Talk:Paper - Some Observations on the Development of the Ventral Pancreas in Man

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An Early Human Ovum (Thomson) in situ. Odgers PN. J Anat. 1937 Jan;71(Pt 2):161-168.3. No abstract available. PMID 17104634

An Early Human Ovum (Thomson) in situ

IN March 1913 the late Prof. Arthur Thomson received the uterus, which contained this ovum, with its adnexa from Prof. H. M. Turnbull of the LondonHospital. HebrieflydemonstratedittotheAnatomicalSectionofthe XVIIth International Congress of Medicine, which was held in London in the summer of that same year, but he never published any detailed description ofit.Somefiveyearsagohesuggestedtome thatitwaswellworthfurther examination,butitwasnotuntilafterhisdeaththattheslidesofthisspecimen with his serial microphotographs and notes about it were brought to my notice. Ishallcallitinthispaperafterhisname,andIhopethat,ifitisever referred to in the future, itmay be known as the Thomson ovum.


Prof. Turnbull discovered it in the decidua of the posterior wall of the uterus of an unmarried woman, aged 20, who died 5 hours after severe burns on 4 March 1913. The right ovary contained a corpus luteum, which measured 24*5x17-5mm. The woman's menstrual history was as follows. It first occurred at 15 years of age and had always been regular. Her last period commenced on3February,exactly29 daysbeforeherdeath. No datesofcoitus were available. The post-mortem examination took place 26 hours after death. The specimen was fixed in 10 per cent. formalin and cut into sections 10 thick.


From the menstrual history the maximal age of this ovum must be about 16 days. The inside measurements of the blastocyst are 2-1x 151 x 0-7 mm.; theembryonicdiscmeasures0-26x0-31(?)x0-16mm. (?).


In the following list it is placed among its contemporaries. DrS.Zuckerman has kindly computed for me from the inside measurements of the blastocyst cavity a rough estimate of its cubic capacity in the several ova, the cavities beingregardedforthispurposeasperfectspheres,ofwhichthediameterswere taken as the mean of the dimensions given. These figures are added in a separate column.


As is seen in the general view (PI. I, fig. 1), the blastocyst cavity and the trophoblast are well preserved, but the embryonic disc, particularly the ectoderm, has suffered a good deal of disorganization, the amniotic cavity being partly obscured by cellular detritus. While this forbids any detailed description of the disc itself, I wish to draw attention here to:

  1. the formation of the extra-embryonic coelom, and
  2. the wall of the yolk sac.


I. THE EXTRA-EMBRYONIC COELOM

On either side of the amnio-embryonic vesicle a space is seen (P1. I, fig. 2), the one on the left being triangular in section, while the right one appears more quadrangular in shape. These are marked off from the rest of the blastocyst cavity by a regular layer - of mesothelial cels, which vary in differentsectionsintheirnumberandtheir shape, most of them being round, while aminorityareflatterandspindle-shaped. They are partially filed by coagulum similar to thatofthemagma. Ifthealmostsymmetricalchannelsthus formed are traced in one direction, they areseentocoalesceand surroundthisendoftheembryonicdisc(PI.I,fig.1), the common channel extending for thirteen sections beyond the disc: traced in the opposite direction they are observed to narrow and finallydisappear, the right before the left, sixteen sections from the disc, but here they are always separated from each other by a strand of mesenchyme and never join (PI.I,fig.4).


Text-fig. 1 is a diagram drawn roughly to scale to show the disposition of these channels (ExC) around the embryonic disc (ED) viewed from the dorsalaspect. ThelettersA,B,Ccorrespondtothelevelsofthemicro- photographs (PI. I, fig. 1; PI. I, fig. 2; and PI. I, fig. 4). While, as I have said, these channels are well marked out from the rest of the blastocyst cavity for the most part, there are in the case of either of them gaps in their parietes where they become continuous with the adjacent magma spaces. When tracedinserialsections,suchspacesareseentobeincreasinglyoutlined by mesothelialcelsso thateventuallytheyappear as litlebaysinthelateral wall of the channel which is thereby correspondingly enlarged.


The only other specimen which shows anything exactly like this one is the Peters ovum, but that of Tennant & Ramsey presents a condition very com- parabletoit.Theformerofthesewas re-examinedbyProf.0.Grosserin1908, andhe-hasbeengoodenoughtosendme hismicrophotographofthisovum andverykindlypermitsme toreproduceithereforcomparison(PI.I,fig.2).


The only differences in the appearance presented by these two sections are that in the Peters ovum (1) the lateral wall of the right channel has been torn away, (2) the containing wall of the intact left channel appears to be more celular, and (3) itextends on to the ventral wall of the yolk sac, while in the Thomson ovum, although they are wider, these channels are confined totheneighbourhoodoftheamnio-embryonicvesicle. Grosserdescribedthe channel in the Peters ovum as horseshoe-shaped; itsurrounds the cranial end of the disc as a small fissure. This is rather different from the wide com- munication presented by the present specimen.


