Paper - Observations upon Young Human Embryos
James Thomas Wilson
Challis Professor of Anatomy in the University of Sydney, Australia.
<pubmed>17233002</pubmed>| PMC1288949
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Part 1
The appearance in recent years of Keibel and Mall's Manual of Human Embryology (1), following upon Keibel and Elze's Normentafein (2), marked an epoch in the formulation of our knowledge of specifically human development.
The comprehensive summary there offered of our knowledge of the earlier human ontogenetic processes provided for the firsttime a more or less connected account of these phenomena, but it also served to accentuate the still very sketchy and incomplete character of that knowledge.
Much of our belief in regard to the method of establishment of the human blastocyst is stil quite hypothetical, even if probable; and as regards the appearance of the earliest rudiments of the body itself, our knowledge isbased on a very few human specimens separated by intervals which it is important to fill in with the aid of intermediate or allied stages.
In the present paper I propose to give an account of the three youngest human embryos in my collection.
Previous writers have described and figured specimens of a stage of development more or less similar to those exhibited by the two older of the embryos which form the subject of this communication. Nevertheless, well-preserved specimens, of ages nearly corresponding to these, are of such comparative rarity that for some time to come it will still be desirable to have accurate records published of the form and structure of any that may become available for detailed examination. As a matter of fact, no two specimens hitherto described, however apparently similar in stage of development, have proved to be precisely identical in detail. It will appear in the course of the paper that each of the individual embryos under consideration presents features entitling it to independent description.
A more special interest attaches to the youngest of the three specimens, inasmuch as it would seem to exhibit a phase of development hitherto unrepresented in the records of early human embryos.
It possessed probably two, possibly three, pairs of somites, and may thus be determined as occupying a position in the gap between stages 2 and 3 of Keibel and Elze's Norrnentafel. These stages are represented respectively by Spee's embryo " Gle " (3), and the Kroemer-Pfannenstiel embryo " Klb."
Hitherto, or as far as I am aware, no human embryo has been recorded as exhibiting a smaller number of somites than five (in " Klb ").
I am inclined to believe that the embryo "E," No.1 of His' Normentafel (4), would have turned out to be of very similar character to that now about to be described, inspire of the somewhat greater length of the former (2.1 mm. as against 1.68 mm.).
Eternod's well-known embryo of 1-3 mm. (his "No. 7 Vuill.") (5) may well represent a somewhat earlier phase.
The youngest embryo now to be described -the first of the three referred to- appears in my list of human embryos under the designation of "Hdr." I shall, however, refer to it in future simply under its catalogue number "H 3."
History of the Human Embryo "H3"
The specimen[1] was received by me so long ago as 25th May 1898, from my late friend Dr H.V.C. Hinder of Sydney, who had obtained it from a case of abortion on the previous day. The unopened chorionic vesicle had been placed in diluted alcohol. It was to outward appearance well conserved and perfectly intact when I received it, and it was at once transferred to picrosulphuric acid and then passed through graded alcohols.
From notes procured at the time by my friend Dr A.E. Mills, who was also associated with the case, it appears that the last menstruation period had begun on 12th April 1898 and ended on 16th April 1898. Abortion actually took place on 24th May 1898, but hemorrhagic discharge had appeared on 22nd May 1898. Thus the period that had elapsed since the beginning of the last menstruation up to the commencement of abortion was 40 days, or 36 days from the end of the last menstruation. The period that had elapsed since the due date of the lapsed menstrual period was 12 days.
According to present-day criteria the age of this embryo may be estimated as included in the period 18-21 days, and probably in the earliest part of this period.[2]
As originally received by me, the specimen was to al appearance in excellent condition. No defect in its preservation was recognisable throughout the period of its examination as an entire specimen prior to embedding, except that a portion of the yolk-sac was accidentally broken away during manipulation.
After embedding in paraffin a complete series of sections at 10 , was obtained. Unfortunately the histological condition of the sections was most disappointing. The attempt at an adequate fixation must, after all, have been too belated. Possibly also there may have been some over-heating in the paraffin oven. So unsatisfactory was the result at the time, that after a somewhat cursory examination the series was put aside for a considerable number of years. Now, however, on re-examination in connexion with the investigation of more recent specimens, ithas appeared to me to be well worth while to describe this early embryo in some detail. More especially perhaps do the photographic records of the entire specimen, which shows no external sign of structural deterioration, constitute original documents of some value for comparative purposes. And even the sectional series, although not fully adequate for the purposes of plastic reconstruction, turns out, on closer examination, to be of no little interest and value.
Characters and Dimensions of Chorionic Vesicle
The chorionic vesicle of "H3 " is illustrated in the photograph reproduced in PL. I. fig. 1, which was taken after the vesicle had been rendered transparent in cedar oil. It was flattened in its polar axis and measured 5mm. in its (practically a villous) polar diameter. Its equatorial diameter was about 8.5mm., inclusive of the vili; or, without vili, 6.4mm. in its longer and 5.7 mm. in its shorter equatorial diameter.
