Paper - A presomite human embryo with a neurenteric canal (embryo R.S.)

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A presomite human embryo with a neurenteric canal (embryo R.S.)

By P. N. B. Odgers

Department of Human Anatomy, University of Oxford

THE paraffin block containing the embryo described in this communication was given to me by Dr Robb Smith, Pathologist to the Radcliffe Infirmary, who was also good enough to furnish me with the following information about its origin. Hefounditinauteruswhichhadbeenremovedbyhysterectomy the same day from a woman, aged 40, and which contained several intramural fibroids undergoing red degeneration. The whole of the endometrium was thickened, and a small cyst was noticed in the lower posterior quadrant; this was excised with the corresponding portion of the uterine wall. The excised specimen was fixed in 5 % formol saline and embedded in paraffin. When a few sections had been cut and stained, Dr Robb Smith discovered the presence of a blastocyst; he therefore handed the rest of the block to me.

The menstrual history was as follows. The last period began on 5 October, and the period which was due on 2 November was missed. The uterus was removedon15November. Fromthesedatatheminimalageoftheembryo must have been 22 days.

Theblockwascutintoserialsections10o inthickness,andthesewere stainedwithhaematoxylinandeosin.Theywerefoundtocontainthecomplete embryonalrudiment. Eachsectionwasphotographedatamagnificatiopof 150, and a reconstruction model of the embryonal disc was made at this magnification by the blotting paper and beeswax method. This latter has been used by myself for many years in this Department, and I consider that it is unrivalledforitsaccuracyanddurability. Fromthereconstructionthesections werefoundnottobetrulytransversetothelongitudinalaxisoftheembryonal plate. The angle which the plane of section makes with that transverse to the embryonalaxisincreasesinthecranialhalfofthedisc. Itis100midwayalong the primitive streak, 150 at the level of the neurenteric canal, while it is 400 half-way along the chorda plate and almost 800 in the prochordal area. These differences are partly due to the angle which the chorda plate makes with the primitivestreakatthesiteoftheneurentericcanal(seebelow). Norarethe sections accurately vertical: they are cut with an obliquity which is most marked in the cranial end of the embryonal plate.

From theaccountofitwhichfollows,theembryowillbeseentofalinto the group to which the following four previously described specimens belong:

• Frassi (1907, 1908), length 1-17 mm.
• V. Spee, Embryo Gle (1889, 1896), length 15 mm.
• Triepel, Embryo Dy (1917), length 1.6mm.
• Boerner-Schwarzacher (1923), length 0.47 mm.

Whilealoftheseareverysimilartothepresentspecimenintheirsize(with the exception of the Boerner-Schwarzacher embryo) and in the appearances they present, the Frassi embryo seems to be the most like, and many of his drawingsmightwellillustrate,my description.Theonlysubstantialdifferences between the two are the length and shape of the embryonal disc, the Frassi discbeingbothshorter(1.17mm.)andnarrower(0.6mm.)thanmine. Hill& Florian (1931) have drawn attention to the two varieties of embryonal shield, the one small and comparatively narrow, the other large and comparatively broad. My embryo,R.S.,evidentlybelongstothelattertype.

Description Of Embryo

The germ disc (P1. 1, fig. 1 and Text-fig. 1). The embryonal area is roughly triangular in shape and its greatest breadth is not far short of its axial length. The following measurements of the disc and of the axial structures were made with a flexible tape measure on the reconstruction model (x 150) and reduced:

Length of disc from the cranial amniotic reflexion to the caudal end of the primitive streak . . . .. . Greatest breadth of disc . . . . . . Length of primitive streak, ... ... ... ... Length of chorda plate . . . Length of prochordal plate . . . . . Length of area in front of prochordal plate . . . 1*5mm. 136mm. 04 mm. 07mm. 0-29mm. 0-08mm.

The embryonal disc exhibits a marked dorsal convexity in.either plane, the greatest height of this being about half-way along the chorda plate where it risesto037mm. above the baseline. While in the transverse plane this convexity forms an even curve, along the cranio-caudal axis of the disc (Text-fig. 2) it is definitely angulated midway along the chorda plate. In Triepel's embryo this angulation, which occurs at about the same situation, isincreased to a right angle.

