. Tansey EM 1995 A brief history of Gray’s Anatomy. In: Gray’s Anatomy, 38th ed. London: Churchill Livingstone.
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Williams PL, Warwick R (eds.) 1973 Preface. In: Gray’s Anatomy, 35th ed. London: Churchill Livingstone.THE DOCTORS’ BIBLE Neither Gray nor Carter, the young men who – by their committed hard work between 1856 and 1858 – created the original Gray’s Anatomy, would have conceived that so many years after their deaths their book would not only be a household name, but also be regarded as a work of such pre-eminent importance that a novelist half a world away would rank it as cardinal – alongside the Bible and Shakespeare – to a doctor’s education (Sinclair Lewis 1925, Richardson 2008). From this forty-first edition of Gray’s Anatomy, we can look back to appraise the long-term value of their efforts. We can discern how the book they created tri - umphed over its competitors, and has survived pre-eminent. Gray’s is a remarkable publishing phenomenon. Although the volume now looks quite different to the original, and contains so much more, its kinship with the Gray’s Anatomy of 1858 is easily demonstrable by direct descent,
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every edition updated by Henry Gray’s successor. Works are rare indeed that have had such a long history of continuous publication on both sides of the Atlantic, and such a useful one. Ruth Richardson, MA, DPhil, FRHistS Senior Visiting Research Fellow, Centre for Life-Writing Research, King’s College London; Affiliated Scholar in the History and Philosophy of Science, University of Cambridge, UK REFERENCES Anon 1908 Henry Gray. St George’s Hospital Gazette 16:49–54. Carter HV 1855 Diary. Wellcome Western Manuscript 5818; 25 Nov.Gray H 1858 Preface. In: Anatomy: Descriptive and Surgical. London: JW Parker & Son. Holmes T (ed.) 1860 I: Preface. In: A System of Surgery. London: JW Parker & Son.ACKNOWLEDGEMENTS For their assistance while I was undertaking the research for this essay, I should like to thank the Librarians and Archivists and Staff at the British Library, Society of Apothecaries, London School of Hygiene and Tropical Medicine, Royal College of Surgeons, Royal Society of Medi -
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cine, St Bride Printing Library, St George’s Hospital Tooting, Scarbor - ough City Museum and Art Gallery, University of Reading, Wellcome Institute Library, Westminster City Archives and Windsor Castle; and the following individuals: Anne Bayliss, Gordon Bell, David Buchanan, Dee Cook, Arthur Credland, Chris Hamlin, Victoria Killick, Louise King, Keith Nicol, Sarah Potts, Mark Smalley, and Nallini Thevakarrunai. Above all, my thanks to Brian Hurwitz, who has read and advised on the evolving text.
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xviANATOMICAL NOMENCLATURE with the median plane; although often employed, ‘parasagittal’ is there - fore redundant and should not be used. The coronal (frontal) plane is orthogonal to the median plane and divides the body into anterior (front) and posterior (back). The horizontal (transverse) plane is orthogonal to both median and sagittal planes. Radiologists refer to transverse planes as (trans)axial; convention dictates that axial anatomy is viewed as though looking from the feet towards the head. Structures nearer the head are superior, cranial or (sometimes) cephalic (cephalad), whereas structures closer to the feet are inferior; caudal is most often used in embryology to refer to the hind end of the embryo. Medial and lateral indicate closeness to the median plane, medial being closer than lateral; in the anatomical position, the little finger is medial to the thumb, and the great toe is medial to the little toe. Specialized terms may also be used to indicate medial and lateral. Thus, in the upper limb, ulnar and radial are used to mean medial and
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lateral, respectively; in the lower limb, tibial and fibular (peroneal) are used to mean medial and lateral, respectively. Terms may be based on embryological relationships; the border of the upper limb that includes the thumb, and the border of the lower limb that includes the great toe are the pre-axial borders, whilst the opposite borders are the post-axial borders. Various degrees of obliquity are acknowledged using com- pound terms, e.g. posterolateral. When referring to structures in the trunk and upper limb, we have freely used the synonyms anterior, ventral, flexor, palmar and volar, and posterior, dorsal and extensor. We recognize that these synonyms are not always satisfactory, e.g. the extensor aspect of the leg is anterior with respect to the knee and ankle joints, and superior in the foot and digits; the plantar (flexor) aspect of the foot is inferior. Dorsal (dorsum) and ventral are terms used particularly by embryologists and neuroanato - mists; they therefore feature most often in Sections 2 and 3.
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Distal and proximal are used particularly to describe structures in the limbs, taking the datum point as the attachment of the limb to the trunk (sometimes referred to as the root), such that a proximal structure is closer to the attachment of the limb than a distal structure. However, proximal and distal are also used in describing branching structures, e.g. bronchi, vessels and nerves. External (outer) and internal (inner) refer to the distance from the centre of an organ or cavity, e.g. the layers of the body wall, or the cortex and medulla of the kidney. Superficial and deep are used to describe the relationships between adjacent struc - tures. Ipsilateral refers to the same side (of the body, organ or structure), bilateral to both sides, and contralateral to the opposite side. Teeth are described using specific terms that indicate their relation - ship to their neighbours and to their position within the dental arch; these terms are described on page 517.Anatomy is the study of the structure of the body. Conventionally, it is divided into topographical (macroscopic or gross) anatomy (which may be further divided into regional anatomy, surface anatomy, neuro -
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anatomy, endoscopic and imaging anatomy); developmental anatomy (embryogenesis and subsequent organogenesis); and the anatomy of microscopic and submicroscopic structure (histology). Anatomical language is one of the fundamental languages of medi - cine. The unambiguous description of thousands of structures is impos - sible without an extensive and often highly specialized vocabulary. Ideally, these terms, which are often derived from Latin or Greek, should be used to the exclusion of any other, and eponyms should be avoided. In reality, this does not always happen. Many terms are ver - nacularized and, around the world, synonyms and eponyms still abound in the literature, in medical undergraduate classrooms and in clinics. The Terminologia Anatomica, 1 drawn up by the Federative Com- mittee on Anatomical Terminology (FCAT) in 1998, continues to serve as our reference source for the terminology for macroscopic anatomy, and the text of the forty-first edition of Gray’s Anatomy is almost entirely TA-compliant. However, where terminology is at variance with, or, more likely, is not included in, the TA, the alternative term used either is cited
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in the relevant consensus document or position paper, or enjoys wide - spread clinical usage. Synonyms and eponyms are given in parentheses on first usage of a preferred term and not shown thereafter in the text; an updated list of eponyms and short biographical details of the clini - cians and anatomists whose names are used in this way is available in the e-book for reference purposes (see Preface , p. ix, for further discus - sion of the use of eponyms). PLANES, DIRECTIONS AND RELATIONSHIPS To avoid ambiguity, all anatomical descriptions assume that the body is in the conventional ‘anatomical position’, i.e. standing erect and facing forwards, upper limbs by the side with the palms facing forwards, and lower limbs together with the toes facing forwards (Fig. 1). Descrip - tions are based on four imaginary planes – median, sagittal, coronal and horizontal – applied to a body in the anatomical position. The median plane passes longitudinally through the body and divides it into right and left halves. The sagittal plane is any vertical plane parallel
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1Terminologia Anatomica (1998) is the joint creation of the Federative Committee on Anatomical Terminology (FCAT) and the Member Associations of the Interna - tional Federation of Associations of Anatomists (IFAA).
