Paper - Description of a 5 mm human embryo (1900)
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Description of a 5 mm Human Embryo
By H. E. Jordan,
Adjunct Profeasor of Anatomy, University of Virginia.
The material upon which the following contribution is based is a human embryo 5 mm. in greatest length. The specimen, for which I am indebted to Dr. Stephen H. Watts, Professor of Surgery, came into my hands in normal salt solution two hours after hysterectomy. It was immediately transferred to 95 per cent alcohol. Subsequent measurement showed a shrinkage of 1 mm. The specimen was stained in toto in Delafield's hematoxvlin and sectioned at 10 microns. The tissues are excellently preserved and the degree of development is very similar to that previously described for embryos of aproximately this length. Thus in respect to the vascular and alimentary systems, it appear* similar to embyro No. 148 of the Mall collection (length 4.3 mm., myotomes 28) carefully studied by Mall,^ Bardeen and Lewis^ and more recently by Mrs. Gage ;* and to embryo G. 31 of O. Hertwig's collection (length 4.9 mm., myotomes 35) studied and described by Ingalls.* It differs markedly, however, from these embryos in respect to the brain and the nephric system, the difference probably representing a slight advance in development. In external form it appears similar to embryo R of the His* collection (length 5 mm., myotomes 35), and is probably of about the same age, i. e., between 22 and 24 days.
I have been able to procure the following history of the case: The woman wa^ 33 years of age and had previously had four children, the youngest of whom is 9 years old, and two miscarriages. Menstruation had been regular. Her last period began September 16th. The operation was performed October 27th.
The above dates leave an interval of 13 days between the first omitted menstruation and the time of operation. The minimum and maximum ages of the embryo are probably about 21 and 25 days, respectively. Fertilization must have occurred at least 12 days after the last menstruation or at least 8 days before the omitted one. These data indicate independence of ovulation and menstruation.
The following points are important concerning the external appearance: The head turns slightly to the right and the tail to the left (see text figure). The umbilicus appears to be median. The nuchal bend is almost a right angle. There are 35 (perhaps 36) somites (lo, plus 8c, plus 12 1, plus 51, plus 5s, plus 4 or 5c). External corrugations simulate segmentation anterior to the single occipital somite. The sections show that these represent remnants of three additional occipital somites. The arm-buds extend from the 4 to 8 cervical somite inclusive ; the leg-buds from the 1 lumbar to the 1 sacral inclusive. The left arm-bud is turned back, thus presenting its medial face. Four gill-arches appear externally; a fifth lies within the sinus praecervicalis. The maxilla is just beginning to take shape.
Mall, F. P., Johns Hopkins Hospital Bull., xii, 1901.
Bardeen CR. Born's method of reconstruction by means of wax plates as used in the Anatomical Laboratory of the Johns Hopkins University. (1901) Johns Hopkins Hospital Bulletin, vol. 12.
Bardeen, C. R., and Lewis, W. H., Amer. Jour. Anat., 1901.
Gage SP. A three weeks' human embryo, with especial reference to the brain and nephric system. (1905) Amer. J Anat. 4: 409-443.
Gage, S. P., Amer. Jour. Anat., IV, 1905.
IngaUs, N. W., Archlv f. Mik. Anat., Ixx, 1907.
His, W., Anatomie menschliclier Embryouen. Text and Atlas. Leipzig, 1880-1885.
Three corrugations appear on the anterior face of the diencephalic region. The extent of the thin roof of the fourth ventricle is plainly visible. In the mid-region there is a small notched elevation of folded ectoderm. Eye, ear, auricle, ventricle, bulbus and truncus arteriosus, liver. Wolffian ridge and ganglia (5, 7 and 8, 9, and 10) are also visible externally. Anterior to the maxilla, the ectoderm of the ventral face of the head is very thin and slightly depressed. On changing the level of focus one sees in this region a sucker-like projection with median groove and central indentation. Sections show that this is the region of the optic recess, and the indentation probably marks the site of final closure of the neuropora The ectoderm consists generally of a single layer of ouboidal cells. Ventral to the eye on either side of the head, occur patches of thickened ectoderm extending through 24 sections representing the anlagen of the nostrils. Thickened patches of ectoderm occur also in relation to the cranial nerves forming so-called "placodes." Sections show that the somites have differentiated into sclerotome (with loose anterior and denser posterior segment) and myotome. The otic vesicles lie between the base of the third gill arch and the ectodermal fold of the roof of the fourth ventricle above mentioned.
