Paper - Two Early Human Embryos

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Morton, W. R. M. 1949. Two early human embryos. J. Anat., 83, 308-314.

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Two Early Human Embryos

By W.R.M.Morton

Department of Anatomy, Queen's University, Belfast

The two embryos described here were given to me by Prof. J. H. Biggart, who had recovered them from curettage material removed by Prof. C. H. G. Macafee; I have named the older specimen the Biggart Ovum and the younger the Macafee Ovum. The material was fixed in formol-saline and later transferred to formol-Zenker; it was embedded in paraffin, cut at 6 µm and stained with haematoxylin and eosin.

The Biggart Ovum

The Biggart ovum was recovered from a woman aged 24 years; married 7 years; one child 4 years 8 months; no miscarriages; menstruation began at 15, 7-day loss recurring very regularly every 21 days; last menstruation stated to have ended 29 June; admitted to hospital 12 July and curetted 16 July; coitus said to have occurred only once between the end of menstruation and admission, probably on 2 July and certainly between 1 and 5 July.

The specimen consists of a piece of endometrium in which the ovum is very superficially implanted (P1.1, fig.1). Examination shows the preservation to be good and that only some peripheral villous material and one section containing embryonic material proper are missing; but some of the sections are a little distorted. It was found to be cut almost vertically to the embryonic shield and obliquely at 138 to its sagittal plane. The primary measurements in millimetres of the ovum and its parts areas follows.

The endometrium shows early decidual reaction and contains peripheral syncytium beyond the limits of the trophoblastic shell; there is no zone of necrosis. The glands are dilated and in the saw-tooth phase; they contain extravasated blood ands ecretion. Large blood sinuses are present beneath the ovum and seem to have limited the villus formation over one part of the vesicle. The site of entry is wide and filled with the thin ab-embryonic wall of the chorionic vesicle covered by a thin layer of schlusscoagulum; there is no operculum.

The trophoblast, which is lined with a chorionic mesoderm, consists of cytotro- phoblastandsyncytiotrophoblast,andviliarepresentovermostofit,butareabsent over the superficial surface and over one part of the deep surface near a large venous sinus(P1.1,fig.1).Manyofthevilicontainamesodermalcoreandsome havebegun to branch.

The chorionic vesicle contains a granular coagulum surrounding the embryonic rudiment,whichconsistsofanembryonicshield,asmallamnioticcavity,andalarger yolk-sac which is continued by a long narrow tubular duct into a large irregular expansion(Text-fig.1).Theembryonicrudimentisjoinedtothechorionicmesoderm firstbyabroadconnectingstalkattachedtotheamnion,andsecondlybymesoderm at the distal expansion of the yolk-sac (Text-fig. 1).

Text-fig.1.Drawing of wax model cut to show the relations of the embryonic mass. A.C. Amniotic cavity. Ch.M. Chorionic mesoderm. Y.S.Yolk-sac.

Text-fig.2. Scale diagram of surface view of the embryonic shield.

The embryonic shield is a broad oval in dorsal view and is not quite symmetrical about its long axis (Text-fig. 2). It is slightly convex towards the amniotic cavity. Itsmarginsarecontinuouswiththethinneramnioticectoderm,thetransitionbeing clearlymarked (P1.2,fig.4).The shieldectoderm consistsofhighcolumnarcels whichinplacesappeartobestratified,butthisisduetotheobliquityofthesection; those of the central area are larger than those nearer the margin and are more vacuolatedandgranular. Attheposteriorendoftheshieldandreachingitsmargin thereisamedianlinearstripoflightermore looselyarrangedpolyhedralcells (PI. 2, fig. 5); the strip extends over six sections and as measured by reconstruction is0-059mm. longand0-036mm. broad,thatis,itisalitlemorethanone-fifthofthe length of the shield (Text-fig. 2). The cells forming it are continuous in front and at the sides with the general shield ectoderm, and below they are in close contact with thegutendodermbutcanbedistinguishedfromit;therecanbelitledoubtthatitis the primitive node and streak.

The main interest of the specimen lies in the condition of the yolk-sac. The form of its three parts is shown in Text-fig. 1. The wall of the whole of the proximal part consists of a single layer of round or oval cels much smaller than the overlying ectodermalcelsandeasilydistinguishablefromthem. Cellsofthesamekindbound asmallwidediverticulumwhichpassesbackwards,belowtheleveloftheconnecting stalk, under the posterior end of the shield, which may represent the allantoic diverticulumormay beapartoftheexpandingyolk-sacwhichislaterincorporated in itas described by Florian (1930). The cels lining the duct part of the sac are cuboidal in form, have large round nuclei and are arranged in a single continuous layer(Pi.1,fig.2);onthewholetheyareslightlylargerthanthecelsoftheproximal part.Thecelsliningthedistalexpansion(PI.1,fig.3)graduallychangefromcuboidal above to thin flattened squamous-like cels below, and over the greater part are easily distinguishable from the covering layer of mesoderm; but the central part of the ab-embryonic wall is very thin and consists of a single layer of very flattened celsthenatureofwhich,endoderm ormesoderm, Iam unabletodecide.