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AnAtomic Al nomencl Ature xvii Fig. 1 The terminology widely used in descriptive anatomy. Abbreviations shown on arrows: AD, adduction; AB, abduction; FLEX, flexion (of the thigh at the hip joint); EXT, extension (of the leg at the knee joint). LEFT LATERAL ASPECTPOSTERIOR ASPECTSUPERIOR ASPECT Lateral InversionEversionMedial (internal) rotationLateral (external) rotationPronationSupinationDistallyProximally DistallyProximallyMedial (internal) rotationLateral (external) rotationMedialPosterior or dorsalAnterior or ventralCoronal plane Median or sagittal plane Transverse or horizontal planeInferior or caudal Superior or cranial INFERIOR ASPECTANTERIOR ASPECTRIGHT LATERAL ASPECT
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xviiiBIBLIOGRAPHY OF SELECTED TITLES Haaga JR, Dogra VS, Forsting M, Gilkeson RC, Ha KH, Sundaram M 2009 CT and MR Imaging of the Whole Body, 5th ed. St Louis: Elsevier, Mosby. Lasjaunias P, Berenstein A, ter Brugge K 2001 Surgical Neuroangio­ graphy, vol 1. Clinical Vascular Anatomy and Variations, 2nd ed. Berlin, New York: Springer. Meyers MA 2000 Dynamic Radiology of the Abdomen: Normal and Pathologic Anatomy, 5th ed. New York: Springer. Pomeranz SJ 1992 MRI Total Body Atlas. Cincinnati: MRI ­EFI. Spratt JD, Salkowski LR, Weir J, Abrahams PH 2010 Imaging Atlas of Human Anatomy, 4th ed. London: Elsevier, Mosby. Sutton D, Reznek R, Murfitt J 2002 Textbook of Radiology and Imaging, 7th ed. Edinburgh: Elsevier, Churchill Livingstone.
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Whaites E, Drage N 2013 Essentials of Dental Radiography and Radiol ­ ogy, 5th ed. Edinburgh: Elsevier, Churchill Livingstone.Wicke L 2004 Atlas of Radiologic Anatomy, 7th ed. Philadelphia: Elsevier, WB Saunders. CLINICAL Birch R 2010 Surgical Disorders of the Peripheral Nerves, 2nd ed. Edin ­ burgh: Elsevier, Churchill Livingstone.Bogduk N 2012 Clinical and Radiological Anatomy of the Lumbar Spine, 5th ed. Edinburgh: Elsevier, Churchill Livingstone. Borges AF 1984 Relaxed skin tension lines (RSTL) versus other skin lines. Plast Reconstr Surg 73:144–50. Burnand KG, Young AE, Lucas JD, Rowlands B, Scholefield J 2005 The New Aird’s Companion in Surgical Studies, 3rd ed. Edinburgh: Elsevier, Churchill Livingstone. Canale ST, Beaty JH 2012 Campbell’s Operative Orthopaedics, 12th ed. Philadelphia: Elsevier, Mosby.
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Cormack GC, Lamberty BGH 1994 The Arterial Anatomy of Skin Flaps, 2nd ed. Edinburgh: Elsevier, Churchill Livingstone. Cramer GD, Darby SA 2013 Clinical Anatomy of the Spine, Spinal Cord, and ANS, 3rd ed. MO: Elsevier, Mosby. Dyck PJ, Thomas PK 2005 Peripheral Neuropathy: 2 ­Volume Set with Expert Consult Basic, 4th ed. Philadelphia: Elsevier, WB Saunders. Ellis H, Mahadevan V 2013 Clinical Anatomy: Applied Anatomy for Students and Junior Doctors, 13th ed. Wiley ­Blackwell. Ellis H Feldman S, Harrop ­Griffiths W 2004 Anatomy for Anaesthetists, 8th ed. Oxford: Blackwell Science.Morris SF, Taylor GI 2013 Vascular territories. In: Neligan PC (ed.) Plastic Surgery, vol. I. Principles, 3rd ed. London: Elsevier, Saunders. Rosai J 201 1 Rosai and Ackerman’s Surgical Pathology, 10th ed. London: Elsevier, Mosby.
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Shah J 2012 Jatin Shah’s Head and Neck Surgery and Oncology: Expert Consult Online and Print, 4th ed. London: Elsevier, Mosby. Zancolli EA, Cozzi EP 1991 Atlas of Surgical Anatomy of the Hand. Edinburgh: Elsevier, Churchill Livingstone. CLINICAL EXAMINATION O’Brien M 2010 Aids to the Examination of the Peripheral Nervous System, 5th ed. London: Elsevier, WB Saunders. Lumley JSP 2008 Surface Anatomy: The Anatomical Basis of Clinical Examination, 4th ed. Edinburgh: Elsevier, Churchill Livingstone.The following references contain information relevant to numerous chapters in this edition. They are therefore cited here rather than at the end of individual chapters. For an extended historical bibliography, all references from the thirty ­eighth edition (which includes all references cited in earlier editions, up to and including the thirty ­eighth edition) are available in the e ­book that accompanies Gray’s Anatomy . TERMINOLOGY Federative Committee on Anatomical Terminology 1998 Terminologia Anatomica: International Anatomical Nomenclature. Stuttgart: Thieme.
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Dorland WAN 201 1 Dorland’s Illustrated Medical Dictionary, 32nd ed. Philadelphia: Elsevier, WB Saunders. BASIC SCIENCES Abrahams P, Spratt JD, Loukas M, van Schoor A ­N 2013 McMinn and Abrahams’ Clinical Atlas of Human Anatomy: with STUDENT CONSULT Online Access, 7th ed. London: Elsevier, Mosby. Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P 2007 Molecu ­ lar Biology of the Cell, 5th ed. New York: Garland Science. Berkovitz BKB, Kirsch C, Moxham BJ, Alusi G, Cheeseman T 2002 Interactive Head and Neck. London: Primal Pictures. Boron WF, Boulpaep E 2012 Medical Physiology: with STUDENT CONSULT Online Access, 2nd ed. Philadelphia: Elsevier, WB Saunders. Crossman AR 2014 Neuroanatomy: An Illustrated Colour Text, 5th ed. Edinburgh: Elsevier, Churchill Livingstone.
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Fitzgerald MD 201 1 Clinical Neuroanatomy and Neuroscience: with STUDENT CONSULT Online Access, 6th ed. Edinburgh: Elsevier, Saunders. Hall JE 2010 Guyton and Hall Textbook of Medical Physiology: with STUDENT CONSULT Online Access, 12th ed. Philadelphia: Elsevier, Saunders. Kerr JB 2010 Functional Histology, 2nd ed. London: Elsevier, Mosby. Kierszenbaum AL 2014 Histology and Cell Biology: An Introduction to Pathology, 4th ed. St Louis: Elsevier, Mosby. Lowe JS, Anderson PG 2014 Stevens & Lowe’s Human Histology, 4th ed. London: Elsevier, Mosby. Male D, Brostoff J, Roth D, Roitt I 2012 Immunology: with STUDENT CONSULT Online Access, 8th ed. London: Elsevier, Mosby. Moore KL, Persaud TVN, Torchia MG 2015 Before We Are Born: Essen ­ tials of Embryology and Birth Defects, 9th ed. St Louis: Elsevier.
