Paper - A Human Embryo with Seven Pairs of Somites Measuring about 2 mm in Length: Difference between revisions

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==Ectoderm==
==Ectoderm==


Nervous system. The extent of development of the nervous system is a medullary groove, open throughout its entire length. The brain region is divided into three primary Vesicles (Plate VI), the anterior being equal in size to the other two combined. The first or
Nervous system. The extent of development of the nervous system is a medullary groove, open throughout its entire length. The brain region is divided into three primary Vesicles (Plate VI), the anterior being equal in size to the other two combined. The first or anterior vesicle is long, wide and deep with edges everted and projecting outward over the anterior and lateral Walls of the anterior body elevation. The walls of the second and third vesicles have a tendency toward inversion and the enclosed vesicles are more Sharply defined laterally than the first. The brain passes insensibly into the spinal cord, which is much smaller in all diameters, but is nowhere closed. Posteriorly the medullary groove forms a shallow dilatation gradually fading into the flattened surface of the primitive streak. (Plate IV.) M
 
 
No neurcnteric canal is visible, though present in the younger embryos of Frassi, Graf Spec and Eternod and possibly the Kroemer-Pfannenstiel Klb. No traces of spinal or cerebral ganglia or nerves are visible. N o suggestion of anlage of the lens, optic or otio vesicles couldloe detected.
 
 
Primitive streak. Posterior-ly and dorsally in the region of the termination of the notochord and neural groove, ectoderm and mesoderm gradually losing their individual morphological characteristics, fuse to form a mass of a single variety of simple undifferentiated cells, which extends to the posterior termination of the embryo. This is the primitive streak (Fig. 7 The entoderm also seems to be a part of this mass since its cells are directly continuous with it, although the charac-tcristic lining of the hinclgut is still maintained throughout. There is no groove except a very shallow flattened
dorsal depression which is gradually lost posteriorly and which is continuous anteriorly with the neural groove. This primitive streak region suggests a storehouse of simple undiiferentiated cells supplying mesoderm, ectoderm and entoderm in the earlier stages, later becoming difierentiated into the characteristic morphology and arrangement of the different layers.
 
==Entoderm==
 
Entoderm lines the Ventral surface of embryo, ‘fore and hind gut, allantois and the inner surface of the umbilical vesicle. In the ventral entoderm of the embryo is a median longitudinal groove corresponding to the location of the notochord and caused by its adherence to the ectoderm of the medullary groove. On either side of this groove and parallel to it is a ridge caused by
 
 





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Dandy WE. A Human Embryo with Seven Pairs of Somites Measuring about 2 mm in Length. (1910) Vol. 10

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A Human Embryo with Seven Pairs of Somites Measuring about 2 mm in Length

By

Walter E. Dandy.

From The Anatomical Laboratory, Johns Hopkins University.

With 6 Plates.

Among the youngest human embryos and one of the youngest in Professor Mall’s collection, Embryo No. 391, which he has very kindly permitted me to reconstruct and describe. In general development this embryo is almost identical with, probably a trifle older than, the Kroenier-Pfannestiol embryo, Klb, which measures 1.8 mm. and has six ‘pairs of somites. It is older than E’rernod’s embryo measuring 1.3 mm., Grraf Spee’ s Gle 1.54 mm., and Embryo Frassi 1.17 mm., in the order named. It is younger than the embryos of Unger and Bulle, of 9 and 14 somites respectively, Eternod’s 2.1 mm. embryo, and embryos XLIV (Eff), LXVIII (Lg. 2.15 mm.), VI (B. R. 2.2 mm.) and VII (E 2.1 mm.) of His.


This very rare specimen came into Professor Mall’s possession through the kindness of Dr. R. W. Pearce, of Albany, New York, with the following history from the physician who handed the specimen to him. “The woman had passed her period about two weeks when she performed an abortion with a stick about 8 inches long, which she whittled out for the purpose. This she passed into the uterus and 24 hours later this specimen was aborted. Her purpose in calling me was to see if her object had been attained. I have kept the specimen two years in a bottle of weak formaldehyde.”


