Paper - Observations upon Young Human Embryos

James Thomas Wilson

Challis Professor of Anatomy in the University of Sydney, Australia.

Medicine Museum

<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 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.

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.)

Micrometer Measurements

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.

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.

The lateral portions of this hood gradually thin out as they clothe the sides of the amnion. In consequence of this extension of body-stalk tissue over the sides of the amniotic sac, the amnion exhibits in this region the appearance ofa more or less vascular membrane (text-figs.4 and 5).

Within the amniotic chamber the outline of the embryo may be discerned (Pl. I. fig. 2), with its cephalic expansion elevated bilaterally into two prominent medullary folds, separated by a deep and wide medullary groove (cf.text-fig. 6).

There is either no dorsal flexure or kink of the embryonic body, or only a faint indication of one(cf. text-fig. 1).

Behind the broad cephalic region the embryo shows marked constriction and then appears to widen out into a foliate expansion, which forms the hinder third of the apparent embryonic body.

The superficial appearance of the foliate expansion suggests a widening andopening-outofthemedullaryplateinthisregion. This,however,is nottheexactconditionmetwith. Thestilwidelyopenneurenteric aperture, text-fig. 7, is situated near the junction of the middle and posteriorthirdsoftheembryonicregion. Itspositionnearlycoincides with the ventrally open angle recognizable in PI. I. fig. 2, and also in text- fig. 1, at b, where also an arrow points to the site of the neurenteric aperture. Theopenangleatbintext-fig.1reallymarkstheanterior limit of the leaf-like expansion of the hinder part of the embryonic region. Immediately in front of the neurenteric aperture, the sections show the medullary plate as stil markedly infolded so as to form a deep and not very wide medullary groove.

Behind the aperture there is no shallowing out, but, on the contrary, the primitive-streak formation is here actually depressed into a deep cleft which continues backwards between two elongated, bolster-like, caudal swellings containing mesoderm (text-fig. 5).

It is these caudal swellings which form the lateral wings of the posteriorfoliateexpansionoftheembryonicregion. Theectodermcover- ing them is not at al, or only slightly, thickened, whilst the primitive- streak ectoderm lining the deep dorsal furrow between them is thick and columnarlikethatofthemedullaryplateinfront.

Apart from the elongated caudal cushions bounding the deep sulcus in the primitive-streak region,thereisnoposteriortail-prominence. The partoftheembryonic region containing the caudal swellings is,however, placedslightlyatananglewiththerestoftheembryonicarea. Thismay be recognized in the photograph, fig. 2, PI. I., and is well shown in the outline in text-fig. 1.

The caudal cushions gradually fade away posteriorly in the floor of the hinderregionoftheamnioticcavity. Thislattertapersintoanarrow- pointed prolongation and ends at the plane of the hinder limit of the root ofthebody-stalk. Ithasnocontinuationintothebody-stalk. Itsflooris formed solely by the continuation backwards of the floor of the vanishing primitive groove (text-fig. 8).

Close to the termination of the amniotic cavity there is a thick " cloacal membrane" connectingwiththeentodermatthebaseoftheallantoicduct (text-fig.9). Thepositionofthiscloacalmembranemaybecomparedwith that described and figured recently by Grosser in a younger embryo (8, Taf. 27). Therewasnocaudalstalk-likeprolongationoftheamnion,otherthan the above, to correspond with that described by Eternod in his 1P3mm. embryo.

As there is no tail-prominence proper, the posterior limit of the embryo has been reckoned as if coincident with the hinder limit of the furrowed primitive-streak region,although the extremity of the future tail would undoubtedly be formed by hypertrophy of the bilateral caudal cushions far infrontofthispoint. Reckoning,then,fromtheposteriorlimitofthe primitive-streak region to the cranial limit of the cephalic medullary plate, the length of the embryonic area included 143 sections at 10 micron. In the table of measurements the apparent length of the embryo prior to embedding was 1.64mm.; the difference just indicated is doubtless to be accounted for mainly by shrinkage during embedding.

In text-figs. 1, 2, and 3 are reproduced the outlines of freehand sketches, from various points of view, that were made in the course of examination of this embryo, in toto, in cedar oil.

