Paper - Description of a Human Embryo of Twenty-three Paired Somites
Peter Thompson
<pubmed>17232726</pubmed>| PMC1289111
| Historic Disclaimer - information about historic embryology pages |
|---|
 |
Introduction
THE embryo which forms the basis of this work came from Dr Robert Meyer's collection in Berlin. Sent to Professor Keibel, who was accumulating material for his forthcoming Normentafeln of human embryos, the specimen was most kindly lent to me with the object of making a model whilst I was at the Anatomical Institute in Freiburg during the summer of 1906. Working with this specimen, I had an excellent opportunity of becoming acquainted with the reconstruction of embryos by the wax-plate method, as carried out so successfully in that University.
The embryo, obtained at an operation, was recorded as 2.5 mm. long, and was cut transversely into 488 sections, each 5 , in thickness, and stained with borax-carmine. In making the model, every other section was drawn, enlarged 100 diameters, and the wax plates were made 1 millimetre in thick- ness. Whentheplateshadbeencutandlaidinposition,itwasfound, owingr prol)ably to the hot weather and the weight of wax, that the total height of the model was only 220 millimetres, and, in order to correct the error, 24 additional wax plates were made, duplicates of every tenth section, and introduced into the series. In this way the total length of the model was brought up to 244 millimetres, corresponding to the 244 sections used, which indicates a shortage of less than 3 per cent, when compared with the 250 millimetres, the absolutely accurate measurement which the enlargement should have been, taking the length of embryo as 25 min. The difference is probably due to a slight shrinkage, which would most probably occur in preparing the specimen for cutting. It may be stated here that the embryo is histologically in excellent condition, mitosis being readily observed in the multiplying cells, and there seems no valid reason for doubting that the specimen is a normal one. Inaddition to the model of the whole embryo and its yolk sac, other models were made of special organs, namely, the heart and its endothelial tube, the brain, a part of the alimentary canal, and the septum transversum.
It may be noted that the embryo described in this paper resembles in many ways His's embryo Lg., which was2-15mm. in length and estimated to be about fifteen days old.
Descriptions of Models
The Embryo and its Yolk Sac
The head is small, somewhat flattened from above downwards, and pointed. At the side, the opening of the otocyst externally is seen over the upper end of the second post-oral bronchial cleft. Anteriorly, two slight but well-marked bulgings indicate the position of the optic vesicles, between which is a depressed area corresponding to the anterior neuropore, com- pletely closed, but stil recognizable in the sections by the continuity of the general ectoderm and the neural ectoderm. The mouth is a narrow transverse cleft, at the bottom of which the bucco-pharyngeal membrane is seen, perforated in two or three places. Behind the mouth three bronchial clefts are seen externally, and the large prominence below it indicates the position of the heart. There is no trace of limbs.
The alimentary canal is in wide communication with the yolk sac through the vitelline duct, which, with the body stalk, occupies the umbilical orifice. The yolk sac, about as large as the embryo, is globular in form and presents two marked depressions, an upper one for the accommodation of the head, and the prominence of the heart, and a lower one for thetailandthebodystalk. Thebodystalk,aboutthesamethicknessas the vitelline duct, below and to the left side of which it is placed as it emerges from the umbilical orifice, is directed forwards, downwards, and outwardstojointhechorion. Inthecaudalregionthebackoftheembryo curves to end in the prominent tail which is sharply flexed and turned to the right, so that the embryo appears to be somewhat spirally twisted. Here the neural groove is not yet closed, giving rise to a posterior neuro- pore, a gutter, shallow at first but gradually deepening up to the point where it becomes continuous with the central canal of the spinal cord just infrontofthebendoftherump. Thecloacalmembraneisclearlydistin- guishable on the side opposite to the neural groove, and the free extremity ofthecaudaisreceivedintoadepressionontherightsideofthebodystalk, intoalitlecul-de-sacformedbytheamnion. Atthecurveoftherump the mesoblastic somites are recognizable as distinct protuberances, whereas in the region of the back and the neck they are much less prominent. Root of Amnion.-Caudalwards, the root of the amnion is prolonged on to the dorsal aspect of the body stalk, where it has a V-shaped attachment. Thetwolimbsarecontinuedupwardsoneithersideofthe umbilical orifice, that on the right passing between the tail and the stalk and forming, as mentioned above, the recess into which the tip of the tail isreceived. Attheupper(anterior)boundaryoftheumbilicalorificethe amnion is reflected along a line which runs transversely across the embryo at the lower end of the heart prominence, and in this way the attachment oftherootoftheamnioniscompleted. Itwillthusbeseenthat,inthis embryo, a very small part only of the body stalk, on its dorsal surface, is covered with ectoderm.
Nervous System
The nervous system is closed except in the region of the tail, where, as already noted, there is a posterior neuropore. Whilst, therefore, the caudal portion of the neural tube is the last part to close in this embryo,this is not invariably the rule. Indeed there seems to be considerable variations both with regard to the last place of closure of the tube, and the time at which the closure is complete.