Upon receipt the following measurements were. made by Professor Mall; ovum 162: 14x12 mm., embryo about 2 mm. The specimen was placed in fresh formalin, stained with alum-cochineal-eosin, imbedded in paraffin and cut into serial sections 10 microns in thickness.


From these sections (163 in number, excluding the bauehstiel), the embryo was reconstructed upon a scale of 200 magnification, giving the model a total length of 32.6 cm. A shrinkage space between the layers of tissue aided materially in the reconstruction without sacrificing tissue. The model is so constructed that every detail may be exposed by cuts and windows.


Upon presentation of an embryo the question naturally arises, is it normal? The history of a mechanical abortion points very strongly toward a normal embryo. It compares very closely with the few other known embryos of this period. The tissues, including the chorion, are excellently preserved and show no degenerative changes.

Age

From the history that the menstrual period passed about two weeks, the age of 13-14 days would fit very nicely into the conventional Reichert—His theory of fertilization and age of embryos. Recent observations however by Mall in 1000 cases with menstrual history have shown that we underestimate the age of young embryos, on an average about 10 days, that fertilization is not restricted to the period immediately before the menstrual period, but may occur at any time in the intermenstrual period. Bryce and Teacher also came to the same conclusions and illustrate this in a well constructed diagram of the menstrual cycle. This obviates the remarkable distortion which has been necessary to harmonize age with size and progress of development, and makes the human embryo much less precocious in its development. In conformity to the above observations the true age of this embryo is probably about 24 days, which would make the time of fertilization occur about ten days before the time for onset of lapsed menstrual period or about eighteen days after beginning of the last menstrual period.

The Adnexa

Chorion. The chorionic membrane is about 0.1 mm. thick, and is covered with many branching villi varying in size up to 1.25 mm. in length and 0.1 mm, in thickness. These villi are more numerous at the point of attachment of the bauchstiel and gradually fade away on all sides until finally a clear zone results from their absence on the opposite pole.


The chorionie membrane and villi have the characteristic inner loose mesenchymatous layer of beautiful branching spindle and stellate cells with anastomosing processes and a somewhat jelly-like inter-eellular substance; an external double epithelial layer, consisting of the inner Langhans layer of small cells with lightly staining nuclei and cytoplasm, and the outer syneytial layer which stains more deeply with eosin and has larger and more densely staining nuclei, but no definite cell boundaries. From the epithelial layer of the chorionic membrane and villi, numerous buds develop, some from the syneytial layer alone, others from both layers of epithelium. These represent proliferation and new formation of villi. The mesenchymatous layer of the ehorionic membrane contains many newly forming capillaries, some of which extend into the villi. The details of these will be considered later in the description of the vascular system.


The Bauchstiel. The bauchstiel does not diiier from that of other young embryos. It consists of loose mesodermal tissue, lined externally by a single layer of flattened mesodermal cells. It is continuous distally with the chorionic mesoderm and proximally with the mesoderm of the umbilical vesicle, amnion and body of the embryo. It contains the allantois, umbilical arteries and veins and their resulting sinuses and branches.


Umbilical vesicle. Tl1e yolk sac has no stalk but is attached to the entire length and breadth of the embryo, with the exception of small portions of the anterior and posterior ends, which are covered by the short head and tail folds of amnion. At its attachment to the embryo, the walls are very thin, consisting of two layers of flattened cells—mesoderm and entoderm. These walls gradually grow thicker distally, due to the degree of development of‘ the blood islands, which also cause a great distortion and knotty appearance of the mesodermal surface. This vascular development extends throughout the Whole length of the umbilical vesicle, but is practically limited to the ventral or distal half, only a few islands being seen in the dorsal half. The greatest development seems to be near the center of the vesicle and is apparently developing to connect with the future vitelline veins. This is in eontradistinction to the findings in Eternod’s 1.3 mm. embryo, in which the posterior region of the umbilical vesicle is drained by the sinus ensiforme. The entodermal lining of the inner surface of the umbilical vesicle forms a distinct layer in places, whereas in other locations, especially in the ventral half, no definite layer of cells can be made out, so intimately is it fused with the knotty and thickened mesoderm.