There is no bend at the site of the neurenteric canal, as is seen in V. Spee's embryo Gle in which the streak area makes almost a right angle with the cranial portion of the plate at this point.

A dorsalview of the reconstruction model (PI. 1,fig. 1)shows at the junction ofthecaudalthirdwiththeanteriortwo-thirdstheopeningoftheneurenteric canal,whilebehindthisistheprimitivegroove. Immediatelyinfrontofthe neurentericorificeisasmallprojectionwhichmay betheremainsofHensen's node, and in front of this again the general dorsal convexity is broken for a short distance by the commencing medullary groove. Two further points may be observed in PI. 1, fig. 1 and in Text-fig. 1, in botb of which the different portionsoftheembryonalaxisaremappedout. Inthefirstplacethereiscon- siderableasymmetryofthedisc,sothatroughlythree-fifthstotwo-thirdsofit, varying between these limits at different levels, lie to the left side of the embryonalaxiswhenviewedfromabove. Secondly,itisapparentthatthe chorda plate and the primitive streak are not in the same straight line. The chordaplateisinclinedtotheleftandmakesanangleof250withthelineofthe primitivestreakatthesiteoftheneurentericcanal.Thisdeviationisfiguredas being more marked stilinthe Boerner-Schwarzacher specimen and may be readily explained, I think, by the attachment of the massive connecting stalk to the caudal end of the plate. While the streak area is fixed in this way, the openneurentericcanalverypossiblyallowsacertainamountofmobilitytothe anteriorportionofthedisc.

Theprimitivestreak.Themeasurementsoftheextentoftheprimitivestreak and the chorda plate (0.4 and 0 7 mm.) show that the former is of greater length than would be expected at this stage in comparison with most of the otherembryosofaboutthisage. IntheFrassiembryoabouthalfoftheaxial formation isrepresented by the streak so that the latteriseven more extensive thaninmine. InTriepel'sembryo,however,onlyone-seventhoftheembryonal area is stated to be caudal to the neurenteric canal, while in Rossenbeck's embryo Peh-Hochstetter (1923), which is younger than mine, the proportion between streak and head process is 2 to 5 and in Ludwig's specimen (1928), whichisslightlyolderthanmine,itisalmost1to5. Boththeselattermeasure- ments are taken from Florian's.(1934) Schema der Entwicklung derAxialgebilde desmenschlichenEmbryos,inwhichthegradualdiminutionofthestreakareain comparisonwiththesteadyelongationoftheheadprocessisgraphicallyshown. Intosuchaschemeasthismy embryohardlyfits.

PI. 1, fig. 2 is a microphotograph of a typical section of the streak area. A smallpointtowhichattentionmay be-directedistheconditionhereofthe entoderm.Whileitgenerallytakesnopartinthefusionofthegermlayersand formsadistinctlayerthroughoutitsextent,thecellscomposingitarecrowded together and give the appearance of sharing in the cellular activity of the neighbourhood. Inmorethanoneplace,moreover,particularlyinthecaudal endofthestreak,thereareconnexionsbetweenitandtheoverlyingmesoderm, whichalmostsuggestthatitmay evenstilbecontributingtotheformationof thelatter. Inearlierembryosfusionoftheentodermandofthemesodermof the primitive streak has been frequently observed (Florian, 1932-3).