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Pollard TD, Earnshaw WC 2007 Cell Biology: with STUDENT CONSULT Access, 2nd ed. Philadelphia: Elsevier, WB Saunders. Salmon M 1994 Anatomic Studies: Book 1 Arteries of the Muscles of the Extremities and the Trunk, Book 2 Arterial Anastomotic Pathways of the Extremities. Ed. by Taylor GI, Razaboni RM. St Louis: Quality Medical. Young B, O’Dowd G, Woodford P 2013 Wheater’s Functional Histology: A Text and Colour Atlas, 6th ed. Edinburgh: Elsevier, Churchill Livingstone. IMAGING AND RADIOLOGY/RADIOLOGICAL ANATOMY Butler P, Mitchell AWM, Healy JC 201 1 Applied Radiological Anatomy, 2nd ed. New York: Cambridge University Press. Ellis H, Logan BM, Dixon AK 2007 Human Sectional Anatomy: Pocket Atlas of Body Sections, CT and MRI Images, 3rd ed. CRC Press.
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4SECTION 1 CHAPTER 1 Basic structure and function of cells Epithelial cells rarely operate independently of each other and com- monly form aggregates by adhesion, often assisted by specialized inter - cellular junctions. They may also communicate with each other either by generating and detecting molecular signals that diffuse across inter - cellular spaces, or more rapidly by generating interactions between membrane-bound signalling molecules. Cohesive groups of cells con - stitute tissues, and more complex assemblies of tissues form functional systems or organs. Most cells are between 5 and 50 µm in diameter: e.g. resting lym- phocytes are 6 µm across, red blood cells 7.5 µm and columnar epithe - lial cells 20 µm tall and 10 µm wide (all measurements are approximate). Some cells are much larger than this: e.g. megakaryocytes of the bone marrow and osteoclasts of the remodelling bone are more than 200 µm in diameter. Neurones and skeletal muscle cells have relatively extended shapes, some of the former being over 1 m in length. CELLULAR ORGANIZATION
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Each cell is contained within its limiting plasma membrane, which encloses the cytoplasm. All cells, except mature red blood cells, also contain a nucleus that is surrounded by a nuclear membrane or enve - lope (see Fig. 1.1; Fig. 1.2). The nucleus includes: the genome of the cell contained within the chromosomes; the nucleolus; and other sub - nuclear structures. The cytoplasm contains cytomembranes and several membrane-bound structures, called organelles, which form separate CELL STRUCTURE GENERAL CHARACTERISTICS OF CELLS The shapes of mammalian cells vary widely depending on their interac - tions with each other, their extracellular environment and internal structures. Their surfaces are often highly folded when absorptive or transport functions take place across their boundaries. Cell size is limited by rates of diffusion, either that of material entering or leaving cells, or that of diffusion within them. Movement of macromolecules can be much accelerated and also directed by processes of active trans - port across the plasma membrane and by transport mechanisms within the cell. According to the location of absorptive or transport functions,
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apical microvilli (Fig. 1.1) or basolateral infoldings create a large surface area for transport or diffusion. Motility is a characteristic of most cells, in the form of movements of cytoplasm or specific organelles from one part of the cell to another. It also includes: the extension of parts of the cell surface such as pseu - dopodia, lamellipodia, filopodia and microvilli; locomotion of entire cells, as in the amoeboid migration of tissue macrophages; the beating of flagella or cilia to move the cell (e.g. in spermatozoa) or fluids overly - ing it (e.g. in respiratory epithelium); cell division; and muscle contrac - tion. Cell movements are also involved in the uptake of materials from their environment (endocytosis, phagocytosis) and the passage of large molecular complexes out of cells (exocytosis, secretion). Fig . 1 .1 The main structural components and internal organization of a generalized cell . Plasma membraneActin filaments Vesicle Golgi apparatusIntermediate filamentsMitochondrion Smooth endoplasmic
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reticulum Rough endoplasmic reticulumPeroxisomes CytosolSurface invaginationSurface projections (cilia, microvilli) Cell junctions Desmosome Microtubules Centriole pairNuclear envelope Nucleus RibosomeNucleolus Lysosomes Cell surface foldsNuclear pore
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Cell structure 5 CHaPTER 1 charides and polysaccharides are bound either to proteins (glycopro - teins) or to lipids (glycolipids), and project mainly into the extracellular domain (Fig. 1.3). In the electron microscope, membranes fixed and contrasted by heavy metals such as osmium tetroxide appear in section as two densely stained layers separated by an electron-translucent zone – the classic unit membrane. The total thickness of each layer is about 7.5 nm. The overall thickness of the plasma membrane is typically 15 nm. Freeze- fracture cleavage planes usually pass along the hydrophobic portion of the bilayer, where the hydrophobic tails of phospholipids meet, and split the bilayer into two leaflets. Each cleaved leaflet has a surface and a face. The surface of each leaflet faces either the extracellular surface (ES) or the intracellular or protoplasmic (cytoplasmic) surface (PS). The extracellular face (EF) and protoplasmic face (PF) of each leaflet are artificially produced during membrane splitting. This technique has
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also demonstrated intramembranous particles embedded in the lipid bilayer; in most cases, these represent large transmembrane protein molecules or complexes of proteins. Intramembranous particles are distributed asymmetrically between the two half-layers, usually adher - ing more to one half of the bilayer than to the other. In plasma mem - branes, the intracellular leaflet carries most particles, seen on its face (the PF). Where they have been identified, clusters of particles usually represent channels for the transmembrane passage of ions or molecules between adjacent cells (gap junctions). Biophysical measurements show the lipid bilayer to be highly fluid, allowing diffusion in the plane of the membrane. Thus proteins are able to move freely in such planes unless anchored from within the cell. Membranes in general, and the plasma membrane in particular, form boundaries selectively limiting diffusion and creating physiologically distinct compartments. Lipid bilayers are impermeable to hydrophilic solutes and ions, and so membranes actively control the passage of ions and small organic molecules such as nutrients, through the activity of
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membrane transport proteins. However, lipid-soluble substances can pass directly through the membrane so that, for example, steroid hor - mones enter the cytoplasm freely. Their receptor proteins are either cytosolic or nuclear, rather than being located on the cell surface. Plasma membranes are able to generate electrochemical gradients and potential differences by selective ion transport, and actively take up or export small molecules by energy-dependent processes. They also provide surfaces for the attachment of enzymes, sites for the receptors and distinct compartments within the cytoplasm. Cytomembranes include the rough and smooth endoplasmic reticulum and Golgi appa- ratus, as well as vesicles derived from them. Organelles include lyso - somes, peroxisomes and mitochondria. The nucleus and mitochondria are enclosed by a double-membrane system; lysosomes and peroxi - somes have a single bounding membrane. There are also non-membranous structures, called inclusions, which lie free in the cytosolic compartment. They include lipid droplets, glycogen aggregates and pig -
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ments (e.g. lipofuscin). In addition, ribosomes and several filamentous protein networks, known collectively as the cytoskeleton, are found in the cytosol. The cytoskeleton determines general cell shape and sup - ports specialized extensions of the cell surface (microvilli, cilia, flag - ella). It is involved in the assembly of specific structures (e.g. centrioles) and controls cargo transport in the cytoplasm. The cytosol contains many soluble proteins, ions and metabolites. Plasma membrane Cells are enclosed by a distinct plasma membrane, which shares fea - tures with the cytomembrane system that compartmentalizes the cyto - plasm and surrounds the nucleus. All membranes are composed of lipids (mainly phospholipids, cholesterol and glycolipids) and pro - teins, in approximately equal ratios. Plasma membrane lipids form a lipid bilayer, a layer two molecules thick. The hydrophobic ends of each lipid molecule face the interior of the membrane and the hydrophilic
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ends face outwards. Most proteins are embedded within, or float in, the lipid bilayer as a fluid mosaic. Some proteins, because of extensive hydrophobic regions of their polypeptide chains, span the entire width of the membrane (transmembrane proteins), whereas others are only superficially attached to the bilayer by lipid groups. Both are integral (intrinsic) membrane proteins, as distinct from peripheral (extrinsic) membrane proteins, which are membrane-bound only through their association with other proteins. Carbohydrates in the form of oligosac- Fig . 1 .2 The structural organization and some principal organelles of a typical cell . This example is a ciliated columnar epithelial cell from human nasal mucosa . The central cell, which occupies most of the field of view, is closely apposed to its neighbours along their lateral plasma membranes . Within the apical junctional complex, these membranes form a tightly sealed zone (tight junction) that isolates underlying tissues from, in this instance, the nasal cavity . Abbreviations: AJC, apical junctional complex; APM, apical plasma membrane; C, cilia; Cy, cytoplasm; EN,
