Book - The Development of the Albino Rat 3
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2-cell stage. The material on which my own observations of this stage are based is listed in table 2.
Thus in all 40 ova after the completion of the first segmentation division and 2 ova in the 3-cell stage, in each of which the undivided blastomere presents a nucleus in mitosis.
My own material lacks stages showing the formation of the first segmentation spindle, the conjugation of the two pronuclei, and the first segmentation division. I am forced to proceed from the pronuclear stage to that showing the first two blastomeres. It was not possible to supplement my material after this was sectioned and the stages determined, since it was only after leaving The Wistar Institute that this gap in my series was recognized. This is the more to be regretted since neither ^Nlelissinos, Sobotta and Burckhard, nor Kirkham and Burr, all of whom have considered maturation and fertilization a»s observed in the albino rat, discuss these stages in their account. In the albino rat, the fusion of the two pronuclei on the first segmentation spindle, and the first segmentation division would appear to fall to a period ranging from the beginning to near the middle of the second day after the beginning of insemination, probably about 30 to 32 hours after insemination. In the mouse, in which these stages have been very completely and carefully investigated by Sobotta, the conjugation of the pronuclei and the first segmentation spindle formation falls to the end of the first day after copulation. These phenomena appear to be passed through rather quickly in the mouse ovum, covering a period of only about one and a half to two hours.
The 2-cell stage with resting nuclei extends through a relatively long period. In the mouse it extends through nearly an entire day, as shown by Sobotta, who found 2-cell stages present through a period ranging from 25 hours to 48 hours after copulation. Melissinos often observed the 2-cell stage with resting nuclei in both mice and rats in material gathered 24 hours after copulation and to 44 hours thereafter. It is to be regretted that this observer does not differentiate more specifically between ova of mice and rats in his description. As a rule it is impossible to determine except by inference to which of the two varieties of ova his account refers. It may be assumed that the statements made apply equally well to the ova of either the mouse or the rat.
In my own material, the 2-cell stage was observed during a period extending from 1 day, 18 hours to 2 days, 22 hours after the beginning of insemination, thus for a period extending over more than 24 hours. In the albino rat, the first two blastomeres are equivalent cells of essentially the same size and structure, as may be seen from B and C, of figure 1, drawn respectively of ova found in the right and left oviducts of rat No. 59, killed two days after the beginning of insemination, and regarded as representative ova. The two cells of each ovum are not spherical, but of slightly oval form, with relatively large, distinctly membranated nuclei, with fine chromatin granules scattered on the lining network and a number of relatively large chromatoid nucleoli. The cytoplasm presents a granular appearance, the granules being evenly distributed throughout the cell. In my own material, I seldom find the two cells lying in the same plane, but one cell, as a rule, rises slightly higher than the other. This is more clearly seen in reconstructions than in sections. In figure 6 are shown reconstructions of the 2-cell stages, figured in B and C of figure 1. In B, of the figures, the plane of section is at right angles to the vertical axis of the reconstruction as shown in B of figure 6, while in C of figure 1, the plane of section is parallel to the vertical axis of the reconstruction shown in A of figure 6. The equivalence or non-equivalence of the first two blastomeres of the segmenting mammalian ovum has been the subject of discussion since the time of Van Beneden's fundamental observations on the segmentation of the ovum of the rabbit. This discussion has been summarized a number of times in recent years, and need not be entered into here. Suffice to say that the consensus of opinion of the more recent contributors is, that the first two blastomeres of the mammalian ovum are equivalent in size and structure if the stage is observed soon after its formation. As above stated, the 2-cell stage of the mammalian ovum extends through a relatively long period, probably about 24 hours.
The two cells do not as a rule divide synchronously, the division of one preceding the other by some little time, resulting in a 3-cell stage. The cell to divide first increases slightly in size and presents a clearer protoplasm prior to its division. In a 2-cell stage, viewed in this phase of cytomorphosis, one of the cells appears slightly larger with clearer protoplasm than does the other cell, explaining the difference in size and structure observed by Van Beneden and by other observers who concur in his views. I am convinced that a difference in the size of the two cells may be accounted for by the plane of section in which they are cut, even though the nuclei of both cells are included in the section. In the figures of sections of the 2-cell stage of the mouse, given by Sobotta and Melissinos, the nuclei of the two cells lie in about their center and essentially in the same plane. In my own material of the 2-cell stage of the albino rat it is not unusual to find the nuclei of the respective cells nearer the opposite poles of the two cells than at their centers, as shown in C, of figure 1. In B of this figure, where the two nuclei appear as lying much nearer the center of the cells, they are in reality placed much as in C, as is shown by the reconstruction.
