Paper - Formation of single ovum twins (1919)
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|twinning. The embryo is classified as Carnegie Embryo No. Template:CE1522.
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Formation of Single-Ovum Twins
Department of Embryology, Carnegie Institution of Washington
It has recently been the writers privilege to study the Mateer ovum, a very young, Weil-preserved specimen containing twin embryos. The larger embryo is in the primitivegroove stage, having an embryonic plate 0.92 mm. long and 0.78 mm. wide. In it form and attachment to the chorionic tnembrane it is entirely normal and, judging from the best criteria we have at present, it has a development of about 17 days.' The smaller embryo consist* of an amniotic vesicle 0.1 mm. in its largest internal diameter, and a yolk-vesicle (0.03 mm. internal diameter) slightly detached from it, the two being suspended in the l<H)st> me.>;enc)iyme in the region of the bmly-stalk of the co-twin. The position of these vesicles and their relation to the larger embryo are shown in Fig. 1. The chorionic membrane with its villi and tropoblast can be rect»giiized above, while below is shown the yolk-sac of the larger embryo, cut transversely through the region of the body-stalk, Among the loose strands of parietal mesoblast intervening between these can be seen the two defaclierl vesicles which together constitute the very much smaller twin.
- For a complete description of this embryo See " A human embryo (Mateer) of the presomite period." Contributions to Embryology. Vol. 9, 1919. Carnegie Inst. Wash.. Pub. No. 272.
The detailed structure of the amniotic vesicle of the smaller embryo is shown in Fig. 2, where its wall can be seen to be made up of an ectodermal layer clearly subdivided into amniotic ectoderm and a thicker plate Ik'Iow of embryonic ectoderm, which is to form the embryonic shield. The ectodermal layer is everywhere surrounded by an irregular membranous layer of mestMlerm. The wall of the yolk-sac consists of a single layer of endo<lermal cells (see Fig. 3) and, like the amnion, is inclosed in a layer of mesodermal tissue. In general form the anmiotic vesicle appears to be normal and corresponds in many respects to those seen in the ova describeil by Peters, Fetzer. Jung and Strahi-Heneke. It ditTers from those, however, in being itimpleti'ly detachetl from the yolk-sac, and in this respect is probably abnormal.
Among the first 2.">00 specimens in the Carnegie Embryological Collection there are 4.1 instances of twinning, which is about what would be expected in that number of births if we assume the frequency to vary between 1 and 2 per cent. In nine of these specimens the embryos are not over 20 mm. long — that is, less than eight weeks old — but in only two arc they under 5 mm. One of the.se is the Mati-er specimen which I am describing; the other is a patliological specimen ( No. Template:CE1522) which was obtained by Dr. Cecil Vest at operation for tubal pregnancy. In this case the tube contained two chorions undergoing hydatiform degeneration. Sections of these show respective internal diameters of 11x12 mm. and 14x10 mm.
Neither chorion contains an amnion, and in only one is there any remnant of an embryo (a nodule 2.5 mm. long). In size and character the chorions correspond to those seen in normal ova about four weeks old. The specimen represents, therefore, about the same stage of development as the Watt (1915) twins, .in which there were 17-19 paired somites. The Ohidester specimen (mentioned by Newman, 1917, p. 68) was at first thought to be twins, one embryo having a development of one month, while the other was supposedly in the primitive streak stage. Subsequent study, however, has shown that the smaller structure was not really a twin. If, then, we disregard this specimen, that of Watt constitutes the youngest re])orted case of human twinning.
Fig. 1. — Section of the Mateer ovum showing yolk-sac and body-stalk of the primary embryo, between which and the chorionic membrane are two small ectodermic vesicles constituting the amniotic and yolk vesicles of a twin embryo. X 34.