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euchromatic nucleus; LPM, lateral plasma membrane; M, mitochondria; MV, microvilli; N, nucleolus . (Courtesy of Dr Bart Wagner, Histopathology Department, Sheffield Teaching Hospitals, UK .) C MV M Cy LPM N ENMAPMAJCV M C M Cy LPM N ENMAPMAJC Fig . 1 .3 The molecular organization of the plasma membrane, according to the fluid mosaic model of membrane structure . Intrinsic or integral membrane proteins include diffusion or transport channel complexes, receptor proteins and adhesion molecules . These may span the thickness of the membrane (transmembrane proteins) and can have both extracellular and cytoplasmic domains . Transmembrane proteins have hydrophobic zones, which cross the phospholipid bilayer and allow the protein to ‘float’ in the plane of the membrane . Some proteins are restricted in their freedom of movement where their cytoplasmic domains are tethered to the cytoskeleton . Receptor protein Lipid bilayer appearancein electronmicroscopeInternal (intracellular)
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surfaceCarbohydrateresidues TransmembraneproteinIntrinsic membrane protein Transport or diffusion channelExtrinsic proteinTransmembrane pore complex of proteins External (extracellular) surface Polar end ofphospholipidNon-polar tailof phospholipid Cytoskeletalelement
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Basic structure and function of cells 5.e1 CHaPTER 1 Combinations of biochemical, biophysical and biological tech - niques have revealed that lipids are not homogenously distributed in membranes, but that some are organized into microdomains in the bilayer, called ‘detergent-resistant membranes’ or lipid ‘rafts’, rich in sphingomyelin and cholesterol. The ability of select subsets of proteins to partition into different lipid microdomains has profound effects on their function, e.g. in T-cell receptor and cell–cell signalling. The highly organized environment of the domains provides a signalling, trafficking and membrane fusion environment.
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BaSIC STR uCTuRE aNd fu NCTION Of CEllS 6SECTION 1 abundant proteins; SER is abundant in steroid-producing cells and muscle cells. A variant of the endoplasmic reticulum in muscle cells is the sarcoplasmic reticulum, involved in calcium storage and release for muscle contraction. For further reading on the endoplasmic reticulum, see Bravo et al (2013). Smooth endoplasmic reticulum The smooth endoplasmic reticulum (see Fig. 1.4) is associated with carbohydrate metabolism and many other metabolic processes, includ - ing detoxification and synthesis of lipids, cholesterol and steroids. The membranes of the smooth endoplasmic reticulum serve as surfaces for the attachment of many enzyme systems, e.g. the enzyme cytochrome P450, which is involved in important detoxification mechanisms and is thus accessible to its substrates, which are generally lipophilic. The membranes also cooperate with the rough endoplasmic reticulum and the Golgi apparatus to synthesize new membranes; the protein, carbohydrate and lipid components are added in different structural
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compartments. The smooth endoplasmic reticulum in hepatocytes con - tains the enzyme glucose-6-phosphatase, which converts glucose-6- phosphate to glucose, a step in gluconeogenesis. Rough endoplasmic reticulum The rough endoplasmic reticulum is a site of protein synthesis; its cytosolic surface is studded with ribosomes ( Fig. 1.5E). Ribosomes only bind to the endoplasmic reticulum when proteins targeted for secretion begin to be synthesized. Most proteins pass through its membranes and accumulate within its cisternae, although some integral membrane pro - teins, e.g. plasma membrane receptors, are inserted into the rough endoplasmic reticulum membrane, where they remain. After passage from the rough endoplasmic reticulum, proteins remain in membrane- bound cytoplasmic organelles such as lysosomes, become incorporated into new plasma membrane, or are secreted by the cell. Some carbohy - drates are also synthesized by enzymes within the cavities of the rough endoplasmic reticulum and may be attached to newly formed protein (glycosylation). Vesicles are budded off from the rough endoplasmic
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reticulum for transport to the Golgi as part of the protein-targeting mechanism of the cell. Ribosomes, polyribosomes and protein synthesis Ribosomes are macromolecular machines that catalyse the synthesis of proteins from amino acids; synthesis and assembly into subunits takes place in the nucleolus and includes the association of ribosomal RNA (rRNA) with ribosomal proteins translocated from their site of synthesis in the cytoplasm. The individual subunits are then transported into the cytoplasm, where they remain separate from each other when not actively synthesizing proteins. Ribosomes are granules approximately 25 nm in diameter, composed of rRNA molecules and proteins assem - bled into two unequal subunits. The subunits can be separated by their sedimentation coefficients (S) in an ultracentrifuge into larger 60S and smaller 40S components. These are associated with 73 different pro - teins (40 in the large subunit and 33 in the small), which have structural and enzymatic functions. Three small, highly convoluted rRNA strands
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(28S, 5.8S and 5S) make up the large subunit, and one strand (18S) is in the small subunit. A typical cell contains millions of ribosomes. They may form groups (polyribosomes or polysomes) attached to messenger RNA (mRNA), which they translate during protein synthesis for use outside the system of membrane compartments, e.g. enzymes of the cytosol and cytoskel - etal proteins. Some of the cytosolic products include proteins that can be inserted directly into (or through) membranes of selected organelles, such as mitochondria and peroxisomes. Ribosomes may be attached to the membranes of the rough endoplasmic reticulum (see Fig. 1.5E). In a mature polyribosome, all the attachment sites of the mRNA are occupied as ribosomes move along it, synthesizing protein according to its nucleotide sequence. Consequently, the number and spacing of ribosomes in a polyribosome indicate the length of the mRNA mole -
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cule and hence the size of the protein being made. The two subunits have separate roles in protein synthesis. The 40S subunit is the site of attachment and translation of mRNA. The 60S subunit is responsible for the release of the new protein and, where appropriate, attachment to the endoplasmic reticulum via an intermediate docking protein that directs the newly synthesized protein through the membrane into the cisternal space. Golgi apparatus (Golgi complex) The Golgi apparatus is a distinct cytomembrane system located near the nucleus and the centrosome. It is particularly prominent in secretory cells and can be visualized when stained with silver or other metallic of external signals, including hormones and other ligands, and sites for the recognition and attachment of other cells. Internally, plasma mem - branes can act as points of attachment for intracellular structures, in particular those concerned with cell motility and other cytoskeletal functions. Cell membranes are synthesized by the rough endoplasmic reticulum in conjunction with the Golgi apparatus. Cell coat (glycocalyx) The external surface of a plasma membrane differs structurally from internal membranes in that it possesses an external, fuzzy, carbohydrate-
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rich coat, the glycocalyx. The cell coat forms an integral part of the plasma membrane, projecting as a diffusely filamentous layer 2–20 nm or more from the lipoprotein surface. The cell coat is composed of the carbohydrate portions of glycoproteins and glycolipids embedded in the plasma membrane (see Fig. 1.3). The precise composition of the glycocalyx varies with cell type; many tissue- and cell type-specific antigens are located in the coat, including the major histocompatibility complex of the immune system and, in the case of erythrocytes, blood group antigens. Therefore, the glycocalyx plays a significant role in organ transplant compatibility. The glycocalyx found on apical microvilli of enterocytes, the cells forming the lining epithelium of the intestine, consists of enzymes involved in the diges - tive process. Intestinal microvilli are cylindrical projections (1–2 µm long and about 0.1 µm in diameter) forming a closely packed layer called the brush border that increases the absorptive function of enterocytes. Cytoplasm
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Compartments and functional organization The cytoplasm consists of the cytosol, a gel-like material enclosed by the cell or plasma membrane. The cytosol is made up of colloidal pro - teins such as enzymes, carbohydrates and small protein molecules, together with ribosomes and ribonucleic acids. The cytoplasm contains two cytomembrane systems, the endoplasmic reticulum and Golgi apparatus, as well as membrane-bound organelles (lysosomes, peroxi - somes and mitochondria), membrane-free inclusions (lipid droplets, glycogen and pigments) and the cytoskeleton. The nuclear contents, the nucleoplasm, are separated from the cytoplasm by the nuclear envelope. Endoplasmic reticulum The endoplasmic reticulum is a system of interconnecting membrane-lined channels within the cytoplasm ( Fig. 1.4). These channels take various forms, including cisternae (flattened sacs), tubules and vesicles. The membranes divide the cytoplasm into two major compartments. The intramembranous compartment, or cisternal space, is where secre -
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tory products are stored or transported to the Golgi complex and cell exterior. The cisternal space is continuous with the perinuclear space. Structurally, the channel system can be divided into rough or granu - lar endoplasmic reticulum (RER), which has ribosomes attached to its outer, cytosolic surface, and smooth or agranular endoplasmic reticu- lum (SER), which lacks ribosomes. The functions of the endoplasmic reticulum vary greatly and include: the synthesis, folding and transport of proteins; synthesis and transport of phospholipids and steroids; and storage of calcium within the cisternal space and regulated release into the cytoplasm. In general, RER is well developed in cells that produce Fig . 1 .4 Smooth endoplasmic reticulum with associated vesicles . The dense particles are glycogen granules . (Courtesy of Rose Watson, Cancer Research UK .)