Text-fig.1 is the outline of an early profile view. Itmaybelooked at along with the photograph, fig. 2, PI. I., which represents a dorso- lateralviewofthespecimen. Thetext-figureshowsthedegreeofvertical curvatureoftheseveralregionsoftheembryo. Theapproximateposition oftheneurentericapertureisindicatedbythearrow. Theregionato b is the primitive-streak region: the angle of its inclination to the main embryonicaxisistobenoted.

If this figure be compared, e.g., with figs. 19 and 20 of Eternod's mono- graphL'ceufhuman (5)(Geneve,1909),illustratingrespectivelySelenka's outlinefigureofHylobatesRaflesi,and Eternod'soutlineofhis1-3mm. human embryo ("No. 7Vuil."),it will be seen that the most outstanding difference is due to the acute ventral flexure of the primitive-streak region intheseembryosascomparedwithmyspecimen"H3." Andthisdiffer- ence in flexure is correlated with the difference observable in relative positionofthebody-stalk. Theabsenceofaventralflexureofthe primitive-streak region in embryo "H 3" permits of a rather acute reflexion,inacranialdirection,ofthebody-stalk.

Practically the same comparison may be made with embryo "Klb" (Kroemer-Pfannenstiel) of Keibel and Elze's Normentafel (2), which possessed 5-6 pairs of somites and must represent a somewhat similar developmentalphasetothatoftheembryounderconsideration. That " Klb" was slightly more advanced is indicated by its possession of a well- marked tail-prominence, of a closed-in hind-gut, and by several other features to be later commented upon.

The possibility cannot be entirely excluded that the extended position of the hinder part of the embryonic area of " H 3 " is somewhat abnormal, especiallyastheflexedattitudeoftheprimitive-streakregion was already attained in Spee's embryo " Gle" (3), which is certainly less advanced and isprobablyperfectlynormal.

In any case, it is plain that the absence of ventral flexion of the primitive-streak region in "H 3" involves a reconsideration of the true lengthoftheembryo. Ifareliablecomparisonistobeinstitutedwith other human embryos of approximately the same stage, then we must deduct from the apparent embryonic length given above as 1-64 mm., nearly the whole length of the primitive-streak region behind the neuren- teric aperture, since, in the embryos referred to, the neurenteric aperture lies quite close to the angle of a ventrally flexed tail-end of the embryonic body.

The deductionheresuggestedasnecessaryincludesnearlythewholeof theprimitive-streakregion. Inthetotalof143sectionsoftheembryonic area, inclusive of primitive streak, no fewer than 47 lie behind the plane of the caudal boundary of the neurenteric aperture.

Allowing for the slight caudal convexity behind this which is recognisable in comparable embryos, I estimate the true " embryonic length," for comparative purposes,to have been not over 1.25 mm. The actual proportion of the total of 1.64 mm. in front of the neurenteric aperture is almost exactly 1 mm.

Text-fig. 2 represents the outline of embryo "H 3,"as viewed from above and slightly from the caudal direction. The irregular area which has been hatched in the drawing represents the small portion of chorion connected with the body-stalk. The stippled area represents the dorso-caudal aspect of the body-stalk, and shows how it spreads out laterally over thehinderportionoftheamnioticsac. Someindicationsofthechorio- placental vessels may be noted, but their detailed arrangement has not yet been worked out from the sections.

Text-fig. 3 represents an outline sketch giving the frontal "elevation" (Norma frontalis) of the amnion and head of the embryo, as viewed from the front and very slightly from the ventral side. Thewidebulgingarea below the amniotic sac corresponds to what may be termed the "pericardial plate," although at the present stage the pericardium does not actually reach quite to its surface.

Mesodermal Somites

Notwithstanding careful and prolonged examination of this embryo while in cedar oil prior to embedding, I was unable to detect the presence of differentiatedsomites. Theotherfeaturesoftheembryoseemedtoindicate that some at least of the earliest somites should be already in existence.