The Mateer ovum is considerably younger than the Watt specimen. Whereas the latter is in the stage of 17 somites, the primary Mateer embryo is in the presomite stage and has only jtist acquired a primitive groove, while the accompanying twin lias attained an even lesser degree of development. The fact therefore, that it affords the earliest picture of twin formation that has thus far been seen, lends to this specimen a particular interest. Furthermore, it possesses additional importance in that it shows this early stage of the twinning process in a single ovum. Both the Watt specimen and No. 835, mentioned above, are double-ovum twins.
Fig. 2. — Amniotic vesicle of the twin embryo showing the thicker embryonic plate below already differentiated from the thinner amniotic ectoderm. The vesicle is surrounded by an irregular membranous layer of mesoderm, X 400.
The exact character of the mechanism by which human twins are derived from a single ovum is still unknown. There are, however, certain mammals in which polyembryony is the normal occurrence, and where it has been possible to obtain all the stages of tlie process. This is the case in the nine-banded armadillo which has been carefully studied by Newman ( 1917 ) and Patterson (1913). According to these authors, at a time when the single amniotic vesicle has reached a considerable amount of differentiation there occurs a physiological isolation of four secondary areas in its wall which undergo iude])endent growth and development, giving rise to four individual embryos with a common anmion. This departs from the conditions existing in human twins, since in the latter there are two separate amnions. This is also the case in sheep.
Fig. 3. — Yolk-sac of twin embryo enclosed by an irregular layer of mesoderm. X 400.
Very young sheep twins, no more than seven days old, have been described by Assheton. His specimen consists of a blastocyst 0.9 X O.T mm., coutaining two germinal areas, each apparently perfect and presumably capable of forming a perfect cmbrvo. They were completely separated and in each of them the embryonic mtiss consisted of a compact cluster of ectodermal cells with a thin endodermal membrane extending a short distance from its margins — the uncompletinl yolk-sac. There was a slight difference in size between the two areas and both were smaller than normal germinal areas in the same stage of development. Assheton regarded liis specimen as having been produced by fission, or a mechanical splitting apart of the original germinal mass, at the time of the formation of the blastodermic vesicle, which he thinks is rendered more likely in such animals as the sheep, in which the cavity of the blastocyst is produced by an irregular cleft formation which tends to subdi%idc the embryonic mass. The direction of the line of fission was a matter of concern to Assheton, who thought that only sagittal fission could produce twins and double monsters. Transverse fission separating primary and secondary centers of growth of the embryo he believes to be incapable of producing embryos that would subsequently grow and develop.
Although apparently meeting the requirements in the sheep, it is not probable that this simple mechanical explanation presented by Assheton will be sufficient to explain certain features of the problem occurring in other forms. There is doubtless, as Xewman argues, a large physiological element which is not to be explainc<l on morphological grounds. In this connection reference may be made to the interesting experiments of Ijewis (1910), who found that small lateral fragments of the medullary plate in the region of the hind-brain, when transplanted, tend to develop individually into a bilaterally symmetrical medulla oblongata, with a characteristic ventricle, roof, and the typical arrangement of white and gray substance. In my own experiments on the otocyst I always found that fractional parts of the otic cup, when transplanted, tend to form comjdetely closed vesicles, although in this organ there is marked predetermination of tissue. We should, therefore, naturally expect that sub<]ivision into two masses of such a primitive and undifferentiated tissue as the embryonic node would result in the formation of two embryos having all the potentialities of the original node.
The earliest stage of development that has been observed in the human embryo is the one described by Miller. In his specimen the embryo, or embryonic node, consists of a solid cell mass undergoing cleft formation preliminary to the opening up of the amniotic ca\-ity. The embryo and the tissue from which the c.xocteloni is formed arc inclosed by a trojihobla«tic shell of ectoderm possessing irregular syncytial loops which tend to enclose the large blood lacun<-F surrounding the ovum, there being as yet no villi. This stage is shown in Fig. 5. A. Instead of a single embryo, as was the case in the Miller specimen, the embryonic node is represented as having subdivided into two masses — that is, as having given off a hypothetical twin. It is conceivable that the chances of the secondary bud reacliing maturity would depend upon how large a share of the original mass is contributed to it. If the twin is as large as the priuuiry embryo their chances of developing in an orderly manner would be equal, and this is presumably the case in most instances of identical twins, ^^^lee the secondary bud is merely a fragment of the original mass we should expect that, like the fragments of medullary plate in the Lewis experiments, there would be some degree of differentiation; but the process of development would soon be arrested, and at term the stunted bud would be found as a small epithelial cyst on the placenta near the attachment of the timbilical cord. In case the twin-bud is only partially detached from the primary node there would exist the basis for the various types of double monsters and teratomata.