Similarly, Tennant & Ramsey described "a space around the embryonic masswhichislimitedbyasinglelayerofflattenedmesodermalcellshavingthe appearanceofanendothelium(PI.II,fig.3).Thislayernotonlysurroundsthe space but likewise completely encloses those surfaces of the embryonic mass notdirectlyattachedtothemesenchyme." Intheirspecimen,too,thespace is horseshoe-shaped, as must be inferred from the fact that they found that "the left end of the embryonic disc stops bluntly and is entirely free of meso- thelial attachment while the right is completely encased in mesenchyme".


The three ova, Thomson, Peters and Tennant & Ramsey, seem to represent threestagesinthedevelopmentofthesechannels. Inthefirsttheyborder the amnio-embryonic vesicle only, in the second they have extended on to the lateralsideoftheyolksac,whileinthelasttheypracticallyenclosetheyolk saconallsides. Here,althoughtheauthorsdescribeaspace,Ithinkthereisa fine strand in the microphotograph they reproduce between the mesothelial layercoveringtheventralwalloftheyolksacandtheparietesofthespacetothe rightofthemidlinewhichmay indicateitsbilateralorigin(PI.II,fig.3),while in their reconstruction they show the ventral surface of the embryo as almost entirely free of mesenchyme, "with the exception of a small band of attach- ment extending about half way along this surface from the encapsulated end ".


In 1908 Keibel & Elze and Grosser described these channels as the com- mencement of the extra-embryonic coelom. Grosser (1924) suggested that theygraduallygrowventrallyroundtheyolksac,theiropposedwallspossibly forming, when they meet, the "distaler Nabelblasenfaden", the mesodermal strand which may connect the ventral pole of the yolk sac with the chorion. Further, he thought that these exocoelomic channels extend lateralwards by taking up the magma spaces nearest to them so that eventually the whole cavityoftheblastocystbecomestheextra-embryoniccoelom. Hewondered ifthelossofthelateralwallintheleftchannelinthePetersembryomightnot be due to its having already broken down so that on this side the exocoelom wascontinuouswiththeintercellularspacesofthemagma. Ihavementioned above that in the Thomson ovum there is evidence of the commencement of the same process, while Tennant & Ramsey wrote that in the specimen they described "on the surface towards the main chorionic cavity the continuity of thismembrane-likestructureislesswellmaintainedandirregularlydistributed openings in it offer probable avenues of communication between the larger and the smaller spaces ".


These three ova are peculiar then in being the only ones so far described which exhibit definite symmetrical coelomic channels.[1] The Fetzer (Grosser, 1924) and Herzog (Lewis, 1912) ova and the much more advanced embryo described by Waterston (Mall, 1916) have al been considered by these different observers to show something similar, but I would suggest that at al events the illustrations, which accompany the descriptions of these specimens, arefarfromconvincingfromthispointofview. Inalotherspecimensthe whole of the blastocyst cavity seems to represent the extra-embryonic coelom (Bryce,1924-5). An accumulationofmagmafluidappearsaccordingtothis author's account near its centre close to the embryonic disc, which gradually leads to the stretching and absorption of the mesodermal bands traversing itandtotheisolationoftheembryo,sothatultimatelyallthemagma spaces run together and the whole of the magma cavity becomes the exocoelom. Grosser (1913) and v. Mollendorif (1921) both suggested that there are thus two alternative ways in which this coelom may be formed.


It is, however, perhaps pertinent to observe that in the Peters ovum the trophoblast is already lined with "morula" mesoderm (Stieve, 1926), which also covers the ventral wall of the yolk sac beyond the limits of the exo- coelomic channel. The Thomson ovum presents the same appearance, although themesodermalcellscoveringtheyolksacarefew. Ineitherofthesecases, apparently, a gradual liquefaction of the magma would result in a perfect coelom lined with mesoderm without the help of the mesothelium of the channels at all.


With regard to the later fate of these exocoelomic channels there is only Grosser'ssurmisetoguideus,andonecannothelpthinkingthattheirformation as seen in the three specimens in which they occur serves no useful purpose. In any case their appearance in the Tennant-Ramsey and Thomson ova prove that the Peters ovum isno longer quite exceptional.


I. THE YOLK SAC

As seen in PI. I, fig. 2, this appears as a space beneath the amnio-embryonic disc,outlinedfromtherestofthemagma cavitybyathinmembranous wall. Itisflatteneddorsoventrally. On theleftside,itsventralwalliscon- nected by an irregular strand of mesenchyme cels to the chorion, but this is the only evidence of a commencing "Nabelblasenfaden" so obvious in the Schlagenhaufer-Verocay specimen. While the dorsal wall of the yolk sac beneath the embryonic disc has a lining of cubical entodermal epithelium its lateral and ventral walls are formed by a sparse layer of flattened spindle- shaped cells mesothelial in character. These latter appear to be similar to many of the cels lining the extra-embryonic coelomic channels, and the coagulum within the yolk sac seems to be exactly like that filing these

  1. Since this was written K. Hiramatsu (Fol. Anat. Japon, 1936, Bd. XIV, p. 15) has published an account of the ovum, Ei-Ando. In this he describes two small cavities on either side of the dorsal portion of the yolk sac, which he thinks correspond to these exocoelomic channels.