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Basic structure and function of cells 6.e1 CHaPTER 1 The glycocalyx plays a significant role in maintenance of the integrity of tissues and in a wide range of dynamic cellular processes, e.g. serving as a vascular permeability barrier and transducing fluid shear stress to the endothelial cell cytoskeleton (Weinbaum et al 2007). Disruption of the glycocalyx on the endothelial surface of large blood vessels precedes inflammation, a conditioning factor of atheromatosis (e.g. deposits of cholesterol in the vascular wall leading to partial or complete obstruc - tion of the vascular lumen). Protein synthesis on ribosomes may be suppressed by a class of RNA molecules known as small interfering RNA (siRNA) or silencing RNA. These molecules are typically 20–25 nucleotides in length and bind (as a complex with proteins) to specific mRNA molecules via their comple - mentary sequence. This triggers the enzymatic destruction of the mRNA or prevents the movement of ribosomes along it. Synthesis of the encoded protein is thus prevented. Their normal function may have
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antiviral or other protective effects; there is also potential for developing artificial siRNAs as a therapeutic tool for silencing disease-related genes.
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Cell structure 7 CHaPTER 1 Fig . 1 .5 The Golgi apparatus and functionally related organelles . A, Golgi apparatus (G) adjacent to the nucleus (N) (V, vesicle) . B, A large residual body (tertiary lysosome) in a cardiac muscle cell (M, mitochondrion) . C, The functional relationships between the Golgi apparatus and associated cellular structures . D, A three-dimensional reconstruction of the Golgi apparatus in a pancreatic β cell showing stacks of Golgi cisternae from the cis-face (pink) and cis-medial cisternae (red, green) to the trans-Golgi network (blue, yellow, orange–red); immature proinsulin granules (condensing vesicles) are shown in pale blue and mature (crystalline) insulin granules in dark blue . The flat colour areas represent cut faces of cisternae and vesicles . E, Rough endoplasmic reticulum (R), associated with the Golgi apparatus (G) . (D, Courtesy of Dr Brad Marsh, Institute for Molecular Bioscience, University of
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Queensland, Brisbane . A,B,E From human tissue, courtesy of Dr Bart Wagner, Histopathology Department, Sheffield Teaching Hospitals, UK .) M Phagocytic pathway Secretory pathway Receptor-mediated endocytosis Membrane recycling Early endosome Late endosome Secondary lysosome Residual body cis-Golgi network Rough endoplasmic reticulumGolgi cisternaeVesicle shuttling between cisternaeLysosomal fusionClathrin-coated pit trans-Golgi networkA B C D EG VN G G RG G
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BaSIC STR uCTuRE aNd fu NCTION Of CEllS 8SECTION 1 Endocytic (internalization) pathway The endocytic pathway begins at the plasma membrane and ends in lysosomes involved in the degradation of the endocytic cargo through the enzymatic activity of lysosomal hydrolases. Endocytic cargo is internalized from the plasma membrane to early endosomes and then to late endosomes. Late endosomes transport their cargo to lyso - somes, where the cargo material is degraded following fusion and mixing of contents of endosomes and lysosomes. Early endosomes derive from endocytic vesicles (clathrin-coated vesicles and caveolae). Once internalized, endocytic vesicles shed their coat of adaptin and clathrin, and fuse to form an early endosome, where the receptor molecules release their bound ligands. Membrane and receptors from the early endosomes can be recycled to the cell surface as exocytic vesicles. Clathrin-dependent endocytosis occurs at specialized patches of plasma membrane called coated pits; this mechanism is also used to
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internalize ligands bound to surface receptor molecules and is also termed receptor-mediated endocytosis. Caveolae (little caves) are struc - turally distinct pinocytotic vesicles most widely used by endothelial and smooth muscle cells, when they are involved in transcytosis, signal transduction and possibly other functions. In addition to late endo - somes, lysosomes can also fuse with phagosomes, autophagosomes and plasma membrane patches for membrane repair. Lysosomal hydro - lases process or degrade exogenous materials (phagocytosis or hetero - phagy) as well as endogenous material (autophagy). Phagocytosis consists of the cellular uptake of invading pathogens, apoptotic cells and other foreign material by specialized cells. Lysosomes are numerous in actively phagocytic cells, e.g. macrophages and neutrophil granulo- cytes, in which lysosomes are responsible for destroying phagocytosed particles, e.g. bacteria. In these cells, the phagosome, a vesicle poten - tially containing a pathogenic microorganism, may fuse with several lysosomes. Autophagy involves the degradation and recycling within an
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autophagosome of cytoplasmic components that are no longer needed, including organelles. The assembly of the autophagosome involves several proteins, including autophagy-related (Atg) proteins, as well as Hsc70 chaperone, that translocate the substrate into the lysosome (Boya et al 2013). Autophagosomes sequester cytoplasmic components and then fuse with lysosomes without the participation of a late endosome. The 26S proteasome (see below) is also involved in cellular degradation but autophagy refers specifically to a lysosomal degradation–recycling pathway. Autophagosomes are seen in response to starvation and cell growth. Late endosomes receive lysosomal enzymes from primary lysosomes derived from the Golgi apparatus after late endosome–lysosome mem - brane tethering and fusion followed by diffusion of lysosomal contents into the endosomal lumen. The pH inside the fused hybrid organelle, now a secondary lysosome, is low (about 5.0) and this activates lyso -
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somal acid hydrolases to degrade the endosomal contents. The products of hydrolysis either are passed through the membrane into the cytosol, or may be retained in the secondary lysosome. Secondary lysosomes may grow considerably in size by vesicle fusion to form multivesicular bodies, and the enzyme concentration may increase greatly to form large lysosomes ( Fig. 1.5B). Lysosomes Lysosomes are membrane-bound organelles 80–800 nm in diameter, often with complex inclusions of material undergoing hydrolysis (sec - ondary lysosomes). Two classes of proteins participate in lysosomal function: soluble acid hydrolases and integral lysosomal membrane proteins. Each of the 50 known acid hydrolases (including proteases, lipases, carbohydrases, esterases and nucleases) degrades a specific sub - strate. There are about 25 lysosomal membrane proteins participating in the acidification of the lysosomal lumen, protein import from the cytosol, membrane fusion and transport of degradation products to the cytoplasm. Material that has been hydrolysed within secondary lyso -