Fig. 5. — Schematic drawing, showing tlic probable stops in tlie formation of a single-ovum twin. The 3tage.s are drawn at the same scale of enlargement so that they may be directly compared.
A. Stage corresponding to the Miller specimen, showing a hypothetical twin budding off from the primary embryonic node.
B. Stage corresponding to the Bryce-Teacher specimen.
C. The Mateer specimen. The relatively small size of the twin In this specimen, and the detachment of the yolk sac from the amniotic vesicle are Indications of arrest In development.
A slightly more advanced stage is schematically shown in Fig. 5, B, which may be compared to the conditions existing in the well-known Bryce-Teacher specimen. Here, both embryonic mas-^es have developed into ectodermal (amniotic) vesicles, and at the same time small yolk-sacs are forming near them and j)nibably from cells derived from their wall. The transition from stage B to stage C may be readily followed, the latter showing the conditions existing in the Mateer specimen. Here the size of the cavity of the yolk-sac has surpassed that of the amnion, a definite body-stalk has formed, and near it are the two vesicles constituting the twin, which, as compared with the primary embryo, is considerably retarded in development. It is probable that under usual conditions the two embryos would be approximately of the same size, and the small size of the twin is to be accounted for by the incompleteness of the bud given off from the primary embryonic node. A comparison of sections made through the amniotic vesicle of the twin and through a corresponding region of the primary embryo, as shown in Fig. 4, would indicate that the former was essentially normal in form though much retarded in development. The complete detachment of the yolk-sac justifies us, however, in considering it as abnormal. If the pregnancy had not terminated in this case it is probable that the larger embryo would have gone on to maturity and the smaller one would have remained stationary in the form of two minute epithelial vesicles and been entirely overlooked.
Fig. 4. — Photographs of the twin (above) and the primary embryo (below), talten at the same enlargement (lOO diameters) and through similar portions of the embryonic plate.
In closing I would call the particular attention of obstetricians to this point, for it is probable that a careful search at the placental attachment of the umbilical cord would frequently reveal the presence of similar minute epithelial vesicles, the remains of stunted twins. We might thus find that the tendency toward twinning in man is even greater than is now supposed.
Assheton, R., 1898. An account of a blastodermic vesicle of the sheep of the seventh day, with twin germinal areas. Jour. Anat and Physiol., XXXII, 362.
Bryce, T. H. and J. H. Teacher, 1908. Contributions to the study of the early development and imbedding of the human ovum. Glasgow.
Lewis, W. H., 1910. Localization and regeneration in the neural plate of amphibian embryos. Anat. Rec, IV, 19.3.
Miller, J. W., 1913. Corpus luteum und Schwangerschaft. Das jiingste operativ erhaltene menschliche Ei. Berlin klin. Wochenschr., L. S65.
Newman, H. H., 1917. The biology of twins (mammals). University of Chicago Press.
Patterson, J. T., 1913. Polyembryonic development of Tatusla novemcincta. Jour. Morph., XXIV, 559.
Streeter, G. L., 1914. Experimental evidence concerning the determination of posture of the membranous labyrinth In amphibian embryos. Jour. Exper. Zool.. XVI.
Watt, J. C, 1915. Description of two young twin human embryos with 17-19 paired somites. Contributions to Embryology, Vol. 2, Carnegie Inst. Wash., Pub. No. 222.
Cite this page: Hill, M.A. (2021, June 15) Embryology Paper - Formation of single ovum twins (1919). Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_Formation_of_single_ovum_twins_(1919)
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