Book - The Development of the Albino Rat 6
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Blastodermic Vesicle, Blastocyst or Germinal Vesicle
The material on hand is listed in table 6.
|RECORD NUMBER||AGE||NUMBER OF VESICLES|
|75||5 days, 15 hours||6|
|91||5 days, 16 hours||2|
|88||5 days, 21 hours||7|
|89||5 days, 21 hours||6|
During the sixth day, the blastodermic vesicle of the albino rat increases in size relatively rapidly. The greater portion of its wall is, at this stage, composed of a single layer of flattened cells. The vesicles are not as yet attached to the uterine wall, though the uterine mucosa shows a distinct reaction to their presence. Localized thickenings of the uterine mucosa, sufficient to cause localized swellings of the uterine tube, indicating the position of the ova, are evident. I have experienced more difficulty in successfully fixing the vesicles during this stage than any of the earlier or later stages studied. Although my material contains 58 vesicles of the stage under consideration, none of them may be regarded as being well fixed, and the majority of them are so folded as a result of contraction during fixation that they are of little value as objects for especial study. That the vesicles are still unattached to the uterine wall is readily determined by the fact that the shrivelled vesicles are found lying free in the depressions of the uterine mucosa, lined by a low cubic epithelium, intact throughout, and retaining its normal relation to the mucosa. The molding in these mucosal depressions no doubt gives the size of the respective vesicles as in vivo.
It is not my purpose at this time to consider more than superficially the changes affecting the uterine nuicosa during ovum implantation in the albino rat. It is hoped that this may be the subject of a future comnumication. It is the purpose in the present conununication to confine consideration to the development of the ovum itself. Many of the observations recorded by Burckhard on the implantation of the ovum of the mouse and the formation of the decidua, I find equally adapted to similar phenomena in the albino rat. Differences are to be observed in certain details which it is not the purpose to enter into here. Grosser gives a number of excellent figures (67 to 70, and 112 to 116) showing implantation and decidua formation in the albino rat; to these the interested reader is referred for the present. The thickening of the mucosa affects primarily its antimesometrial portion. During this process of thickening, the mucosal fold in which the ovum primarily finds lodgment, becomes deepened and converted into a funnel shaped crypt communicating with the uterine lumen, and surrounded by the 'Eibuckel,' or oval fold. Burckhard 's schematic figures (text figures 2 to 4) may be consulted to make the phenomenon intelligible.
In figure 23, there are reproduced representative sections of five blastodermic vesicles falling to the end of the sixth day after insemination. None of these five vesicles can be regarded as well fixed. All show a certain amount of distortion, much more evident were the entire series of each of the respective vesicles shown. The form of the blastodermic vesicle of the albino rat at this gtage of development, as indicated by the molding of the uterine mucosa, is elipsoid. Their size as in vivo, when distended and of regular outline, again as indicated by the molding of the uterine mucosa, is slightly larger than would be supposed from the drawings presented. By reason of this distortion, exact measurements of size cannot be given.
Fig. 23 Sections of blastodcrniic vesicles or blastocysts of the albino rat. X 200. A and C, rat No. 99, 6 days; B, D, E, rat No. 100, 6 days, ij.ent., yolk entoderm; p.cnl., parietal layer of entoderm; p.ect., parietal or transitory ectoderm.
In A of figure 23, there is reproduced that portion of one of the sections of a blastocyst (rat No. 99, 6 days) which passes through its floor; the thin roof of this vesicle was so folded that its inclusion in the drawing was deemed undesirable. However, its floor or the germinal disc, seems to have retained its normal form and structure, presenting when traced through the series a regular concavo-convex, discoidal form. It consists in the main of three layers of cells of polyhedral type; toward the border of the disc, of two layers of somewhat flattened cells, the peripheral layer being continuous with the single layer of cells fonuiii«;; the roof of the vesicle, not shown in the figure, and known as the parietal or transitory ectoderm. In the floor or germ disc, there is evident a single layer of cells hordciiiig (lie segmentation cavity or blastocele and possessing a more granular i)r<)t()plasni, which stains a little more intensely in Congo red. Their differentiation and characteristic reaction to staining agents is at this stage of development not quite so distinct as in slightly older stages. This layer of cells, similar to that described by Sobotta for the blastodermic vesicle of the mouse in essentially the same stage of development, he has termed the yolk entoderm, 'Dotter entoderm,' a designation which is here followed. In the more superficial layer or layers of cells no characteristic differentiation is observed. In no portion of the floor of this vesicle was a distinct covering or trophoblast layer recognized.