The mesoderm covers the duct and the proximal part of the yolk-sac and is a continuous layer over the amnion; it forms the broad irregular connecting stalk. There is a small amount of intra-embryonic mesoderm in the peripheral rim of the embryonicshieldwhichisdirectlycontinuouswiththeextra-embryonicmesoderm; and in the angle between the anterior end of the shield and the yolk-sac there is amorelooselyarrangedmassofmesodermwhichmay wellbetheprotocardiacarea. Thereareafewprojections,withindicationsofcavityformation,inthemesodermof theproximalyolk-sacandcavitiesinthechorionicmesoderm,especiallyintheregion oftheconnectingstalk(PI.2,figs.4,5);thesehavebeentakentobethefirstevidences ofangiogenesis.

Discussion

The next menstruation, from the history given, should have begun on 14 July, and, taking ovulation to have occurred 14 + 2 days before this, the single coitus probably on2July,falswithintheprobableperiodofovulation28Juneto2July;theprobable coital age is thus 13J days. The general dimensions, structure and differentiation of the embryo, and the appearances of the decidua agree with this estimate and place the ovum in Streeter's (1942) Group VII; and in comparison with known ova it appears to be more developed than embryo No. 7801 (13j days) of Heuser, Rock and Hertig (1945), of about the same stage as the Yale embryo (14 days) (Ramsey, 1938), andyoungerthantheFalkinerovum (15-17days)(Martin& Falkiner,1938).The interestliesinthehithertoundescribedformoftheyolk-sac. %

hepresenceofaduct-likeprocessleadingdistalyfromtheyolk-sacandconnected tothechorionicmesodermwasdescribedbyBryce(1924)intheovum T.B.2andhe notedthatsimilarformationswerepresentinotherova(SchlagenhauferandVerocay, Fetzer,Strahl-Benecke)andthatintheFrassiovum thereisadetachedvesiclelined withendodermwhichisconnectedtotheyolk-sacbymesoderm. Heuseretal.(1945) describe in embryo No. 7801 several large vesicles in the peripheral part of the. chorionic cavity, the walls of which are for the most part composed of a single layer offlatsquamous-likecels,butoccasionallycontainingpatchesofcuboidalcels;the largest vesicle is attached by a faint strand of cytoplasm, which itself contains avacuolatedgroupofcels,totheyolk-sac.Thesameauthorsalsodescribeinembryo No. 7802 (161 days) a tapering funnel-shaped ventral process of the yolk-sac the mesodermal layer of which can be traced, as a fine filament, across the chorionic cavity to the mesoderm of the opposite side. Heuser & Wislocki (1935) have shown inthesloth(Bradypusgriseus)thatinearlystagesthereisachorionicattachmentof the ab-embryonic wall of the yolk-sac and the formation of two yolk-sac vesicles connected,foratime,byanarrowduct. Latertheconnectingductdisappears, leaving a definitive yolk-sac under the shield and an endoderm-lined cyst-like cavity attachedtotheab-embryonicwallofthechorion. Martin& Falkiner(1938)state thattheyolk-sacoftheFalkinerovum ispartiallydividedintotwobyaseptum;and thattheendodermliningthecavityoftheyolk-sacdorsaltotheseptumiscuboidal, and the endoderm of the part ventral to it is thin, flattened and smooth, except at the extreme ventral pole where it also is cuboidal. They also give details of a similar partial subdivision of the yolk-sac in a 15-day-old macaque embryo (No. C 571) and state that the differences of the two types of endoderm are more evident than in the human.Theyoffertwo explanationsfortheoccurrenceofthedivisionsoftheyolk-sac. Theystatefirst(p.257)thatthegrooves(externaltopartsoftheseptum)probably representtheearliestsignofthefolding-offoftheembryo,andthattheyseparatethe gutendodermdorsallyfromtheyolk-sacvesicleendodermventrally,assuggestedby Streeter. Onp.266theystatethatiftheprocessoftakingupevaginationsofthe yolk-sac'hadgonealitlefurthertheseptumpartiallydividingthisregionoftheyolk- sac would have disappeared and the part of the allantois situated in the caudal wall oftheyolk-sac'(i.e.belowtheseptum)'wouldhavebeenincorporatedintheyolk-sac cavity'. Thisimpliesthatsome ofthethinflattenedendodermassignedearliertothe yolk-sacvesiclebecomestransformedintocuboidalgutendoderm. Itisapparent, however, from a study of the Falkiner ovum, that the partial division of the yolk-sac seeninitisofadifferentnaturefromthatseeninthepresentspecimen.Theendoderm celsliningtheductpartoftheyolk-sacoftheBiggartovum areslightlylargerthan thecelslyingbelowtheshieldectodermandlining.theproximalpartoftheyolk-sac cavity. Itisonlyinthedistalexpansionoftheyolk-sacthattheliningcelsbecome thin, flat and smooth, and in part of the ab-embryonic wall they are even more flattened and are either endoderm alone or mesoderm (P1. 1, fig. 3). The appearances intheBiggartovum are much more suggestiveoftheconditionfoundinthe12-somite sloth. This stage could easily lead to the condition found in the 138-day human (No.7801)byHeuseretal.ortothatinthe38-somiteslothembryo.Theappearance of the endoderm cels of the ab-embryonic parts of the vesicle possibly is due to a slowrateor absenceofdivisionofthecels,orperhapstoafallingoffoftheactivity gradientandtorelativelypoorernutritionattheab-embryonicregion.The'stretching- out' of the yolk-sac by the expansion of the whole chorionic vesicle, to which the yolk-sacisattached,probablyisthedirectcauseofthethinningofthedistalyolk- sac cels, the duct cels remaining, or becoming, cuboidal because of the small diameter of the duct relative to the rest of the yolk-sac.