(a) Alimentary canal
The mouth forms a broad transverse slit bounded laterally by the mandibles and maxillae, anteriorly by the fore-brain and posteriorly by the fused mandibles. No remnants of an oral plate can be found. The mouth leads into a broad wide pharynx bounded laterally by the gill arches. Between the latter are the entodermal extensions of the pharynx, or gill pouches, forming with the apposed invaginated ectoderm thin membranes stretching between the arches. A dorsal cephalic extension of the pharynx is the anlage of the hypophysis, posterior to which lies a second extension of the pharynx, SesseFs pocket. Closely applied to the curved anterior border of the hypophysis, rests a projection from the diencephalon, the infundibulum. In this same region the notochord terminates with a sharp ventral curve.
On the floor of the pharynx in the region of the second arch the tuberculum impar is well developed. The alveolo-lingual grooves and the lateral rudiments of the tongue are not well marked in the sections. In the same region the median thyroid, unconnected with the pharyngeal epithelium is present, consisting of a small irregular mass of spheroidal cells. Anlagen of lateral thyroids and the thymus are just recognizable in the sections.
The larynx, represented merely by a slight depression, leads into the trachea, about one half millimeter in length, which bifurcates into two branches each with a terminal expansion. The latter are enclosed by extensive mesoderm forming the lung-buds.
The pharynx leads into a short oesophagus with thick epithelial wall and narrow lumen. On the dorsal border of the liver, the tube enlarges slightly, forming a short spindle-shaped stomach. This in turn leads into the duodenum with wider lumen and finally into the caudal intestines. About the level of the fourth thoracic somite the vitelline duct arises. The gall-bladder and bile-duct lie on the lower border of the liver ventral to the intestine. About 50 microns anterior of this point the anlage of the dorsal pancreas can be recognized as a shallow outpocketing. The intestine can be traced to the cloaca, which is closed by the anal plate. From the cloaca the allantois extends outward into the umbilical cord. Proximally the allantois shows a slight expansion, the anlage of the urinary bladder. Pericardial and pleural coelon are continuous dorsally with the abdominal coelom. The liver has grown out in all directions into the septum transversum. The hepatic tissue is invaded by the omphalomesenteric veins forming sinusoids.
(b) Vascular System
The heart appears very similar to the His model for a 5 mm. embryo. The two atria are continuous, forming a single large sac with lateral expansion, and opening by the atrial canal into thq left ventricle. The sinus venosus has moved to the right. The left ventricle passes into the bulbus arteriosus which makes a sharp turn dextrally and cephalward, and passes into the trimcus arteriosus which extends along the ventral aspect of the atrial part of the heart. The wall of the heart consists of a loose mesh of undifferentiated muscular tissue. The wall is thickest in the left ventricle. The endothelial tube is loosely applied in the heart proper, but more closely in the bulbus and truncus arteriosus.
The truncus passes into the floor of the pharynx where it expands into a wide sinus. From the latter extend two anterior branches (ventral aortal) each with two lateral twigs. The most anterior of these breaks up into capillaries in the mandible and the second supplies an aortic arch to the second gill-arch. From the sinus, and projecting backwards arises the larger third aortic arch. Caudally from this point the fourth and sixth aortic arches take origin. Slender backward extension of the ventral aorta behind the sixth arch supply capillaries to the floor of the larynx. The aortic arches on each side unite dorsally to form two dorsal aortas which in turn unite about the level of the fifth cervical myotome, or the cephalic border of the liver, into the single dorsal aorta. Just anterior to this point the subclavian arteries, and immediately beyond it the coeliac artery, are given off."
A large ventral branch, the right omphalomesenteric or superior mesenteric artery leaves the dorsal aorta at about the level of the fifth thoracic myotome. Near its termination it bifurcates into two branches which accompany the vitelline duct for a short distance on either side. The inferior mesenteric artery arises as a delicate ventral branch about the level of the seventh thoracic myotome. The dorsal aorta again divides into two (the hypogastric or umbilical ffrteries) before it passes into the umbilical cord. Frequent renal twigs are supplied to the glomeruli of the Wolffian ridge. From each dorsal aorta there extends cephalad a twig (internal carotid) to a short distance beyond the hypophysis. The vertebral arteries can be traced forward as far as the diencephalon.
The jugular veins (precardinals) arise from the union of numerous twigs lying close to the surface in the anterior head region. Posterior to the eye, where the vein becomes an elongate vessel, it passes mesad of the gasserian ganglion and laterad of the ganglia of the 7th and 8th nerves and the otic vesicle. Ramifying within the ganglia of the 9th and 10th nerves it passes laterad of the former and mesad of the latter and unites with the duct of Cuvier. The umbilical veins arise at the umbilicus from a single large vessel and thence forward through the somatopleure to the Cuverian ducts. The ductus venosus, formed by the union of the vitelline veins in the liver, also connects with the sinus venosus. A single vein, the "primary ulnar" (subclavian) drains each arm-bud. The leg-bud also contains only one vein, the "primary fibular" or "vena.ischiadica" (common iliac). All blood vessels and the heart contain well preserved erythroblasts ; occasionally these are seen in mitosis.