To determine the position of the segmented ovum in the 2-cell stage in the oviduct, reconstructions were made of two oviducts. In figure 7 is shown a reconstruction of the right oviduct of rat No. 59, killed two days after the beginning of insemination. In preparing the material for embedding, this oviduct was cut not quite at its insertion into the uterine horn. The portion of the oviduct reconstructed measures 2.29 cm. Nine major loops are shown. The four ova in the 2-cell stage found in this tube are situated in the sixth to the seventh loop at a distance of about 1.4 cm. from the fimbriated end. This portion of the oviduct falls to segment three of Sobotta' s designation. It is lined by non-ciliated epithelium resting on a mucosa with inconspicuous secondary folds, but presenting four or five characteristic major folds. This portion of the oviduct is closely applied to the outside of the ovarian capsule, and consjiicuous in all of the figures of models of the oviducts here presented. The detail of the distribution of the ova in the tube is given in figure 8, a reconstruction under a higher magnification of the segment of the oviduct containing the ova. The lumen is exposed so that the character of the mucosal folds may be seen. The ova are spaced in a segment of the tube measuring 3 mm., and are in this case more widely separated than is usual for this stage.
In figure 9, there is reproduced a reconstruction of the left oviduct of rat No. 62, killed 2 days, 22 hours after the beginning of insemination. This tube was also cut a little before its insertion into the uterine horn. The portion reconstructed measures 2.45 cm. In it there are found five ova in the 2-cell stage, situated about 2 cm. from the fimbriated end, and in the last loop of the third segment of the oviduct. The five ova are closely grouped between two opposing folds of the mucosa. Their general relations are shown in figure 10, a reconstruction under higher magnification of the segment of the oviduct containing the ova, cut so as to expose the lumen. At the magnification used it was not possible to reproduce in the model the exact shape of the several ova, their relative position is, however, correctly given. In all, ten oviducts, containing 40 ova in the 2-cell stage, are included in my series. Of these, two, as above given, were reconstructed by the Born method. The other eight were reconstructed graphically, beginning with the uterine end of the tulles. In six of these, the ova are (juile closely grouped as given in the reconstructions shown in figures 9 and 10. In the remaining two they were more widely spaced, about as shown in figures 7 and 8. In the oviducts taken from rats Nos. 58, 61, 62, killed respectively 2 days, 17 hours, 2 days, 18 hours, and 2 days, 22 hours, after insemination, the ova are found in a portion of the tube which corresponds very closely to that shown in the reconstruction presented in figure 9. In rat No. 60, killed 1 day, 18 hours after insemination, the ova are more widely spaced and are situated in a segment of the oviduct approximately one loop nearer the fimbriated end than that given in figure 7, a model of the oviduct of rat No. 59, killed two days after insemination.
In one of the segmented ova of rat No. 60, the two blastomeres resulting from the first segmentation division are distinctly separated by a space equal to about one-half of the diameter of each of the cells. No oolemma is discernible. The two separated cells appear normal in size, shape, and structure, as do also their nuclei. They lie free in a slightly distended portion of the lumen, and appear not to have been separated as a consequence of manipulation. The possibility^ of each developing separately is suggested, and may be offered as a possible explanation of the occurrence of very small embryos now and then found among others showing normal development. King states that On dissecting pregnant females (rats) one frequently finds one or more embryos that are much smaller than the rest. While in some instances such small embryos appear normal and are presumably either runts or embryos that have resulted from superfecundation, in the majority of cases they are pathological, probably because of faulty implantation of the ovum." JMy own material contains pathologic ova and embryos in different stages of development. This portion of the material will be considered in Part II, where the possibility of the occurrence of half embryos will be discussed.