The neurenteric canal (PI. 1, fig. 3). This is seen to be patent in two sections and is oval in shape, its longer axis being in the antero-posterior plane. The lipsofthecanalshelvemoregraduallyonitsventralthantheydoonitsdorsal aspect,thisbeingparticularlymarkedinthecraniallip. Fillingthelumenisan amorphous coagulum stained with eosin. The canal is a true neurenteric canal, beingaverticalconnexionbetweentheamnioticandyolksaccavities. Grosser (1924) insisted that the term should be confined to such a channel and should not be used synonymously with the chorda canal. The Boerner-Schwarzacher embryo showed a neurenteric canal with a larger lumen than mine: it was similarly oval in shape and measured 40 x,30p in its longitudinal and transverse diameters respectively. Sternberg(1927) suggested that such a widely open canal perpendicular to the disc was due to post-mortem changes and was only found in abortions (e.g. V. Spee's Embryo Gle.and Triepel's Embryo Dy), or in cases such as the Boerner-Schwarzacher embryo, in which 40 hr. had elapsed post-mortem before the specimen was found in the uterine wall, and he supposed,therefore,thatitmightnotbetrulycharacteristicofhumanembryos, He contrastedwiththesetheobliquecanalsoftheFrassiandM'Intyre(Bryce, 1925) specimens, but in the former of these the obliquity, as shown in the originaldrawings,isveryslight,andFrassistated,moreover,thathecouldnot determine its patency owing to a damaged section. Again, the M'Intyre embryoisnotcomparablewiththegroupunderconsideration,asthiscontains the remains of a chorda canal rather than the true neurenteric canal of Grosser's definition. On the other hand, both my own and Ludwig's Da I embryos were obtained by operation, and while the neurenteric canal in the latter had a narrower lumen it was, like mine,-perpendicular to the embryonal disc.

Thechordaplate(P1.1,fig.4).Thisformsaflattenedplateofvaryingthick- nesscomposedofroundcels. Itisthickeratitscentrethanatitsperiphery, one to three layers of cels being counted in the different portions in any one section. The plate is interpolated in the entoderm with which its edges are continuous,butinnocasedoesanyentodermclotheitsventralaspect. Nor can,Iseeinanyofmy sectionsthatthechordaplateisrecessedandboundedby distinctlipsoftheentodermasBryce(1925)describedintheM'Intyreembryo (thebourrelets16citho-enteriquesofVanBeneden). Insomesectionstheplate isindirectcontinuityatitsperipherywiththestreakmesoderm,andexception- ally the latter just overlaps the margin of the plate to separate it from the overlyingectoderm. Forthemostpart,however,theyaredistinctfromeach othersothatthereislitle,ifany, evidenceinthisembryo thatthechbrda platecontributesanythingtotheformationofthemesoderm. Onecannotbut beimpressedbythebreadthoftheplate. Allowingfortheobliquityofthe sections in this specimen, it seems impossible to think of it as being merely the remainsofthedorsalwallofthechordacanal. AsfarasIknow,Triepelisthe only author who states that the plate increases in circumference after the disruptionofthecanal,butonemustbelievethatthisisthecase. Bythesame token, as Bryce and others have insisted, the breadth suggests that it must yield something more4than simply the notochordal rudiment, i.e. that it probably helps to form the entoderm of the digestive tract.

Caudally the plate becomes continuous with the ectoderm lining the craniallipoftheneurentericcanal. Icannotmakeoutanycertainevidenceof either chorda plate or of the remains of the head process posterior to the neurentericcanal. Ludwig(1928)describedinhisembryoDa Itwinanlagenof theplatejoinedinthemidline,whichheobservedinthefirstsectioncaudalto this opening.

In the sixth,seventh and eighth sections from the cranial end of the chorda platecan be seen a small collection of cels similar to those ofthe plate,which forms a short oval mass lying between its centre and the ectoderm and con- tiguouswithbothofthese(P1.1,fig.5).This appears to be solid throughout its extent. Does this represent a premature attempt at the formation of a notchord? Triepel suggested that in his embryo Dy there was possibly an indica- tion of a commencing notochord, but this consisted of a small narrow dorsally directed evagination of the chorda plate close to its junction with the entoderm and therefore not median in position, which occurred in some sections not far cranialtotheneurentericcanal. If these appearances in my own embryoorin theembryoDy haveanysuchsignificance,theymustberegardedaspremature efforts. The earliest true evidence of the formation of a notochord from a chordaplatethathasbeenrecorded,sofarasIam aware,isfurnishedby Corner's (1929) embryo with ten somites.