The originof the endoderm celsliningthe yolk-sac stilremains uncertain. The spreadround oftheendoderm celsfrom theperipheryoftheembryonicendoderm asseeninthemacaqueyolk-sac,must,inthehuman,beeitherveryrapidoraltogether omitted. Ifthespreadingofendodermcelsroundtheinterioroftheexo-coelomic membrane is absent or only partial, then the yolk-sac cels, as distinct from the endoderm cels below the shield ectoderm, must arise by delamination from the precocious mesoderm forming the exo-coelomic membrane as isstated to be the case byStreeter(1937).Theabsenceofadefinitedoublelayerofcellsovertheab-embryonic pole of the yolk-sac may suggest a migration of endoderm cels from the original embryonicendodermcels,themigrationnothavingbeencompleted;thesinglelayer presentwouldthenbedescribedasmesoderm. But,alternatively,thedelamination process may have been delayed over the less well-nourished ab-embryonic region or thesecondlayerofcelsmayhavealreadydisappeared. Itisunfortunatethatthe histological appearances are such that one cannot be certain whether this layer belongs to endoderm or mesoderm.

Whatever the exact mode of origin of the endoderm cells may be, it is evident that this ovum illustrates a stage in the formation of the human definitive yolk-sac intermediate between that described by Hertig & Rock (1941) for ovum No. 7700 (121days)andthatbyBrewer(1938)forthe15-dayEdwards-Jones-Brewerembryo. Alsothatthisstage,whichhasnotpreviouslybeendescribedforman,resemblesthe similar stage in the 12-somite sloth, and closely resembles the condition postulated by Hamilton, Boyd & Mossman (1945) in their scheme for the development of the yolk-sac.

The Macafee Ovum

The Macafee ovum was recovered from a woman aged 26 years; her last menstruation was from 20 to 26 January; coitus occurred on 22, 25 and 30 January; she was admitted to hospital on 3 February and was curetted on 5 February; the expected date of her next menstruation was 11 February.

The material obtained from the curetting was cut into a diagnostic section and fifty-one serial sections separated from the former by a small interval due to loss of sections. The diagnostic section, of which P1. 2, fig. 6 is a photograph, is nearest the centre of the ovum, and shows an endometrium well prepared for implantation. The endometrium is not grossly oedematous and there is no marked leucocytosis and no largehaemorrhagicareas. Somecapillarysinusoidswerefoundtocommunicatewith the spaces in the outer part of the trophoblast. The glands are dilated, but do not contain blood or pent-up secretion; some of them are being engulfed by the syncytiotrophoblast and their cells are disintegrating in the trophoblast spaces. There is no necrotic zone, and no syncytial masses of peripheral trophoblast were seen.

The ovum, which from the wax-plate reconstruction is seen to be ovoid in form, is very superficially embedded so that a large part of it is exposed and uncovered by uterine tissue. The measurements of the largest section are: trophoblast, maximum external1413x0-63mm. andtrophoblastcavity0-58x033mm. Thetrophoblast wallconsistsoftwo layers,an inneroflargecuboidalcelsliningthecavity,and projectingoutwards as celcolumns, and an outer thin coveringlayerofsyncytium. Thissyncytiumboundslacunar-likespacesandperipherallyformsacontinuouslayer which constitutes the junctional zone with the maternal stroma. There is a general absenceofactivitypenetratingsyncytiumfromthislayerandthelineofdemarcation between it and the uterine stroma is quite definite. The trophoblast spaces contain a coagulum and only a small amount of maternal blood cells. The trophoblast is lined internally with a layer of extra-embryonic mesoderm which does not indent the trophoblastcolumns. Inthesectionnearestthecentreoftheovum (PI.2,fig.6)the mesoderm forms what appears to be an exo-coelomic membrane, but this cannot be tracedinthemoreperipheralsections;therethemesodermissmallerinamountand forms a much thinner layer. There isno embryonic rudiment.

