Book - The Development of the Albino Rat 4
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Summary of Segmentation Stages, Rate, and Volume Changes
The following summary of the data (table 3) gained by a study of the models of oviducts containing ova in stages from the pronuclear to 12-cell to 16-cell stages in which latter stage transit to the uterine horn occurs, is presented to indicate rate of transit within the oviduct. The regularity of the rate of transit as ixn'oalcd in tlu^ suiniiKiry may jxirhaps speak for the trustworthiness of the age data as concerns my material.
It will be observed that the ova approach the uterine end of the oviduct while in the 2-cell stage; transit through the last portion of the oviduct, where the greater part of the segmentation occurs, being relatively slow. It is hoped that these data, for the accuracy of which I am dependent on reconstructions, may be of service to others who may desire to collect segmentation stages of the albino rat.
- Not the entire length of oviduct was available for reconstruction.
In order to obtain the volume changes of the ova during transit through the oviduct, beginning with the pronuclear to 8-cell to 11-cell stages, the following procedure was adopted. As has been shown by my figures, reconstructions were made at a magnification of 1000 diameters of ova presenting the stage in question. The sections of my series measure 10 m in thickness.
In order, therefore, to obtain the correct third dimension, it was necessary to use wax plates 10 mm. thick, in actual practice, five superimposed 2 mm. plates. For the majority of the sections of my series this procedure w^as relatively simple. However, there was usually a question as to the thickness to be ascribed to the first and last section of any given series, since it was evident, both from the appearance of the section, as seen under the magnification used, and the appearance of the model, that the end sections did not measure 10 ^ in thickness, and it was necessary to reduce proportionately the thickness of the wax plate representing them. As a rule, these were made about one-half the thickness of the other plates. The irregularities revealed by the rough model after superimposing the respective plates, not so marked as might be supposed considering the thickness of the plates used, were adjusted, not by trimming the model and cutting away wax, but by smoothing with warm irons. The possibility of error is admitted, but since all of the models were made in the same way, errors if committed were probably essentiall}^ the same for all of the models. The volumes of the models were obtained by weighing the water displaced by each, and after making the necessary temperature corrections, reducing weight of water displaced to volume. The average of several determinations is given in table 4.
The uniformity of the figures giving the actual volume of the egg mass, as determined by the weight of the water displaced by the models of the respective ova reconstructed, leads me to feel that the errors committed in reconstruction were not serious. The last column of the table, giving averages, is of interest since it shows a very slight increase in the volume of the egg mass during segmentation and transit through the oviduct. Following the pronuclear stage, which, as has been seen, extends through a relatively long period and into the beginning of the second day, by which time the ova have niigi-ated about onefourth of the length of the oviduct, there occur only three successive mitotic divisions, including the first segmentation division, namely mitoses resulting in 2-cell, 4-cell and 8-cell stages while the ova are in transit in the oviduct. In making this statement it is assumed that in the successive segmentations, the several cells divide synchronously, which is not in conformity with the fact. These three mitotic divisions are spaced at intervals of about 18 hours. In the next following division, the fourth, the ovum passes from the oviduct to the uterine horn. Since the normal gestation period of the non-lactating albino rat is only 21 to 23 days, this slow rate of increase in volume and multiplication of cells during the first four days of develoi:)ment is of especial interest and is very probably to be accounted for by the inadequacy of the food supply of the ovum during its transit through the oviduct.
The presence or absence of the oolemma has not been considered in discussing the segmentation stages of the albino rat. In my own material, the oolemma was clearly observed in certain of the 2-cell stages, but not in the 4-cell nor 8-cell stages. A\'idakowich reports that he has observed in the albino rat, loss of the oolemma even in the 2-cell stage. Since all of the material covering these stages was fixed in Carnoy's fluid, a fluid with a relativel}' large glacial acetic acid content, it may be questioned as to whether the fixative used may not be in part responsible for the early disappearance of the oolemma, though neither Hubrecht nor Sobotta considers the presence or absence of an acid in the conserving fluid of special moment in the fixation of the oolemma. Sobotta finds that the oolemma disappears in the ova of mice during the 8-cell stage. The early disappearance of the oolemma in the albino rat may be offered as an explanation of the fact that the egg mass during segmentation and transit through the oviduct does not, as a rule, present a spherical form but appears compressed and molded to fit the form of the lumen. A similar explanation is offered by Sobotta to account for the irregularity of form assumed by the ovum of the mouse after loss of the oolemma. In the forms in which the oolemma persists through the later stages of segmentation, as for instance in the rabbit, the morula mass presents a spherical form. The transit of the ova through the oviducts is effected, very probably, through peristaltic action of the muscular coat, since only a relatively short portion is lined by ciliated epithelium. Whether or not there exists a rhythmic periodicity in the peristaltic action, it is impossible to state. The fairly regular rate of transit argues for the presence of some regulatory mechanism. The compact grouping often presented by a series of ova in transit through the oviduct, especially after reaching the portion with narrower lumen, suggests peristaltic action.