The prochordal plate (PI. 2, fig. 6). The thickening of the entoderm with its characteristic chromatophilic granules (Bonnet, 1901) is seen to extend for 0-29 mm. cranial to the commencement of the chorda plate (PI. 1, fig. 1 and Text-figs. 1, 2'. As the latter appears to be more extensive on its left side, the prochordal plate overlaps it on the right for 0-13 mm. Hill & Florian (1931) described it in the embryo Dobbin as being prolonged backwards for 70/, on either side of the head process. In my embryo the prochordal plate does not reach the cranial margin of the embryonal disc but is separated from it by a region0-13mm. in extent in which the entodermis unaltered.The plate shows in four sections some irregular cavitation, and in its caudal portion is traversed in two sections by what appears to be a short canal. This channel, which is seen to be obliquely sectioned, may or may not be patent, but can be observed inPI.2,fig.8 to be continuous by its dorsal wall with the chorda plate,whilein the next section cranial to this (PI. 2, fig. 7) it opens ventrally and its dorsal wall becomes continuous with the entoderm, the whole canal passing dorsal to the prochordal plate. The site of this canal is marked by a cross in Text-fig. 1 and, while it is median in position, it is not at the extreme cranial end of the chorda plate which extends more anteriorly to the left of it. I am tempted, however, to regard it as the remains of the chorda canal. Suchremainshave been described by Eternod (1899) in the embryo Vull, while Rossenbeck (1923) in the earlier Peh-Hochstetter specimen, where there was a chorda canal throughout, found that this passed anteriorly immediately dorsal to the pro- chordalplate. Bryce (1925) described the chordaplate in the M'Intyre embryo as being continuous cranially with twin 'diverticula', which were dorsal to the prochordal area, but suggested that a cavity in the central mass ventral to these was the blind anterior end of the chorda canal. The channel in my embryo is similar in position to his diverticula, while I suggest that his ventral cavity may well be an instance of the irregular cavitation in the prochordal plate which has been frequently found in it by others. This interpretation would accord with the findings in the Eterjiod and Rossenbeck embryos.

The cloacal membrane.This begins directly at the caudal end of the primitive streak: there isno interval between the two as-in some earlier embryos. Two sections only show a real fusion of ectoderm with entoderm, which Florian (1932-3) insisted should be regarded as the true definition of the-cloacal membrane, but two sections on the cranial side and one on the caudal side of these show outgrowths of ectoderm which in places almost reach the entoderm. In one of the two sections (P1. 2, fig. 9) a-solid outgrowth of the amniotic ectoderm just dorsal to the junction of the amnion and the shield ectoderm is seen to reach and to fuse with the entoderm of the allantois. The other section (P1.2,fig.10) shows columns ofcelswhich, arisingfrom the shield ectoderm close to its junction with the amnion, fuse with the yolk-sac entoderm. These two outgrowths, the one entirely amniotic and the other from the shield ecto- derm, are separated from each other by some stalk mesoderm. This is shown diagrammatically in Text-fig. 2. The cloacal membrane is less extensive here than in the earlier Heuser (1932) embryo in which it forms a continuous plate involving both allantoic and yolk-sac entoderm.This is to be expected,sinceit is generally agreed that the membrane is more extensive in the earlier stages and that its caudal portion gradually disintegrates later. It is, however, unexpectedly more extensive in the Sternberg (1927) embryo with four somites. Here there are three discrete contacts of amniotic ectoderm with the entoderm of the allantois together with one contact of shield ectoderm with theyolk-sac entoderm.

The mesoderm of the disc. There are a few small cavities in the peripheral portions of the streak mesoderm, some of which look more like vascular channels than commencing coelom. These are exceptional, and for the most part the mesoderm appears to be unsplit. In places quite apart from the streak area small bridges of mesoderm unite it to the underlying entoderm, which suggests that the latter may be contributing to its formation.

Yolk sac, body stalk and chorionic cavity. There is nothing in any of these structures that has not been fully described previously. Very marked angiogenesis is to be observed both in the mesoderm covering the collapsed yolk sac and in that of the body stalk. In the chorionic cavity is a very small cyst lined with a cubical epithelium. This is situated close to the chorion and 48 sections (480I)awayfromthecranialborderoftheyolksac. Suchentodermalcysts have been frequently described (Frassi, Grosser (1912-13) and Heuser) in embryosofaboutthisstage,but,asarule,theyareclosetotheyolksacwall.