Discussion

The superficial implantation and the general histological appearance of the speci- men indicate that the ovum was probably dying and being cast offby the maternal tissues. The coital and menstrual histories are believed to be reliable, and assuming that the coitus on 25 January was the fertile one, give to the ovum a maximum coitalageof10 days.Themeasurementsofthisovum,ascomparedwithother pre-villous ova, notably the Hertig & Rock ova (Carnegie Nos. 7699, 7700, 7950 and 8020), the Davies-Harding, Barnes, Miller, Dible and West, Kleinhans, Werner, ScipiadesandMarchettiova,alsoplaceitbetween9and13daysofage. Inappearance itcloselyresemblesthatofCarnegieNo.7950(Rock& Hertig,1942).Theengulfment anddisintegrationoftheuterineglandsbythesyncytiumareunusualinanovum of thisearlystageofdevelopmentandindicatethattheinvasivepowerofthesyncytium must, at one time, have been considerable.

Summary

Two human ova recovered from uterine scrapings are described. The older specimen (14days) isnormal,butisrathersuperficiallyembedded. Itshowsalargeyolk-sac withproximalanddistaldilatationsconnectedbyanarrowtubularduct.Theproximal and duct parts are lined with cuboidal cels. The distal expansion ispartially lined with flattened endoderm celsand also shows an area one celin thickness over the ab-embryonicpole.Thesignificanceoftheappearanceoftheyolk-sacandthegeneral position of the ovum with regard to other early human ova are discussed. The younger ovum (10 days) is incomplete. It is superficially embedded, and probablypathological. Erosionandengulfmentoftheuterineglandshaveoccurred atanearlierstagethanisusuallydescribedinthehuman. Itsprobableageandits generalchronologicalpositionarediscussed.

References

Brewer, J. I. (1938). A human embryo in the bilaminar blastodisc stage. Contr. Embryol. Carneg. Instn,27,85-93.

Bryce, T. H. (1924). Observations on the early development of the human embryo. Trans. roy. Soc. Edinb.58,533-568.

Florian, J. (1930). The formation of the connecting stalk and the extension of the amniotic cavity towards the tissue of the connecting stalk in young human embryos. J.Anat.,Lond.,64,454-476.

Hamilton, W. J., Boyd, J. D. & Mossman, H. (1945). Human Embryology. Cambridge: Heffers.

Hertig. A. T. & Rock, J. (1941). Two human ova of the pre-villous stage, having a developmental age of about eleven and twelve days respectively. Contr. Embryol. Carneg. Instn,29,127-156.

Heuser, C.H., Rock, J. & Hertig, A.T. (1945).Two human embryos showing early stages of the definitive yolk-sac. Contr. Embryol. Carneg. Instn, 81,85-99.

Heuser, C. H. & Wislocki, G. B. (1935). Early development of the sloth (BradypUs griseus) and its similarity to that of man. Contr. Embryol. Carneg. Instn, 25,1-13.

Martin, C. P. & Falkner, N. McI. (1938). The Falkiner ovum. Amer. J. Anat. 63, 251-271. RAMSEY,E.M.(1938).TheYaleEmbryo. Contr.Embryol.Carneg.Inmtn,27,67-84.

Rock, J. & Hertig, A. T. (1942). Some aspects of early human development. Amer. J. Obdtet. Gynaec. 44,973-982.

Streeter, G.L. (1937). Origins of the yolk-sac in Primates. Anat. Rec. 70, Suppl.no.1.53-54.

Streeter, G.L. (1942). Developmental horizons in human embryos. Contr. Embryol. Carneg. Instn, 30,211-245.

Explanation Of Plates

Figs. 1-5 are of the Biggart ovum, fig. 6 of the Macafee ovum.

PLATE 1

Fig. 1. The ovum in 8itu. Sect. 81. x 38.

Fig. 2. The duct part of the yolk-sac. The distal end is inferior. Sect. 77. x 375.

Fig. 3. The distal expansion of the yolk-sac. The end of the duct just appears above, on the left. Sect.77. x375.

PLATE 2

Fig.4. The embryonic rudiment. The distal expansion of the yolk-sac is not figured. Sect.81. x140.

Fig. 5. The opening of the duct part into the distal expansion of the yolk-sac which shows its thin ab-embryonic wall below. Sect.75. x95.

Fig. 6. The Macafee ovum in situ. Diagnostic section. x 65.

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