Examination of the serial sections has, in fact, confirmed this conjecture. Unfortunately,thehistologicalconditionoftheparaxialmesodermisnotso satisfactory as to permit of a wholly reliable determination of the number ofsomitesrepresented. Ihaveonlybeenabletorecognizeinthesection seriesonelineofsegmentalcleavagewithtolerablecertainty. Idarenot assert that it is the only one present, and a critical examination of good silverprintsofthephotographherereproducedon P1.I.fig.2suggests the possible existence of up to three pairs of somites.

In any case, it is evident from the photograph in question that the numberofsomitescouldonlyhavebeensmall. Andinthisconnexionit must be pointed out that only a distance represented by about 25 sections intervenes between the tolerably abrupt posterior limit of the crescentic curve of the cephalic medullary fold and the anterior margin of the neurenteric aperture.

The distance between the probably corresponding points in the second of my specimens, "H 98," which possesses 9-10 pairsof somites,includes no fewer than 65 sections of similar thickness.

We havethereforetoreckoninembryo"H 3"with25sections,against 65inthelatterstage("H 98"),asrepresentingthepossiblelongitudinalfield ofsomitedifferentiation.

Now, an inspection of figures of other early embryos suitable for com- parison will show that the differentiated somites do not extend close up to the anterior, and certainly not nearly to the posterior, limits as above defined, so that we have to deal with a lesser number than 25 sections, which could pass through mesodermal primitive segments.

In the Kroemer-Pfannenstiel embryo, for example (fig. 11D of Taf. I., Keibel and Elze's Normentafel (2), the five pairsofsomnitestherepresentdo not occupy much more than half the distance between the neurenteric apertureandthehinderendofthecephalicmedullaryplate. Fromtheir figure it would appear that the distance from the cranial limit of the first pair of somites to the neurenteric aperture was about 0.7 mm., and the actual segmented zone occupied only about 0.35.

Inasmuch,then,as inembryo "H 3" the entire available length from cephalic medullary plate to neurenteric aperture is represented by only about 25 sections,the actualsegmentedportionwas quiteprobablynot more than about half of that length-say 12 or 13 sections in extent.

I find in the older embryo "H 98" that the somites there met with extend over an average distance of about 5 sections each. If,asishighly probable,there is no great increase in size of the individual somites in these early stages, the conclusion may be drawn that only two, or at most three, pairs of somites could possibly have been differentiated in embryo "H 3."

I have already stated that one cleavage line may be definitely established in the sectional series. I can find no positive evidence in the series of any other, and I see little reason to doubt that here we have an example of a human embryo at about the stage of progressive differentiation of the first two, or possibly three, pairs of somites.

Chorda

The chorda is still in the stage of intercalation in, and is indistinguishable from, the entoderm.

Not only isthere no chord as such, but there isno unequivocal indica- tion of strict delimination, even of a chorda-plate, except for a distance of 3 or 4 sections immediately in front of the neurenteric canal. Itispossible that in the pharyngeal region the longitudinal zone of entoderm, which coversthedorsalmedian prominence,due to thegrooved medullary plate, may actually represent definite chorda-entoderm. But, except that a median strip of entoderm is thus more or less accidentally delimited, there is nothing to suggest the specific character of just that precise area. Text- fig. 13 illustrates the 4th section in front of the anterior margin of the neurentericaperture. Evenherethechorda-plateisevident,notsomuch by its differentiation from the rest of the entoderm, as by the appearance here of an entodermal indentation which leads caudally into the neurenteric opening itself.

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. I.I.fig.2,alongwith text-figs. 1, 2, and 3 and the various sectional text-figures, will sufficiently illustrateitspresentphaseofdevelopment. Thephotomicrographictext- fig. 18 will further elucidate its structural arrangement so far as its rather poorhistologicalconditionwillallow. Text-fig.4illustratestheappearance of the medullary plate and groove a short distance (4-5 sections) caudal to the cephalic expansion of the plate, and 21 sections in front of the anterior lipoftheneurentericaperture. Nowhereisthereanycloserapproximation of the medullary folds than is shown in this section.

The histological conditions are not favourable enough for reliable observations either on neuromeric segmentation or on neural crest formation.

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.

EXPLANATION OF PLATES I.-III. FIGS. 1-6

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