In the vesicle, a section of which is reproduced in Ji of this figure (rat No. 100, 6 days), the floor or germ disc presents essentially the same structure as that shown in A. The vesicle shown under B, was also folded, especially its roof, which was drawn to one side and was thus not cut through its entire length in the section figured. Furthermore, the section chosen for drawing does not pass quite through the center of the germ disc, but a little nearer to one of its edges, which probably accounts for the fact that there is recognized for the greater part only a single layer of cells, superimposed over the yolk endoderm, which layer is continuous with the parietal or transitor}^ ectoderm forming the roof of the vesicle. The cells forming the yolk entoderm constitute a single layer and are quite distinctly differentiated; one of the cells shows a mitotic phase. The roof of the vesicle formed by the parietal or transitory ectoderm, is composed of a single layer of flattened cells with flattened nuclei, the form and structure of which is more correctly shown in the . right half of the roof wall, which in the section is cut transversely, while the left half, owing to the folding, is shown as cut obliquely.
In C of figure 23 (rat No. 99, 6 days), there is shown a greatly compressed blastodermic vesicle, taken from a series of cross sections of the uterine horn. In this figure there is rejiroduced the fifth of a series of 10 sections of 10 ^i thickness; therefore, the third dmiension of the vesicle is approximately 100 /x. It is evident that had this vesicle been cut in a favorable plane at right angles to the present series, or parallel to the mesometrial plane, its form would have approached that of a circle. I have in my possession one vesicle of this stage of devlopment, similarly compressed, cut parallel to the plane of compression, in which almost the entire roof falls within a single section of 10 ^ thickness. The structure of the vesicle shown in C is very similar to that shown in A and B of this figure. The normal form of this vesicle is quite readily reconstructed from a study of the series of sections into which it has been cut. The cells of the yolk entoderm are evident. The parietal or transitory ectoderm constituting the roof consists of a single layer of much flattened cells, with relatively few nuclei, having, as seen in cross section, a long ovoid form, which, when seen in surface view present a regular, nearly circular outline (see lowermost nucleus in the figure). In similarly compressed vesicles cut parallel to the plane of compression, the germ disc may appear as consisting of three to four layers of cells. In an imaginary section passing in a plane at right angles to that figured in C, and having perhaps a slight obliquity, the germ disc would appear as if much thicker than that shown in A and B of this figure. Such sections may readily lead to false conclusions.
It seems evident from a study of the material at my disposal that during the sixth day after the beginning of insemination in the albino rat, the blastodermic vesicle or blastocyst, which has its anlage in the latter part of the fifth day, enlarges relatively rapidlj^; this largely owing to a distension of the segmentation cavity or blastocele. This enlargement is accompanied by a flattening and extension of the enclosing roof cells and by a rearrangement of the cells of the floor, which is reduced in thickness to a discoidal area, the germinal disc or germ area, forming about one-fifth to one-sixth of the wall of the vesicle and consisting of two or three layers of cells. During the rearrangement of the cells which constitute the floor of the vesicle, those adjacent to the segmentation cavity or blastocele differentiate to form the anlage of the yolk entoderm. The remaining cells of the germinal disc, luivin^ all essentially the same structure, are of irregular polyhedral form and are mutually compressed. To designate them as a distinct <i;ei-ni layci- at this stage seems inappropriate. A differentiation into a layer of covering; cells and a layer of formative ectoderm (Selenka) is not to be made. Active cell proliferation as evidenced by mitotic figures does not appear to accompany this enlargement of the vesicle. This phenomenon seems rather to be accomplished by a rearrangenu^nt of the cells constituting its floor, however, primai-ily by an extension and consequent flattening of the cells forming the roof of the vesicle. A similar stage is shown for the mouse by Sobotta ('03) in his figures 3, 4, and perhaps o, of mouse vesicles from the fifth day after fertilization — 'Befruchtung'. Sobotta had at his disposal much more perfectly fixed vesicles than my material contains. The structure of these vesicles as given by this observer, both as depicted in figures and text, is very similar to the presentation given by me. He also recognizes in this stage the anlage of the yolk entoderm. Figure 30, accompanying the account of Melissinos (mouse, 84 hours) presents a similar stage, although he figures fairly distinctly a layer of covering cells, which if I read him correctly, however, is of only transitory existence. None of the figures given by Robinson and Jenkinson is comparable with figures A, B, C, of figure 23 of this account.