The chorionic dimensions were thus rather smaller than those of Spee's embryo "Gle" and almost identical with those of His' embryo "E."
As just indicated, the vili were unequally distributed over the surface of the vesicle. There was a richer equatorial villous zone, whilst the polar areas were freer from vili, though at no place completely bald. Oneof the polar areas, the antembryonic, was barer than the other.
The vili showed a very moderate degree of branching (cf. P1. I. fig.1).
After having been examined and photographed from both polar aspects, the vesicle was opened and the portion of its chorionic wall carrying the attachment of the body-stalk was separated from the rest. Theembryonic rudiment with its associated appendages was then subjected to closer examination and sketched and photographed from various points of view. (P1.I. fig.2, taken in cedar oil; also text-figs. 1-3.)
The following micrometer measurements were obtained of the embryo and its immediate appendages. The measurements were taken while the specimen was in cedar oil.
- Maximum cranio-caudal length of amnion - 1.78 mm.
- Maximum cranio-caudal length of yolk-sac - 2.26 mm.
- Apparent cranio-caudal length of embryo - 1.64 mm.
- Dorso-ventral extent of amniotic cavity from its dorsal convexity to line of reflection of somatopleure in the head region - 0.6 mm.
- Dorso-ventral extent o fyolk-sac 1.66 mm.
N.B. The yolk-sac was partly collapsed and crumpled, as shown in the photograph (fig. 2).
The specimen was next embedded in paraffin and cut in series of 10 micron sections in a plane intended to be transverse to the long axis of the embryo, but which turned out to be distinctly oblique. The sections constitute a practically unbroken series, but their histological condition is not very satisfactory. They are tolerably well stained in haematoxylin. The series was both cut and numbered in caudocranial succession. When the sections and figures are viewed with the dorsal embryonic surface away from the observer, the right and left embryonic surfaces are right and left, respectively, to the observer.
Micrometer Measurements
Form and Characters of Embryo and its Appendages
In PI.I.fig.2 the embryo, along with its immediate appendages, amnion and yolk-sac, is seen attached to a fragment of the chorion by the body-stalk. The latter is rather acutely reflexed in a cranial direction, thus arching over a considerable extent of the amnion so as to form the actual roof of the caudal portion ofthatcavity. Theserialsections,when followed from behind forwards, show that the amnion only gradually becomes free from super incumbent cellular tissue of the stalk. Rather more than the caudal third of the amnion is thus intimately connected with the body-stalk, whose vascular mesodermal tissue, indeed, spreads out over this portion of the amnion like a hood.
Mesodermal Somites
Chorda
The chorda is still in the stage of intercalation in, and is indistinguishable from, the entoderm.
Excretory Apparatus
There is no ascertainable pronephric rudiment, nor would one expect to find any at this early stage. In any case, the histological conditions are unfavourable to any critical verification.
Medullary Plate
The general characters of the medullary plate have already been alluded to in connexion with the form of the embryo.
Absence of Sense Organs
There is no trace whatever of optic groove formation nor of auditory areas, although there is a diffuse and rather extensive thickening of the head ectoderm in the posterior cephalic region, which may possibly foreshadow the appearance of such areas.
Oral Region
On the ventral aspect of the head, immediately in front of the line of reflexion of the ectoderm from the free head to the "pericardial plate" swelling (text-fig.1), the ectoderm forms a plate, slightly recessed bilaterally, the depressions being separated from one another by a slight median prominence.
Atlimentary Canal
The fore-gut shows a general agreement in character with that of the important Kroemer-Pfannenstiel embryo "Klb" of Keibel and Elze's Normentafel.
Allantoic Duct
The proximal portion or vestibule of the allantoic duct has already been referred to, as also its continuation into a narrow canal of uniform calibre prolonged not the reflexed body-stalk.
Pericardum
The pericardium lies chiefly ventral to the pharynx, but it extends for a distance of about 005 mm. in front of the cranial limit of the later.
Heart
The heart itself is still almost completely duplex as regards not only its endothelial but also its myoepicardial components.
Summary of Characters of Embryo "H3"
Second Embryo "H 98"
LIST OF REFERENCES.
EXPLANATION OF PLATES I.-III. FIGS. 1-6
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Cite this page: Hill, M.A. (2024, April 18) Embryology Paper - Observations upon Young Human Embryos. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_Observations_upon_Young_Human_Embryos
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- ↑ A lantern demonstration of slides of this embryo was given at a meeting of the Anatomical Society of Great Britain and Ireland at a meeting held on 16th January 1914.
- ↑ Keibel and Elze (Normentafel (2), p.90) quote Born's estimate of the age of embryo "Klb" as10-14 days. But if we take into account the more recently accepted criteria of age in early embryos, this age must be judged to be considerably underestimated. I shall show reason to regard embryo " Klb" as distinctly more advanced in development than embryo "H 3."