According to Hertwig's Hanzdbuch, the medullary groove in the human embryo at the end of the second week is not yet closed, and attention is drawn to the fact that the series, from the second week to the time when the closure is complete, have not yet been described. The specimen under consideration forms another link in that series, of which a few may be briefly noted.
In Eternod's (1) embryo of eight paired somites and a length of 2-1 mm., the medullary folds are united in the cervical and thoracic regions, but the groove is open in front and behind. InKollmann'sembryo(vonBulle)(2) of thirteen paired somites and a length of 2 3 mm., the groove is closed posteriorly, but open for a considerable distance in front. In Janosik's (3) embryo of twenty-four paired somites and a length of 3 mm., the whole length of the medullary canal is closed except in front, in the neighbour- hood of the cerebral vesicles, where an incomplete fusion of the edges of the medullarytubeisvisible. IntheembryodescribedbyHis(4),2'4mm. long, the medullary groove, with the exception of a short portion, is closed. Lastly, J. L. Bremer(5), in an embryo of 4mm., which he has recently described, found the neural groove open both in front and behind. With differences such as these, it seems impossible to state any definite time, length of embryo, or the number of somites with which complete closure might be associated.
The brain consists of three cerebral vesicles. The fore-brain, with its optic outgrowths, but with no trace of cerebral hemispheres or hypophysis; the mid-brain, the smallest of the three, and distinctly marked of, both in front and behind, by two constrictions; and the hind-brain, larger than either of the preceding vesicles, and more over distinguished by exhibiting a series of neuromeres. These are seven in number, of which the second, third, fourth, and fifth are the most prominent.
Certain important structures are in relationship with these neuromeres. The trigeminalganglion,somewhat pear-shaped, is placed opposite the second; the ganglion agoustico-facialis is placed opposite the fourth; and theotocystoppositethefifth. Theotocystisahollowroundedvesicle, with a small aperture opening on to the surface at the side of the head: it isincontactmesiallywiththehind-brain. Theganglionoftheninthand tenth crania] nerves isnot yet visible,and this may explain the imperfect differentiation of the sixth and seventh neuromeres, opposite which the gangliaofthesenervesnormallydevelop. The number of cases in which seven neuromeres have been observed in association with the hind-brain are now so numerous that the arrangement maybeconsiderednormal. C.Bradley(6)in1904describedtheseriesin the pig, and Broman (7) in 1895 fully described the neuromeres of the hind-brain in a human embryo of three millimetres. The flexures of the brain in this specimen are so different from those usually described that a brief reference must now be made to them. At the isthmus rhombencephali, the district between the mid- and hind- brains, there is a prominent flexure, which also involves the anterior extremityofthenotochord. Themid-brainandthefore-brainarebent downwards, but there is no flexure of the fore-brain round the anterior end ofthenotochord. Itsplaceistakenbytheflexureattheisthmus,andthe head-bendoftheembryocorrespondswithit. A pontineflexureoccursbetweentheanteriorpartofthehind-brain, which is placed horizontally, and the posterior portion which slopes down- wardsandbackwards. Theangleofthebendisopendownwards,and is placed opposite the fourth neuromere. This bend does not correspond with the pons flexure formed later as a result of a ventral bending of the floor of the hind-brain. The neck flexure is a very gradual one, and there is no indication of any constriction separating the brain from the cord. Two of the flexures just described are so different from those universally regarded as primary cerebral flexures, that one hesitates to gofurtherthansimplyplacethemonrecord. One point, however, may perhaps be added. The model shows the brain in a very early stage of development,beforethehemispheresorthehypophysishaveappeared. Is itnotpossiblethattheremay becertainflexures,ofatemporarycharacter, which precede the primary flexures usually described? It will be interesting to see if future models of embryos at this age exhibit any flexures at all resembling them.
Notochord and Somites
The notochord lies close to the ventral surface of the spinal cord and brain, the general curve of which it follows. At its cephalic end it terminates oppositethe bucco-pharyngeal membrane under the mid-brain, atitsjunctionwiththefore-brain. Thispartisdistinctlyflexedinassocia- tion with the bend of the neural tube at the isthmus rhombencephali, and a litlefurther back, i.e.under the foremost neuromeres of the hind-brain, it is thicker than elsewhere, and here it retains a direct connection with theentoderm. Atitscaudalendthenotochordcanbe-tracedroundthe region of the rump into the tail, where it ends indistinctly by joining the undifferentiated mesoderm of that structure. There are twenty-three pairs of somites, of which the first three, or three and a half,may be regarded asoccipital,theremainderconstituting thetrunksomites,ofwhichtherearetwenty. Thetwenty-firsttrunk somite is just beginning to form, but there is a region, including the tail, in which the mesoderm is not yet segmented.