In D, of figure 23 (rat No. 100, 6 days) there is reproduced a section of a blastodermic vesicle which on superficial study presents a somewhat later stage of development than those shown in A to C, of this figure. It is, however, only very slightly older than the three vesicles discussed. Vesicle D, cut in good longitudinal direction, is in reality much more folded than appears from the section figured. Its floor or germ disc is compressed in a plane parallel to that of the plane of section, so that the germinal disc is cut obliquely and not transversely, and thus appears thicker in the section than it in reality is a distinct layer of covering cells, continuous with the cells of the parietal ectoderm, is evident. Such a layer of covering cells is figured by Selenka, Jenkinson, and Duval. The j'olk entoderm has differentiated and extends by perhaps three cells, in the section figured, onto the layer of parietal ectoderm. Selenka and Duval, who regard the cells of the primary entoderm as having ameboid properties, are disposed to regard the entodermal cells found lining the parietal ectoderm as having wandered from their seat of origin to the side wall of the vesicle. Sobotta sees no evidence of such wandering of the primary or yolk entodermal cells, but suggests that they are drawn to their position on the wall of the vesicle during its increase in size ; their wandering, therefore, is more relative than absolute. Certain cells nearer the edge of the yolk entoderm, having attachment to the parietal ectoderm, which attachment they retain as the vesicle enlarges, are thought to be drawn from their close relation to the yolk entoderm and to appear as scattered cells lining the parietal ectoderm. Xow and then, such cells may divide, resulting in further distribution. Sobotta's suggestion seems to me to be more in accord with the observed facts. In vesicle D, the roof, consisting of a single layer of flattened, parietal ectodermal cells, presents several major folds as well as minor folds. The latter particular!}" account for the variation in thickness of the wall of the vesicle as seen in sections. At the lower left of the figure is seen a portion of the wall as seen cut on the flat, the shape of the two nuclei here shown as seen in surface view may be compared with the long ovoid form of similar nuclei when seen in cross section.
Vesicle E of figure 23 frat Xo. 100, 6 days) presents a stage that is slightly older than the other four vesicles shown in this figure. The floor of this vesicle, the germinal disc, as seen in cross section, presents the form of a triangle with its base resting on the cavity, the blastocele. When compared with the slightly younger stages this portion of the vesicle presents an increase in the number of constituent cells, arranged in irregular layers to the number of five in its thickest portion. The thickening is no doubt in part due to the slight lateral compression of the vesicle, but this does not wholly account for it. The cells constituting this thickened germinal disc are for the main of irregular polyhedral form with relatively large nuclei rich in chromatin. A distinct covering layer is not evident. On its under surface there is found a single layer of cells of yolk entoderm. The thin-walled roof of this vesicle, the ])urietal or transitory ectoderm, deserves no special consideration, except to state that its variation in thickness, as seen in the section figured, is due to the plane of section — cross or oblique of different portions of the wall, owing to slight folding. This vesicle I believe to be in stage of development and structure very similar to that shown by Sobotta ('03) in his figure G, mouse vesicle of the first half of the sixth day, and perhaps also figure 31, of the account of Melissinos, mouse vesicle, end of fourth day, also figure 7 of Jenkinson's article who, however, describes and figures a distinct covering or trophoblast layer.
The cell rearrangement and proliferation resulting in the thickening of the floor or the germinal disc as noted in K, of figure 23, marks the beginning of a nuich more distinct thickening of this portion of the vesicle, partly due to cell proliferation, in part also due to the rearrangement and enlargement of the constituent cells, during which thickening process this portion of the vesicle grows outward as well as into the cavity of the vesicle, initiating the phenomenon known as the inversion of the germ layers' or as 'entypy' of the germ layers, to be discussed as to its anlage in the following section.
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Cite this page: Hill, M.A. (2021, April 18) Embryology Book - The Development of the Albino Rat 6. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_The_Development_of_the_Albino_Rat_6
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