Amnion. - The amnion is a completely closed cavity, with very thin delicate Walls of flattened cells, the individuality of ec-toderm and mesoderm being everywhere beautifully preserved. The cavity of the amnion is very large, probably due, as suggested by Professor Mall, to the osmosis of large quantities of dilute formalin in which it was preserved for over two years. Anteriorly a rather deep pocket of amnion dips ventral to the heart, evidently a sign of posterior extension of the head fold. The ta.il told is very small, covering the embryo for a distance of only 5 or 6 sections.

General Appearance on the Embryo

The embryo presents an anterior and posterior elevation with a marked dorsal link. (Plate IV.) The anterior elevation is gradual. the posterior very sharp, rising at an angle of about 80”. This kink seems to be partly natural and partly ar1’exaggerated postmortem condition. We should naturally expect a dorsal concavity due to the greater development of the structures in both the anterior and posterior regions of the embryo. The marked aceentuation however may be due, as suggested by Professor Mall, to the large amnion filled with fluid, the weight of which would naturally act upon the point of least resistance. Shrinkage incident to manipulation and imbedding also plays an important role in its aggravation, especially after location of the point of least resistance. This is clearly shown by a comparison of the model with a sketch of the embryo before imbedding, there being an accentuation of the kink by almost 15°.

Ectoderm

Nervous system. The extent of development of the nervous system is a medullary groove, open throughout its entire length. The brain region is divided into three primary Vesicles (Plate VI), the anterior being equal in size to the other two combined. The first or anterior vesicle is long, wide and deep with edges everted and projecting outward over the anterior and lateral Walls of the anterior body elevation. The walls of the second and third vesicles have a tendency toward inversion and the enclosed vesicles are more Sharply defined laterally than the first. The brain passes insensibly into the spinal cord, which is much smaller in all diameters, but is nowhere closed. Posteriorly the medullary groove forms a shallow dilatation gradually fading into the flattened surface of the primitive streak. (Plate IV.) M


No neurcnteric canal is visible, though present in the younger embryos of Frassi, Graf Spec and Eternod and possibly the Kroemer-Pfannenstiel Klb. No traces of spinal or cerebral ganglia or nerves are visible. N o suggestion of anlage of the lens, optic or otio vesicles couldloe detected.


Primitive streak. Posterior-ly and dorsally in the region of the termination of the notochord and neural groove, ectoderm and mesoderm gradually losing their individual morphological characteristics, fuse to form a mass of a single variety of simple undifferentiated cells, which extends to the posterior termination of the embryo. This is the primitive streak (Fig. 7 The entoderm also seems to be a part of this mass since its cells are directly continuous with it, although the charac-tcristic lining of the hinclgut is still maintained throughout. There is no groove except a very shallow flattened dorsal depression which is gradually lost posteriorly and which is continuous anteriorly with the neural groove. This primitive streak region suggests a storehouse of simple undiiferentiated cells supplying mesoderm, ectoderm and entoderm in the earlier stages, later becoming difierentiated into the characteristic morphology and arrangement of the different layers.

Entoderm

Entoderm lines the Ventral surface of embryo, ‘fore and hind gut, allantois and the inner surface of the umbilical vesicle. In the ventral entoderm of the embryo is a median longitudinal groove corresponding to the location of the notochord and caused by its adherence to the ectoderm of the medullary groove. On either side of this groove and parallel to it is a ridge caused by




Cite this page: Hill, M.A. (2024, March 29) Embryology Paper - A Human Embryo with Seven Pairs of Somites Measuring about 2 mm in Length. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_A_Human_Embryo_with_Seven_Pairs_of_Somites_Measuring_about_2_mm_in_Length

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