Paper - Description of a Human Embryo of 13-14 Mesodermic Somites
Alexander Low
Senior Assistant to the Professor of Anatomy and Lecturer on Embryology, University of Aberdeen. (From the Anatomy Department, University of Aberdeen.)
<pubmed>17232769</pubmed>| PMC1289161
Historic Embryology
This is a slightly edited version of the original 1908 paper published in Journal of Anatomy and Physiology. The full paper is still available as a PDF document through PubMed Central.
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Introduction
THE human embryo Pfannenstiel- III. was placed at the disposal of Pro- fessor Keibel, for publication in his Normentafel zur Entwickelu4qgs- geschichtedes Menschen,by Professor Pfannenstiel of Griefswald. Inthe NormentafeltheembryoisNo.6(fig.Vr.andVv.) ProfessorKeibel kindly afforded me the opportunity of studying this embryo in the Anatomical Institute in Freiburg, and at his suggestion I undertook to reconstruct models of the embryo according toBorn's method. The work of modelling was begun in the Anatomical Institute in Freiburg, and completed in the Anatomy Department, Aberdeen University.
The embryo Pfannenstiel I. was obtained at an operation, and as measured through the amnion had a length of 2-6mm. The embryo was fixed in formalin-Muller's fluid, stained in paracarmine, and sectioned at a thickness of 10MA. Histologically the embryo is in an excellent state of preservation. A model of the whole embryo has been reconstructed at an enlargement of 100. A separate model of the nervous and alimentary systems has also been made, and three models of pharynx, heart with its blood-vessels, and septum transversum at an enlargement of 150.
External Form
(Plates I. and II.) The embryo is somewhat similar to the embryo von Bulle of Kollmann (7), which shows fourteen mesodermic somites and measured 25mm. in length, but is not so far developed; indeed, its outer form more nearly approaches that of the embryo of eight paired somites and a length of 2.15 mm. described and modelled by Professor Eternod (4). Like the Eternod embryo, it is spirally twisted in its long axis, and shows the same three back curvatures -a convex neck bend, a concave dorsal, and a convex rump. The dorsal flexure is slight, while the neck bend makes a right angle with the body, approaching in this respect the form of the embryo of twenty-three somites and a length of 2.15mm. described by Professor Peter Thompson(9). The medullary plate is still open at its head and caudal ends. The front of the head is directed down and slightly to the right over the bulging heart region,and thus is formed a well-developed stomodaeum. Three visceral clefts are visible-the first and second being deep, the third a shallow groove. The first and second visceral clefts lie dorsal to the rounded swelling caused by the commencing mandibular arch. The first cleft occupies a position more to the cranial end of the embryo. The second cleft is separated by a convex ridge from the first, and its caudal end extends to the convexity of the body wall caused by the heart. The third visceral cleft is caudal and somewhat more dorsal, and separated by a considerable interval from the other two clefts. Its ventral end leads into a depression of the body wall behind the pericardial region. The ear pit is represented by an oval depressed area of thickened epithelium just cranial to the dorsal end of the third visceral cleft. There is a large elongated umbilical opening through which the embryonic and extra-embryonic coelom are in communication. The yolk sac is constricted at its communication with the primitive intestine to form a distinct neck. The body stalk arises from the ventral aspect of the embryo and passes to the right, lying first in front and then to the right of the caudal end of the embryo, the tail being directed to the left. The body stalk is separated from the neck of the yolk sac by a narrow interval. The caudal end of the embryo is swollen out, and terminates in a blunt point; a trace of the primitive streak is prolonged from the medullary groove to the cloacal membrane. Along the line of attachment of the amnion to the lateral body wall on each side there is a ridge caused by the underlying umbilical vein; this ridge bounds the umbilical orifice laterally. The amnion extends on to the dorsal aspect of the body stalk, leaving its lateral and ventral aspects uncovered-a disposition similar to that in the embryo described by Professor Thompson.
Mesodermic Somites and Notochord
There are thirteen pairs of well-formed inesodermic somites, and in addition to these there is an anterior pair incompletely formed. Thismost anterior and rudimentary pair of somites is situated just at the level of the neck bend. The last pair of somites are well formed, and situated at the level of the commencement of the rump bend. As the ganglia are not developed, it is not possible to differentiate the regions to which the somites belong. Each somite has a uniformly thick wall three or four cells deep, enclosing a distinct cavity (myocoel). Towards the cavity many of the cells are undergoing mitotic division. Themorecranial-placedsomites are triangular on cross-section, while the more posterior ones are quad- rangular(fig.1). The notochord extends from a point behind the buccopharyngeal membrane, to end in the caudal region just short of the hind-gut and between the cloaca and medullary plate, and not extending into the tail.
FIG. 1.-Right half of a transverse section of the embryothroughtheregionoftheeleventhmeso- dermicsomite. Ec.,ectoderm; XI.M.S.,mesodermic somite; St.,segmental tubule; Coe., coelom; d.Ao., dorsal aorta; Sp.c., medul- larycanal; En.,entodermn;Ch.,notochord.
The chorda lies in close relation with entoderm forming the roof of the intestine, and only toward its caudal termination is it quite separated of, and here lies in the mesoderm midway between the gut and the medullary plate. At its cranial commencement the chorda is at first not separated of, but appears more as a heaping up of entoderm cels in the roof of the pharynx; soon, however, it becomes more differentiated, as represented in fig.2. At the level of the first pair of mesodermic somites the chorda appears more as an evagination of the entoderm of the root of the gut, and this condition obtains almost to its caudal end, where it gets quiteseparatedofffromentoderm(fig.3). Thecellsofthechordaare large, oval, and clear, and contain small, distinctly staining nuclei; in some ofthecellsmitosisisseen. Thereisnotraceofacuticularmembrane.