As may have been seen, the 2-cell stage of the albino rat covers a period of somewhat more than 24 hours, extending from about the middle of the second day until toward the end of the third day after the beginning of insemination. During this period the segmented ova migrate in the oviduct for a distance equahng nearly half its length. The trustworthiness of the material, it would seem to me, is shown by the fact that in the shorter time stages the segmented ova are situated nearer the fimbriated end, while in the longer time stages they approach the region of the insertion of the oviduct into the uterine horn. This is clearly shown in the reconstructions shown in figures 7 and 8. A 3-cell stage was observed only twice: in one of eight ova contained in the oviducts of rat No. 58 (2 days, 17 hours) and in one of eleven ova found in the oviducts of rat No. 62 (2 days,
22 hours). All the other ova found in these two animals were in the 2 -cell stage. In the two 3-cell stages noted, the undivided blastomeres of each ovum presented a nucleus in mitosis; in one, in the monaster phase, in one, in the diaster phase. The division of the first two blastomeres, resulting in the 4-cell stage, it would appear, occurs in the albino rat toward the end of the third day. The material gathered at the beginning of the fourth day after insemination presents throughout a 4-cell stage. In D of figure 1 is shown reproduced one of the sections of a series of six sections including one of the ova in the 3-cell stage. Only one of the two cells resulting from the division of one of the first two blastomeres is included in the section; the cell in mitosis represents the undivided blastomere.
The material includes the oviducts of two rats, Nos. 50 and 03, killed 3 days and 1 hour after the beginning of insemination, with twelve ova in the 4-cell stage. In figure 11, there are shown two views of each of the models obtained by reconstruction after the Born method, at a magnification of 1000, of the three 4-cell stages found in the oviducts of rat No. 50. The drawing of the reconstructions do not present the conventional figures of the 4-cell stage of the mammalian egg. In none of the twelve ova of this stage was the plane of section such as to include all of the four cells in one section. Nearly all lie in a portion of the tube which presents a relatively narrow lumen, and appear as if slightly compressed between the folds of the mucosa. I am not disposed to regard this as a resultant of fixation, due to contraction at the time of fixation. In figure 12 is reproduced a cross section of the right oviduct of rat No. 50, passing through a 4-cell stage. It is evident that in shape the two cells included in the section, conform in the main to the form of the lumen, the mucosa appearing as slightly retracted to one side of the egg mass.
This conformity in shape of cell mass to the form of the lumen I find quite general in my material showing segmentation stages of the albino rat, to some extent even in the 2-cell stage, more clearly shown in the 4-cell and later segmentation stages, as will appear from further reconstructions presented. It would seem to me reasonable to assume that these cell masses are of such plasticity that they are molded by the tubal mucosa rather than they would compress the mucosa and maintain an inherent form. A number of segmented ova in presumably the 6- and 8-cell stages were removed from oviducts by injection and studied in warm normal salt solution, in a living state. In the warm normal salt solution the morula masses presented a nearly spherical form, conforming to the conventional illustrations of the same. In none of the sections of fixed material of my series was this the case. The form of the cell mass, assumed by the segmenting mammalian ovum in early stages of segmentation, therefore, seems to me a question more for academic discussion than one of fundamental importance. The right oviduct of rat No. 50 (3 days, 1 hour) was reconstructed after the Born method.
This model is reproduced in figure 13, and includes the uppermost end of the uterine horn. The oviduct measures 2.8 cm. and contains four ova in the 4-cell stage, situated at the beginning of the last loop leading to the uterine horn, 2.25 cm. from the fimbriated end, thus in the fourth segment of the oviduct as of Sobotta's designation. In figure 14 is reproduced a detailed reconstruction of the segment of the oviduct containing the ova, with the convex portion of the wall of this loop, as shown in figure 13, removed. The section reproduced in figure 12, passes through the lower of the three upper ova, shown in reconstruction in figure 14. In the figure of the reconstruction as also in that of the section, is shown the groove in which these three ova lie. The other oviducts containing 4-ceri stages were reconstructed graphically, beginning with the uterine end. The position of the ova in each is essentially as given in the model reproduced in figure 13.