SUMMARY

The embryonal disc described above measures 1-5x1-36mm. and is reckoned to be 22 days old. In it can be distinguished primitive streak, neurentericcanal,tihordaplateandprochordalplate;inthelatterarea-thereis apparently a remainder of the chorda canal. The cloacal membrane consists of two distinct portions, the first where amniotic ectoderm fuses with allantoic entoderm, and the second where shield ectoderm extends into yolk sac ento- derm. The embryo belongs to the group of which the Frassi embryo is the best known example.

I am indebted to Mr W. Chesterman of this department for much skilled help in my reconstruction and for the preparation of the microphotographs.

References=

BOERNER-PATZELT, D. & SCHWARZACHER, W. (1923). Z.ge8.Anat. 1.Z.Anat.EntwGesch.68,204.

BONNET, R. (1901). Anat.Hefte,IteAbt.16,23.

BRYCE, T. H. (1925). Tran8. R. Soc. Edinb. pt. Im,53, 533.

CORNER,G.W. (1929). Contr. Embryol. Carnfg. Inst.no.112, 20, 81.

ETERNOD, A.C.F. 1899). Anat. Anz.16,131.

FLORIAN,J.(1934). Biol.Gen.10,Lief2,533. (1932-33). J.Anat.,Lond.,67,263.

FRASSt,L.(1907). Arch.mikr.Anat.70,492. (1908). Arch.mikr.Anat.71,667.

GROSSER,0. (1924). Z. ges. Anat. 3. Ergebn. Anat. Entw Gesch. 25, 391. (1912-13). Anat.Hefte,lteAbt.47,653.

HEUSER, C. H. (1932). Contr. Embryol. Carneg. Inst. no. 138, 23, 251.

HILL, J. P. & FLORIAN, J. (1931). Philos. Trans. B, 219, 443.

LUDWIG, E. (1928). Jb. morphol. micro. Anat. 1te Abt. 59, 41.

ROSSENBECK, H. (1923). Z.ges.Anat. 1.Z.Anat. EntwGesch.68,325.

V.SPEE, F. (1889). Arch. Anat. Physiol. p.159. (1896). Arch.Anat.Physiol.p.1.

STERNBERG, H. (1927). Z. ges. Anat. 1. Z. Anat. EntwGesch. 82,142.

TRIEPEL, H. (1917). Anat. Hefte, ite Abt., 54,151.

Explanation Of Plates 1 And 2

Plate 1

Fig. 1. Photograph of the dorsal aspect of a reconstruction model of the embryonal disc (x 75). The amniotic sac has been removed and the allantois dissected out of the body stalk, while the differentportionsoftheaxialformationsareportrayedonthesurface. P.S.primitivestreak. N.C.neurentericcanal. C.P. chorda plate. P.R.prochordal plate.

Fig. 2. A section (x 250) through the primitive streak. It shows the activity of the entoderm whichatX may be contributing to the mesoderm.

Fig. 3. A section (x250) through the neurenteric canal, N.C. C.P. indicates the commencement of the chordplate. Y.S. yolk sac.

Fig. 4. A typical section (x250) of the chordaplate,C.P.

Fig. 5. A section (x370) showing a mass of cels at X contiguous ventrally with the dorsal surface of the chord plate and dorsally with the ectoderm.

Plate 2

Fig. 6.A typical section through the prochordal plate,P.R.(x370)with its characteristic chromatophilic granules.

Fig. 7. Section showing the cranial end of the presumed remains of the chord canal,0.C. (x370). Its dorsal wall is seen to become continuous with the entoderm, while ventral to it is the prochordal plate,P.R.

Fig. 8. Section (immediately caudal to that shown in fig. 7) showing the caudal end of the same canal, C.C. (x370). Its dorsal wall is seen to be continuous with the chordaplate, C.P. Fig.9. Section 112 (x250) showing an outgrowth of the amniotic ectoderm, Am. fusing with the allantoic entoderm, AL.

Fig.10. Section 113 (x250) showing strands from the shield ectoderm reaching and fusing with the entoderm to form the cloacal membrane, C.M.

Fig.11. A section of the embryo in situ (x6).

Reference

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Cite this page: Hill, M.A. (2024, April 24) Embryology Paper - A presomite human embryo with a neurenteric canal (embryo R.S.). Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_A_presomite_human_embryo_with_a_neurenteric_canal_(embryo_R.S.)

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