Vascular System
Cardinal Veins.-The anterior cardinal veins, containing blood-cells, arepresentintheregionofthefirsttothefifthmesodermicsomites. A connection with the heart cannot be established from a careful examination ofthesections. Notraceofposteriorcardinalveinsistobeseen.
Vitelirne Veins.-Although inthe wall of the yolk sac are numerous blood-vesselscontainingblod-cels,stilthe vitelline veins are dificultto trace. Theirterminalpartscanbedefinedwheretheylieintheseptum transversum on the ventral aspect of the gut, and open into the sinus venosusmesialtotheumbilicalveins. Beforetheyopenintothesinus venosus, a transverse piece connects the two vitelline veins.
UmbilicalVei'ns.-Theumbilical veinsarelargevesels,andlieinthe body stalk at first dorsal to the umbilical arteries; here they communicate, and then run cranialwards in the body stalk to gain the lateral body wall. From the lateral body wall the cranial end of each vein bends abruptly, passingmesiallyandventrallyeachtojoinitsown hornofthesinus venosus; here each vein lies embedded in the septum transversum and closetotheflooroftheparietalrecess(fig.12).
Aortac.-The aortic stem divides into a pair of aortic arch vessels, which pass on either side of the median thyroid anlage backwards in the first visceral arch, and each of these is continued caudally as a dorsal aorta. Therearetracesalsoofasecondpairofaorticarchvessels. The dorsal aortas rest on the roof of the gut on either side of the notochord, and do not unite in any part of their course, remaining paired throughout andterminatingonthesidesofthecloaca. A shortdistancebeforethe termination of the dorsal aortfe the umbilical arteries are given off and pass on either side of the allantoic duct into the body stalk, where they unite, forming a single trunk, which again divides into two vessels.
The Heart.-The heart is in a stage very similar to that represented by Born's model (1) of the heart of a rabbit embryo of 095 mm. length- model1(Ziegler). The heart as a whole is S-shaped, and shows sinus venosus, auricle, ventricle, and just at the base of the aortic stem a slight dilatation which may represent a bulbus cords.
The sinus venosus is crescentic in shape, with dilated ends, and is situated in the septum transversum in close relation with the ventral wallofthegut. Openingintoitshornsaretheterminationsofthelarge umbilicalveins,andmesialtothesethevitellineveins. Thesinuspasses into the auricle without any very distinct constriction except on the left side, where there is a slight furrow between the two.
Explanation of Plates
Plate I
Model of the embryo, left side (enlarged 60 times).
Plate II
Model of the ernbiyo. frontil view (enlarged 60 times).
Plate III
The same model in sagittal section, viewed from the left.
Al., alanitoic duiet; Am., anmnion; Ao., origin of first aortic arch; B.S., body stalk ; Ch., chorda dorsalis ; (ltr., choriol ; Cl., cloaca; Cae., ewlom ; D.M., dorsal miesentery; F.B., fire-braii; F.G., fore-gut; G.af., ganglion acustico-facialis; H.G., biind-gut; Ily., hypophysis; AM., mouth ; M.B., midl-brain; Ml.G., mid-gut; Op.,01)ticvesicle; (it.,oticpit;P.C.,l)elicardialceloom;S.T'.,septutnitransversum $1., sinus venosus; T'h., mediant thyroid ; 1.(Ta., lefGuumbilical artery; 1.U.r.%, left umbilical vein; r.U.r. right umbilical vein; Y.S., yolk sac; I. ald III., firstand third visceral clefts; I.-YVIJ., first to seventh neuronieres.
Bibliography
(1) BORN, G., " Beitrige zur Entwicklungsgeschichte des Saugethierherzens," Arich.f/.nik/r. Anat., Bd. xi., 1889.
(2) BREMER, J. L., "Description of a 4 mnm. Human Embryo," The American JowanalofAnatomy, vol.v.,1906.
(3) BROMAN, J., " Beschreibung eines menschlichen Embryos von beinahe 3 mm. Lange, mit spezieller Bemerkung uber die bei demselben befindfichen Hirnfalten," Morph. Arbeiten, Bd. v., 1896.
(4) ETERNOD,A.,"Surun ceufhumainde1-3mm.avereimbryonde,21mm.," Actes de la Societe helcetique des Sciences naturelles, Zurich, 1896.
(5) His, WV., Anatomie menscilicher Embr-yonen, Leipzig, 1885.
(6) INGALLS, N. W., "Beschreibung eitiesmenschlicheniEmbryos von 4.9inm.," Archivf. mikr. Anat., Bd. lx., 1907.
(7) KOLLMANN, J., " Die Ktrperform menschlicher normaler und pathologiselmer Embryonen," Arch.f. Anat. u.Phys., Suplemi.Bd., 1889.
(8) MALL, F. P., "Onl the Development of the Human Diaphragm," Johns Hopkins Hospital Bulletin, vol. xi., 1901.
(9) THOMPSON, P.,"Description of a Human Embryo of Twenty-three Paired Somites," Jour-n. of Anat. and Phys., xli., 1907.
(10) THOMPSON, P., " A Note on the Development of the Septum rransversum and the Liver," Joumn. of Anat. and Phy1s.,xli., 1908.
References
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