In rat No. 57, killed 3 days, 17 hours after the beginning of insemination, there are found in the left oviduct, six segmented ova in the 8-cell stage and one segmented ovum in the 11-cell stage. The right ovary and oviduct was injured in the process of embedding and could not be used for sectioning. The ova are spaced, in the loop of the oviduct which terminates in the uterine horn. Six of the segmented ova were reconstructed, the seventh was not detected at the time the reconstructions were made. The six models obtained are reproduced in figure 15, two views of each model being shown. Five of the models, A to E, show 8-cell stages. In F, there is figured an 11-cell stage, three of the cells having completed the next following division. As may be seen from the figures, the form of these morula masses is not spherical but in the main slightly oval, with further irregularities better shown in the models than in the illustrations, due to the fact that the egg masses conform to the shape of the lumen of the oviduct in the region in which they are found. The mucosa lining the segment of this oviduct containing the ova presents four quite regular longitudinal folds. In figure 16, there is presented a model of a detailed reconstruction of the segment of the oviduct containing the ova, their relative position in the tube and their relation to the major folds is clearly shown. One of these folds it was necessary to in part remove so as to bring to view in the drawing certain of the ova. In figure 17, there is reproduced a portion of one of the sections of the series from which the model shown in figure 16 was made. The fold of the mucosa occupying the center of the drawing, and greatly occluding the lumen, is the fold removed, in the model. In this very fortunate section four of the morula masses are included; all are of the 8-cell stage and represent in section the four ova which are placed closely together as seen in the model figured in figure 16. In figure 19, A, there is reproduced at higher magnification another of the sections of the series, including the right one of the three ova in close apposition as seen in figure 17, showing six of the eight cells, each cut in the plane of its nucleus. In both of these figures (figs. 17 and 19) the morula masses, as seen in the sections drawn, present a quite regular oval outUne. In succeeding sections, in which the mucosal fold and the wall of the oviduct approximate, the cross diameter of each of the four morula masses becomes greatly reduced, they appearing in the final sections of the series in which they are included as narrow, non-nucleated bands of protoplasm.
This series, it seems to me, corroborates the stidcmeut previously made, that the detail of form of the Hving segmenting ova of certain mammals, while in transit through the oviduct, is in a great measure dependent on the configuration pi-escMited by the lumen of the oviduct in the particular i-egion in which the}' are found.
12 to 16-ceIl stages
Rat No. 51, killed 4 days after the beginning of insemination, presents the end of the segmentation stages in the oviduct. In the genital tract of this animal there were found eight morula masses, five on the left side and three on the right side. It is somewhat difficult to determine definitely the number of cells constituting each of the morula. The number appears to vary between 12 and 18, though nearly all of the morula masses show certain nuclei in mitosis. The left oviduct with a short adjoining segment of the uterine horn was reconstructed. Slight tension was applied to the tissue prior to fixation, which accounts for the elongation of the proximal loop of the oviduct. The model is reproduced in figure 18. As is evident on study of this figure, three of the morula masses are situated in a portion of the oviduct just prior to its insertion in the uterine tube. These are closely grouped between folds of the mucosa. A fourth morula is found in the uppermost part of the uterine cavity, just distal to the opening of the oviduct, lying free in a slightly distended portion of the lumen. This morula is of irregular discoidal form, presenting an appearance which suggests that it was fixed soon after it escaped from the oviduct. A fifth morula, of regular oval form, comprising very probably 18 cells, all of which present resting nuclei, is lodged in a shallow pit of the uterine mucosa a little over 1 cm. from the tubal opening. This portion of the uterine horn was not included in the reconstruction, the position of this morula is not, therefore, indicated in the figure. It is evident that this tube was fixed while the several morula masses were in transit from the oviduct to the uterine horn, which occurs, to judge from the material at my disposal, at the end of the fourth day after the beginning of insemination. The morula masses of the right tube are situatedin the oviduct just before its point of insertion into the uterine horn, in about the same relative position as are the three upper morula masses of the left side, as shown in the reconstruction. They are of discoidal form, in close relation and appear to comprise, the one 12, the other two 14 to 10 cells. In B, C and D of figure 19 are reproduced sections of each of these three morula stages. The figures, however, are delusive in that the section for each passes through the greatest diameters of the respective morula.
Fig. 19 Sections of morula stages of the albino rat. X 2UU. A, 8-cell stage, rat No. 57, 3 days, 17 hours; six of the eight cells, each cut in the plane of its nucleus, are included in the section figured. B, C, and D, 12-cell to 16-cell stages, from right oviduct, rat No. 51, 4 days.
The material at hand permits the conclusion that in the albino rat the segmenting ova pass from the oviduct to the uterine horn at the end of the fourth day after the beginning of insemination, probably in the 12-cell to 16-cell stages. With the beginning of the fifth day, as will appear from further discussion, all of the ova are to be found in the uterine horn.
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Cite this page: Hill, M.A. (2021, April 12) Embryology Book - The Development of the Albino Rat 3. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_The_Development_of_the_Albino_Rat_3
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