- I am unable to And any evidence of multiple subclavian arteries such as Evans (Evans, H. M., Anatomical Record. Vol. 2, No. 0, 190S) reiwrts from embryo No. 148 of the Mall collection (two segmental vesicles), and as I have myself observed in very young turtle embryos (three segmental vessels).
(c) Nephric System
In the nephric and central nervous systems the greatest variations are found between this embryo and the several above mentioned. The Wolffian ridge extends from the level of the mid-region of the arm-buds to the cloaca. Anlagen of the genital ridge and metanephros have not yet appeared. The Wolffian ducts appear continuous from end to end of the ridge. There are approximately 30 nephric tubules. The anterior 18 or 20 consist of well-developed glomeruli with Bowman's capsule connected by patent S-shaped tubules with the Wolffian duct. Several of the most anterior glomeruli lie very close to the coelomic epithelium; the remainder lie deeply embedded within the ridge. The tubules have comparatively thick walls and narrow lumen. The posterior 12 or 14 end distally in expanded vesicles, but no true glomeruli have formed ; and these tubules also are connected with the Wolffian duct. Several delicate tubules at the cephalic end of the ridge which are difficult to trace in sections may represent the remnant of a pronephros. However, no tubules connect either with the coelom or the myotomes. The nephric system at this stage consists essentially of a mesonephros apparently considerably less generalized than that of the embryos described by Mrs. Gage and by Ingalls.
(d) Central Nervous System
The eyes at this stage are represented by evaginations from the diencephalic margin of the forebrain and are already slightly cupped. The ectoderm has thickened into a plate of tall cells over the region of the cup forming the anlage of the lens. The posterior roots of the spinal nerves are represented by delicate bundles of neuroblasts. The wall of the neural tube, which consists of an internal ependymal layer of tall cells, a middle layer of neuroblasts and a peripheral marginal velum, appears in the sections to have a perfectly smooth internal contour. Study of a carefully constructed model of the brain (by the blotting paper method described by Mrs. Gage in the Anatomical Record for November 10, 1907) together with the sections disclose the following conditions: Externally the brain tube gives no clear evidence of folds. As already stated, however, three ectodermal corrugations appear in the region representing the diencephalon and the mesencephalon; and the same number appear anterior to the occipital somite. The fact that these latter corrugations correspond to >occipital somites in process of disappearance shows that metamerism had at an earlier developmental stage extended into the region between the first cervical and vagus nerves. The ganglia of the 5, 7 and 8, 9 and 10 nerves are symmetrical in position and similarly developed except that the right gasserian ganglion is considerably larger than the left. The segmental arrangement of ganglia indicates a still more anterior extension of metamerism. When one regards a slight external bulging just anterior to each gasserian ganglion as the "cerebellar folds," and the ganglia of the 7 and 8 nerves as a fusion of two segments, and the region of the otic vesicles as the fifth "neuromere" the full number considered typical (seven) for the mammalian hind-brain (Bradley^) is accounted for. In six distinct regions, also, there is a decided thinning of the neuroblast layer of the wall of the hind-brain and a reciprocal thickening (in sections conforming to a blunt wedge-shaped area) of the marginal fibre layer corresponding more or less closely with ganglia of the 6, 7 and 8 nerves, the otic vesicle, and the ganglia of the 9 and 10 nerves. Moreover, there is in these regions the merest indication of a bulging of the wall. In the model, however, one seeks in vain for distinct evidence of folds, with the exception of a wide embayment in relation to the gasserian ganglion. This "neuromere" is by various writers on different forms described as the most pronounced and least transitory fold. I can find no evidence of distinct internal folds in the fore and mid-brain. The foregoing facts seem to indicate, then, that my embryo has attained to a slightly later stage of development than that of the embryos described by Mrs. Gage and by Ingalls. It seems probable, accordingly, that folds anterior to the gasserian ganglion have already disappeared, as also those related to nerve roots posterior to this region including the vagus. The only persisting "neuromere" at this stage is the one associated with the trigeminal nerve, or the second of the rhombencephalon.
'Bradley, O. C, Rev. Neurol, and Psychiatry, il, 1904.
Cite this page: Hill, M.A. (2021, April 13) Embryology Paper - Description of a 5 mm human embryo (1900). Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_Description_of_a_5_mm_human_embryo_(1900)
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