Alimentary Canal
The pharynx is separated from the mouth by the bucco-pharyngeal membrane, which is already perforated, but there is no trace of the diverticulum of Rathke. Behind the membrane the pharynx is relatively capacious in a trans- verse direction, and exhibits on each side four pouches, of which the first is the largest, the succeeding ones diminishing in size from before backwards. Theyarecompressedantero-posteriorly, and thefirstthreeterminatein bluntextremities,whicharemoreorlessverticallydisposed. Thefourth pouchisverysmallandpointed. Asstatedabove,thereareonlythree depressions externally behind the mouth, forming the bronchial clefts. In the roof of the pharynx there is a prominent ridge continuous with the notochord, and connected with the floor is a small, hollow, rounded diverticulum,themedianrudimentofthethyroidbody. Itopensintothe pharynx by an exceedingly small aperture some distance behind the remains of the bucco-pharyngeal membrane, and opposite to the first pharyngeal pouch. Thereisnoindicationofthetuberculumimparorthefurcula. Behind the fourth pair of pharyngeal pouches the alimentary tube suddenly narrows, and becoming compressed laterally, forms a cleft-like lumendirectedantero-posteriorly. Thepartofthetube,however,where the gradual change from a transversely disposed lumen to an antero- posterioroneismanifest,isparticularlyinterestingandimportant,asbeing theregionatwhichtheanlageofthelungsmayberecognized. Thetwo lung buds are already indicated as outgrowths from the entodermal tube, the epithelium of which is markedly thickened in this situation, and they aresituatedashortdistancebehindthefourthpairofpouches. The left lung bud, more prominent, is placed in advance of the light, and both are growing towards the pleural passages, the communications between the pericardialandperitonealportionsofthecelonm. The lung rudiments are placed opposite either the hindmost occipital or first trunk somite in front (cephalad) of the dorsal inesocardium of the heart.
It is significant that the lung buds in such an early embryo should already be bilateral, particularly in connection with the debatable question of the nature of the lung anlage and the changes leading to the formation ofthelungsacs. Whilst,asJ.W.Flint(8)hasrecentlypointedout,thereis almost unanimity of opinion on the question of an unpaired origin, amongst those who have contributed to our knowledge of the development of the lungs, there are, however, a few who believe in a paired anlage, and regard the mammalian respiratory apparatus as arising from primitively paired structures. Insomuchthatthepairedbudsarealreadypresentinthis embryo, it lends some support to the latter view.
It is a very interesting question, seeing that a paired anlage for the lungswouldbringthemintolinewiththebronchialpouches. "Thelungs," writes J. M. Flint (8) when dealing with this view, " while not representing actually existing bronchial pouches (would) indicate the reappearance of endoderimic evaginations of the head gut which have carriedgilsamong theancestorsofvertebrates." Below the origin of the lung buds is the anlage of the Esophagus and stomach, and in this part of the tube, beginning with the region where the lung rudiments are found, and extending backwards as far as the liver bud, theepithelialliningismarkedlythickened. Oppositethethirdandfourth trunksomnitestheliverisseengrowingintotheseptumtransversum. The hepatic bud is a median structure with thick walls enclosing a cavity in communicationwiththealimentarycanal. Thereisnotraceofthe pancreas.
The succeeding part of the alimentary canal is in wide communication with the yolk sac, and then follows the hind gut, a very narrow part with aroundedlumen. Thisterminatesinthecloaca,arelativelycapacious cavity, pear-shaped in transverse section, opposite the point of junction with the allantois,triangular in section,beyond this,towardsthetailgut. The Wolffian ducts have not yet reached the cloaca, but terminate in connection with the ectoderin some distance fromt it.
Along its greater curvature the cloaca is in relation with the notochord, and along its lesser curvature it comes into contact with the ectoderm at therootofthetailtoformthecloacalmembrane. Theterminalpartof the gut extends beyond the ineimbrane into the tail,and forms the post- analogue.
Connected with the cloaca is the allantois, an extremely small tubular structure which extends outwards for some distance through the umbilical orificeembeddedin-themesodermofthebodystalk Atitscloacalendit is somewhat funnel-shaped, and at the opposite extremity, where it ends blindly, is a small swelling bent upon itself.
The allantois at its origin is situated, like the -cloaca, between the two primarycaudalarches. Fromthisoriginitisdirecteddorsalwards,and forwards, towards the umbilical orifice, being placed between the umbilical arteries. Thispartoftheductisverynarrowandoffairlyuniform calibre until it changes its direction, and runs ventralwards through the umbilicalorifice. Atthebendisawell-markedswelling,andanotherone is present just outside the orifice, a short distance from the blind extremity. This part of the tube is also accompanied by the umbilical arteries, which fuse to form a single vessel soon after they pass out of the embryo into the body stalk.
Glossary Links
- Glossary: A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | Numbers | Symbols | Term Link
Cite this page: Hill, M.A. (2024, June 2) Embryology Paper - Description of a Human Embryo of Twenty-three Paired Somites. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_Description_of_a_Human_Embryo_of_Twenty-three_Paired_Somites
- © Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G