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somes may be completely degraded to soluble products, e.g. amino acids, which are recycled through metabolic pathways. However, degra - dation is usually incomplete and some debris remains. A debris-laden vesicle is called a residual body or tertiary lysosome (see Fig. 1.5B), and may be passed to the cell surface, where it is ejected by exocytosis; alternatively, it may persist inside the cell as an inert residual body. Considerable numbers of residual bodies can accumulate in long-lived cells, often fusing to form larger dense vacuoles with complex lamellar inclusions. As their contents are often darkly pigmented, this may change the colour of the tissue; e.g. in neurones, the end-product of lysosomal digestion, lipofuscin (neuromelanin or senility pigment), gives ageing brains a brownish-yellow colouration. Lysosomal enzymes salts. Traffic between the endoplasmic reticulum and the Golgi appara - tus is bidirectional and takes place via carrier vesicles derived from the donor site that bud, tether and fuse with the target site. Golgins are long coiled-coil proteins attached to the cytoplasmic
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surface of cisternal membranes, forming a fibrillar matrix surrounding the Golgi apparatus to stabilize it; they have a role in vesicle trafficking (for further reading on golgins, see Munro 201 1). The Golgi apparatus has several functions: it links anterograde and retrograde protein and lipid flow in the secretory pathway; it is the site where protein and lipid glycosylation occurs; and it provides membrane platforms to which signalling and sorting proteins bind. Ultrastructurally, the Golgi apparatus (Fig. 1.5A) displays a contin- uous ribbon-like structure consisting of a stack of several flattened membranous cisternae, together with clusters of vesicles surrounding its surfaces. Cisternae differ in enzymatic content and activity. Small transport vesicles from the rough endoplasmic reticulum are received at one face of the Golgi stack, the convex cis-face (entry or forming surface). Here, they deliver their contents to the first cisterna in the series by membrane fusion. From the edges of this cisterna, the protein is transported to the next cisterna by vesicular budding and then
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fusion, and this process is repeated across medial cisternae until the final cisterna at the concave trans-face (exit or condensing surface) is reached. Here, larger vesicles are formed for delivery to other parts of the cell. The cis-Golgi and trans-Golgi membranous networks form an inte - gral part of the Golgi apparatus. The cis-Golgi network is a region of complex membranous channels interposed between the rough endo - plasmic reticulum and the Golgi cis-face, which receives and transmits vesicles in both directions. Its function is to select appropriate proteins synthesized on the rough endoplasmic reticulum for delivery by vesicles to the Golgi stack, while inappropriate proteins are shuttled back to the rough endoplasmic reticulum. The trans-Golgi network, at the other side of the Golgi stack, is also a region of interconnected membrane channels engaged in protein sorting. Here, modified proteins processed in the Golgi cisternae are packaged selectively into vesicles and dispatched to different parts of the cell. The packaging depends on the detection, by the trans-Golgi
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network, of particular amino-acid signal sequences, leading to their enclosure in membranes of appropriate composition that will further modify their contents, e.g. by extracting water to concentrate them (vesicles entering the exocytosis pathway) or by pumping in protons to acidify their contents (lysosomes destined for the intracellular sorting pathway). Within the Golgi stack proper, proteins undergo a series of sequen - tial chemical modifications by Golgi resident enzymes synthesized in the rough endoplasmic reticulum. These include: glycosylation (changes in glycosyl groups, e.g. removal of mannose, addition of N-acetylglucosamine and sialic acid); sulphation (addition of sulphate groups to glycosaminoglycans); and phosphorylation (addition of phosphate groups). Some modifications serve as signals to direct pro - teins and lipids to their final destination within cells, including lyso-somes and plasma membrane. Lipids formed in the endoplasmic reticulum are also routed for incorporation into vesicles. Exocytic (secretory) pathway Secreted proteins, lipids, glycoproteins, small molecules such as amines
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and other cellular products destined for export from the cell are trans - ported to the plasma membrane in small vesicles released from the trans-face of the Golgi apparatus. This pathway either is constitutive, in which transport and secretion occur more or less continuously, as with immunoglobulins produced by plasma cells, or it is regulated by exter- nal signals, as in the control of salivary secretion by autonomic neural stimulation. In regulated secretion, the secretory product is stored tem - porarily in membrane-bound secretory granules or vesicles. Exocytosis is achieved by fusion of the secretory vesicular membrane with the plasma membrane and release of the vesicle contents into the extracel- lular domain. In polarized cells, e.g. most epithelia, exocytosis occurs at the apical plasma membrane. Glandular epithelial cells secrete into a duct lumen, as in the pancreas, or on to a free surface, such as the lining of the stomach. In hepatocytes, bile is secreted across a very small area of plasma membrane forming the wall of the bile canaliculus. This region is defined as the apical plasma membrane and is the site of
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exocrine secretion, whereas secretion of hepatocyte plasma proteins into the blood stream is targeted to the basolateral surfaces facing the sinusoids. Packaging of different secretory products into appropriate vesicles takes place in the trans-Golgi network. Delivery of secretory vesicles to their correct plasma membrane domains is achieved by sorting sequences in the cytoplasmic tails of vesicular membrane proteins.
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Basic structure and function of cells 8.e1 CHaPTER 1 Carrier vesicles in transit from the endoplasmic reticulum to the Golgi apparatus (anterograde transport) are coated by coat protein complex II (COPII), whereas COPI-containing vesicles function in the retrograde transport route from the Golgi apparatus (reviewed in Spang (2013)). The membranes contain specific signal proteins that may allocate them to microtubule-based transport pathways and allow them to dock with appropriate targets elsewhere in the cell, e.g. the plasma mem - brane in the case of secretory vesicles. Vesicle formation and budding at the trans-Golgi network involves the addition of clathrin on their external surface, to form coated pits. Specialized cells of the immune system, called antigen-presenting cells, degrade protein molecules, called antigens, transported by the endocytic pathway for lysosomal breakdown, and expose their frag - ments to the cell exterior to elicit an immune response mediated ini - tially by helper T cells.