The literature dealing with the segmentation stages of the albino rat is very meagre. Grosser figures what is presumably an S-cell stage. His figure 27 is referred to only incidentally in the text, but in the accompanying legend it is stated that the figure shows "three ova of the white rat in process of segmentation, with zona pellucida, in transit through oviduct, three and one-half days after insemination." If I am right in interpreting these ova as in the 8-cell stage, this corresponds very closely to my own observation on rat No. 57, 3 days, 17 hours (figs. 15-17). It is impossible to draw definite conclusions as to the segmentation of the ova of rats from the account of ]\Ielissinos. This observer while he states that his material includes the ova of mice and rats, and while considering segmentation mentions the ova of both forms, discusses them without differentiating between the two. His figures all refer to ova of the mouse. Selenka, Robinson, and Widakowich, who have contributed to our knowledge of the embryology of the albino rat, do not include the segmentation stages, to be found in the oviduct, in their account.
The rate of segmentation and the time of transit through the oviduct, as given in the literature for certain other mammals is as follows: Sobotta has shown for the mouse that the 2-cell stage is reached about 24 hours after copulation, the ovum remaining in this stage to about the 48th hour. The 4-cell stage was observed at about 50 hours, the 8-cell stage at 60 hours, and the 16-cell stage at 72 hours 'post coitum.' The ova of the mouse pass into the uterine horn about 80 hours post coitimi, thus the besiniHiig- of the fourtli day, in a stage in wiiich 10 cells up to 32 cells may be enumerated; the oolemma having been lost in the 8-cell stage. The data furnished by Melissinos as concerns the mouse, are as follows: The 2-cell stage is obtained at the end of 24 hours after copulation, the ()-cell stage during the first 12 hours of the second day, and the 28-cell stage during the second 12 hours of the second day.
The ovum is said to pass into the uterine horn at the end of the third day after copulation, retaining its oolemma. The account of Sobotta seems the more reliable. Hensen describes a 2-cell stage in the guineapig 22 to 24 hours after copulation, and Bischoff records that the ovum of the guinea-pig passes into the uterine horn while in the 8-cell to 16-cell stage, toward the end of the third day. Heape, who has described very fully the segmentation stages of the mole (Talpa europea) gives no data as to the rate of segmentation. In the explanation of the figures presented it may be noted that the ova figured, showing 2-cell to 15-cell stages, were taken from the oviduct. His figure 20, showing an ovum 'fully segmented' was obtained from the anterior end of the uterus. Assheton gives for the rabbit the following data: The 2-cell stage is obtained about 24 hours and the 4-cell stage about 26 hours after coitus. The third series of divisions begins about 28 hours after coitus, so that by the end of the second day a typical morula of 16 cells to 20 cells is to be found. Between 78 hours and 96 hours the beginning of the blastodermic vesicle formation is to be noted. Ova obtained 80 hours after coitus, still surrounded l^y the oolemma, were removed from the uterine horn. Data as to the relative position of the ova in the oviduct in the several stages of development discussed, are given. As concerns the sheep, Assheton states that the ova pass into the uterine horn early on the third day after mating. The pronuclear stage is to be observed the second day, and the first segmentation at the end of the second day.
By the fourth day, with the ova in the S-cell stage, they are found in the upper end of the uterine horn. The blastodermic vesicle formation begins with the 16-cell stage. Again, according to Assheton, the ova of the pig pass to the uterus about the third day after fertilization, if I read him rightly, reaching the uterus in the 4-cell stage, although ova in the 2-cell and 3-cell stages were obtained from the upper end of the uterine horn. The presence of 2-cell stages in the uterine horn has also been noted by Keibel, in Erinaceus europaeus, by Van Beneden in the bat, and by Hubrecht in the insectivor Tupaya javanica. Finally, it may be noted that according to the observations of Bischoff, the segmenting ovum of the dog occupies 8 to 10 days after insemination in transit through the oviduct.
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Cite this page: Hill, M.A. (2020, June 5) Embryology Book - The Development of the Albino Rat 4. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_The_Development_of_the_Albino_Rat_4
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