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Cell structure 9 CHaPTER 1 Mitochondria In the electron microscope, mitochondria usually appear as round or elliptical bodies 0.5–2.0 µm long ( Fig. 1.6), consisting of an outer mitochondrial membrane; an inner mitochrondrial membrane, sepa- rated from the outer membrane by an intermembrane space; cristae, infoldings of the inner membrane that harbour ATP synthase to gener - ate ATP; and the mitochondrial matrix, a space enclosed by the inner membrane and numerous cristae. The permeability of the two mito - chondrial membranes differs considerably: the outer membrane is freely permeable to many substances because of the presence of large non-specific channels formed by proteins (porins), whereas the inner membrane is permeable to only a narrow range of molecules. The pres - ence of cardiolipin, a phospholipid, in the inner membrane may con - tribute to this relative impermeability. Mitochondria are the principal source of chemical energy in most
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cells. Mitochondria are the site of the citric acid (Krebs’) cycle and the electron transport (cytochrome) pathway by which complex organic molecules are finally oxidized to carbon dioxide and water. This process provides the energy to drive the production of ATP from adenosine diphosphate (ADP) and inorganic phosphate (oxidative phosphoryla - tion). The various enzymes of the citric acid cycle are located in the mitochondrial matrix, whereas those of the cytochrome system and oxidative phosphorylation are localized chiefly in the inner mitochon - drial membrane. The intermembrane space houses cytochrome c, a molecule involved in activation of apoptosis. The number of mitochondria in a particular cell reflects its general energy requirements; e.g. in hepatocytes there may be as many as 2000, whereas in resting lymphocytes there are usually very few. Mature may also be secreted – often as part of a process to alter the extracellular matrix, as in osteoclast-mediated erosion during bone resorption. For further reading on lysosome biogenesis, see Saftig and Klumperman (2009). lysosomal dysfunction
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Lysosomal storage diseases (LSDs) are a consequence of lysosomal dysfunction. Approximately 60 different types of LSD have been identi - fied on the basis of the type of material accumulated in cells (such as mucopolysaccharides, sphingolipids, glycoproteins, glycogen and lipo - fuscins). LSDs are characterized by severe neurodegeneration, mental decline, and cognitive and behavioural abnormalities. Autophagy impairment caused by defective lysosome–autophagosome coupling triggers a pathogenic cascade by the accumulation of substrates that contribute to neurodegenerative disorders including Parkinson’s dis - ease, Alzheimer’s disease, Huntington’s disease and several tau-opathies. Many lysosomal storage diseases are known, e.g. Tay–Sachs disease (GM2 gangliosidosis), in which a faulty β-hexosaminidase A leads to the accumulation of the glycosphingolipid GM2 ganglioside in neu - rones, causing death during childhood. In Danon disease, a vacuolar skeletal myopathy and cardiomyopathy with neurodegeneration in hemizygous males, lysosomes fail to fuse with autophagosomes because
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of a mutation of the lysosomal membrane protein LAMP-2 (lysosomal associated membrane protein-2). 26S proteasome A protein can be degraded by different mechanisms, depending on the cell type and different pathological conditions. Furthermore, the same substrate can engage different proteolytic pathways (Park and Cuervo 2013). Three major protein degradation mechanisms operate in eukaryotic cells to dispose of non-functional cellular proteins: the autophagosome–lysosomal pathway (see above); the apoptotic procaspase–caspase pathway (see below); and the ubiquitinated protein–26S proteasome pathway. The 26S proteasome is a multicata - lytic protease found in the cytosol and the nucleus that degrades intra - cellular proteins conjugated to a polyubiquitin chain by an enzymatic cascade. The 26S proteasome consists of several subunits arranged into two 19S polar caps, where protein recognition and adenosine 5 ′- triphosphate (ATP)-dependent target processing occur, flanking a 20S central barrel-shaped structure with an inner proteolytic chamber
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(Tomko and Hochstrasser 2013). The 26S proteasome participates in the removal of misfolded or abnormally assembled proteins, the deg - radation of cyclins involved in the control of the cell cycle, the process - ing and degradation of transcription regulators, cellular-mediated immune responses, and cell cycle arrest and apoptosis. Peroxisomes Peroxisomes are small (0.2–1 µm in diameter) membrane-bound organelles present in most mammalian cells. They contain more than 50 enzymes responsible for multiple catabolic and synthetic biochemi - cal pathways, in particular the β-oxidation of very-long-chain fatty acids (>C22) and the metabolism of hydrogen peroxide (hence the name peroxisome). Peroxisomes derive from the endoplasmic reticu - lum through the transfer of proteins from the endoplasmic reticulum to peroxisomes by vesicles that bud from specialized sites of the endo - plasmic reticulum and by a lipid non-vesicular pathway. All matrix proteins and some peroxisomal membrane proteins are synthesized by cytosolic ribosomes and contain a peroxisome targeting signal that
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enables them to be imported by proteins called peroxins (Braverman et al 2013, Theodoulou et al 2013). Mature peroxisomes divide by small daughter peroxisomes pinching off from a large parental peroxisome. Peroxisomes often contain crystalline inclusions composed mainly of high concentrations of the enzyme urate oxidase. Oxidases use molecular oxygen to oxidize specific organic substrates (such as L-amino acids, D-amino acids, urate, xanthine and very-long-chain fatty acids) and produce hydrogen peroxide that is detoxified (degraded) by per - oxisomal catalase. Peroxisomes are particularly numerous in hepato - cytes. Peroxisomes are important in the oxidative detoxification of various substances taken into or produced within cells, including ethanol. Peroxin mutation is a characteristic feature of Zellweger syn - drome (craniofacial dysmorphism and malformations of brain, liver, eye and kidney; cerebrohepatorenal syndrome). Neonatal leukodystro -
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phy is an X-linked peroxisomal disease affecting mostly males, caused by deficiency in β-oxidation of very-long-chain fatty acids. The build-up of very-long-chain fatty acids in the nervous system and suprarenal glands determines progressive deterioration of brain function and suprarenal insufficiency (Addison’s disease). For further reading, see Braverman et al (2013). Fig . 1 .6 A, Mitochondria in human cardiac muscle . The folded cristae (arrows) project into the matrix from the inner mitochondrial membrane . B, The location of the elementary particles that couple oxidation and phosphorylation reactions . (A, Courtesy of Dr Bart Wagner, Histopathology Department, Sheffield Teaching Hospitals, UK .) A B Elementary particlesCristae (folds)Inner membraneOuter membrane
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Basic structure and function of cells 9.e1 CHaPTER 1 The transcription factor EB (TFEB) is responsible for regulating lyso - somal biogenesis and function, lysosome-to-nucleus signalling and lipid catabolism (for further reading, see Settembre et al (2013)). If any of the actions of lysosomal hydrolases, of the lysosome acidification mechanism or of lysosomal membrane proteins fails, the degradation and recycling of extracellular substrates delivered to lysosomes by the late endosome and the degradation and recycling of intracellular sub - strates by autophagy lead to progressive lysosomal dysfunction in several tissues and organs. Experimentally, TFEB activation can reduce the accumulation of the pathogenic protein in a cellular model of Huntington’s disease (a neurodegenerative genetic disorder that affects muscle coordination) and improves the Parkinson’s disease phenotype in a murine model. Cristae are abundant in mitochondria seen in cardiac muscle cells and in steroid-producing cells (in the suprarenal cortex, corpus luteum and Leydig cells). The protein steroidogenic acute regulatory protein (StAR) regulates the synthesis of steroids by transporting
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cholesterol across the outer mitochondrial membrane. A mutation in the gene encoding StAR causes defective suprarenal and gonadal steroidogenesis.
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BaSIC STR uCTuRE aNd fu NCTION Of CEllS 10 SECTION 1 ent and its electronic charge, and the potential difference across the membrane. These factors combine to produce an electrochemical gradi - ent, which governs ion flux. Channel proteins are utilized most effec - tively by the excitable plasma membranes of nerve cells, where the resting membrane potential can change transiently from about −80 mV (negative inside the cell) to +40 mV (positive inside the cell) when stimulated by a neurotransmitter (as a result of the opening and sub - sequent closure of channels selectively permeable to sodium and potassium). Carrier proteins bind their specific solutes, such as amino acids, and transport them across the membrane through a series of conforma - tional changes. This latter process is slower than ion transport through membrane channels. Transport by carrier proteins can occur either pas - sively by simple diffusion, or actively against the electrochemical gradi - ent of the solute. Active transport must therefore be coupled to a source of energy, such as ATP generation, or energy released by the coordinate movement of an ion down its electrochemical gradient. Linked trans -
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port can be in the same direction as the solute, in which case the carrier protein is described as a symporter, or in the opposite direction, when the carrier acts as an antiporter. Translocation of proteins across intracellular membranes Proteins are generally synthesized on ribosomes in the cytosol or on the rough endoplasmic reticulum. A few are made on mitochondrial ribosomes. Once synthesized, many proteins remain in the cytosol, where they carry out their functions. Others, such as integral membrane proteins or proteins for secretion, are translocated across intracellular membranes for post-translational modification and targeting to their destinations. This is achieved by the signal sequence, an addressing system contained within the protein sequence of amino acids, which is recognized by receptors or translocators in the appropriate membrane. Proteins are thus sorted by their signal sequence (or set of sequences that become spatially grouped as a signal patch when the protein folds into its tertiary configuration), so that they are recognized by and enter the correct intracellular membrane compartment. Cell signalling Cellular systems in the body communicate with each other to coordi -
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nate and integrate their functions. This occurs through a variety of processes known collectively as cell signalling, in which a signalling molecule produced by one cell is detected by another, almost always by means of a specific receptor protein molecule. The recipient cell trans - duces the signal, which it most often detects at the plasma membrane, into intracellular chemical messages that change cell behaviour. The signal may act over a long distance, e.g. endocrine signalling through the release of hormones into the blood stream, or neuronal synaptic signalling via electrical impulse transmission along axons and subsequent release of chemical transmitters of the signal at syn - apses or neuromuscular junctions. A specialized variation of endocrine signalling (neurocrine or neuroendocrine signalling) occurs when neu - rones or paraneurones (e.g. chromaffin cells of the suprarenal medulla) secrete a hormone into interstitial fluid and the blood stream. Alternatively, signalling may occur at short range through a paracrine mechanism, in which cells of one type release molecules into the inter - stitial fluid of the local environment, to be detected by nearby cells of a different type that express the specific receptor protein. Neurocrine
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cell signalling uses chemical messengers found also in the central nervous system, which may act in a paracrine manner via interstitial fluid or reach more distant target tissues via the blood stream. Cells may generate and respond to the same signal. This is autocrine signal - ling, a phenomenon that reinforces the coordinated activities of a group of like cells, which respond together to a high concentration of a local signalling molecule. The most extreme form of short-distance signalling is contact-dependent (juxtacrine) signalling, where one cell responds to transmembrane proteins of an adjacent cell that bind to surface recep - tors in the responding cell membrane. Contact-dependent signalling also includes cellular responses to integrins on the cell surface binding to elements of the extracellular matrix. Juxtacrine signalling is impor - tant during development and in immune responses. These different types of intercellular signalling mechanism are illustrated in Figure 1.7. For further reading on cell signalling pathways, see Kierszenbaum and Tres (2012). Signalling molecules and their receptors
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The majority of signalling molecules (ligands) are hydrophilic and so cannot cross the plasma membrane of a recipient cell to effect changes erythrocytes lack mitochondria altogether. Cells with few mitochondria generally rely largely on glycolysis for their energy supplies. These include some very active cells, e.g. fast twitch skeletal muscle fibres, which are able to work rapidly but for only a limited duration. Mito - chondria appear in the light microscope as long, thin structures in the cytoplasm of most cells, particularly those with a high metabolic rate, e.g. secretory cells in exocrine glands. In living cells, mitochondria constantly change shape and intracellular position; they multiply by growth and fission, and may undergo fusion. The mitochondrial matrix is an aqueous environment. It contains a variety of enzymes, and strands of mitochondrial DNA with the capacity for transcription and translation of a unique set of mitochondrial genes (mitochondrial mRNAs and transfer RNAs, mitochondrial ribosomes with rRNAs). The DNA forms a closed loop, about 5 µm across; several
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identical copies are present in each mitochondrion. The ratio between its bases differs from that of nuclear DNA, and the RNA sequences also differ in the precise genetic code used in protein synthesis. At least 13 respiratory chain enzymes of the matrix and inner membrane are encoded by the small number of genes along the mitochondrial DNA. The great majority of mitochondrial proteins are encoded by nuclear genes and made in the cytosol, then inserted through special channels in the mitochondrial membranes to reach their destinations. Their membrane lipids are synthesized in the endoplasmic reticulum. It has been shown that mitochondria are of maternal origin because the mitochondria of spermatozoa are not generally incorporated into the ovum at fertilization. Thus mitochondria (and mitochondrial genetic variations and mutations) are passed only through the female line. Mitochondria are distributed within a cell according to regional energy requirements, e.g. near the bases of cilia in ciliated epithelia, in the basal domain of the cells of proximal convoluted tubules in the renal cortex (where considerable active transport occurs) and around the proximal segment, called middle piece, of the flagellum in sperma -
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tozoa. They may be involved with tissue-specific metabolic reactions, e.g. various urea-forming enzymes are found in liver cell mitochondria. Moreover, a number of genetic diseases of mitochondria affect particu - lar tissues exclusively, e.g. mitochondrial myopathies (skeletal muscle) and mitochondrial neuropathies (nervous tissue). For further informa - tion on mitochondrial genetics and disorders, see Chinnery and Hudson (2013). Cytosolic inclusions The aqueous cytosol surrounds the membranous organelles described above. It also contains various non-membranous inclusions, including free ribosomes, components of the cytoskeleton, and other inclusions, such as storage granules (e.g. glycogen), pigments (such as lipofuscin granules, remnants of the lipid oxidative mechanism seen in the supra - renal cortex) and lipid droplets. lipid droplets Lipid droplets are spherical bodies of various sizes found within many cells, but are especially prominent in the adipocytes (fat cells) of adipose connective tissue. They do not belong to the Golgi-related vacu -
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olar system of the cell. They are not membrane-bound, but are droplets of lipid suspended in the cytosol and surrounded by perilipin proteins, which regulate lipid storage and lipolysis. See Smith and Ordovás (2012) for further reading on obesity and perilipins. In cells specialized for lipid storage, the vacuoles reach 80 µm or more in diameter. They function as stores of chemical energy, thermal insulators and mechani - cal shock absorbers in adipocytes. In many cells, they may represent end-products of other metabolic pathways, e.g. in steroid-synthesizing cells, where they are a prominent feature of the cytoplasm. They may also be secreted, as in the alveolar epithelium of the lactating breast. Transport across cell membranes Lipid bilayers are increasingly impermeable to molecules as they increase in size or hydrophobicity. Transport mechanisms are therefore required to carry essential polar molecules, including ions, nutrients, nucleotides and metabolites of various kinds, across the plasma mem - brane and into or out of membrane-bound intracellular compartments. Transport is facilitated by a variety of membrane transport proteins,
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each with specificity for a particular class of molecule, e.g. sugars. Trans - port proteins fall mainly into two major classes: channel proteins and carrier proteins. Channel proteins form aqueous pores in the membrane, which open and close under the regulation of intracellular signals, e.g. G-proteins, to allow the flux of solutes (usually inorganic ions) of specific size and charge. Transport through ion channels is always passive, and ion flow through an open channel depends only on the ion concentration gradi -
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