Book - The Development of the Albino Rat 13
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Huber GC. The Development of the Albino Rat (Mus norvegicus albinus). (1915) J. Morphology 26(2).
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Abnormal Segmentation Cavity Formation
The following three ova have been grouped as showing irregularity in the formation of the segmentation cavity.
In figure 5 are reproduced four consecutive sections passing through an abnormal ovum obtained from rat No. 46, 6 days, 14 hours, after insemination. There were obtained from the uterus of this rat ten blastodermic vesicles, two of wjiich :ii-e repiv)duced ill A and B of figure 24, Part I, as showiiifi; 1\ picilly cnly stages of the anlage of the ectoplacental cone and cntypy of the germ layers. The ovum shown in figure 5 is found in a decidual crypt which is in very close proxhnity to the one containing the vesicle figured under B of figure 24, Part I, the two crypts being separated by a distance of approximately 1.3 mm., while the distance between decidual crypts is normally 1 cm. to 1.5 cm. T\\o decidual crypt lodging the abnormal ovum presents a normal aj)pearance, resembling very closely in form, dej)tli and slnielure
Fig. 6 Two ova of the albino rat, interpreted as evidencing retarded or irregular formation of the segmentation cavity. X 200. A, rat No. 90, 6 days, 17 hours, after the beginning of insemination. B, rat No. 90, 6 days, 17 hours, after the beginning of insemination, p.ect., parietal or transitory ectoderm : y.ent., yolk entoderm; p.ent., parietal entoderm.
of the surrounding decidua, the crypt and decidua enclosing the adjacent normal vesicle figured in B of figure 24, Part I. The abnormal ovum in question appeared to have proceeded normally in segmentation, its constituent cells being of about the size and structure of the cells of normal ^Tsicles taken the early part of the seventh day after insemination. The cell-mass encloses a relatively small cavity which may be regarded as an abnormally placed segmentation cavity, in that its position is not eccentric, and that it is surrounded on all sides by more than one layer of cells. There is thus no differentiation of floor and roof as in normal blastodermic vesicles, and no development of ectoplacental cone and egg-cylinder as in the other ova obtained from this uterus. I am for the present unable to offer any plausible explanation or give reasons for such abnormal development of the segmentation cavity. The fate of such a structure may perhaps be conjectured from a study of the abnormal ovum shown in A of figure 6, interpreted as showing a similar abnormality, but obtained in early stages of degeneration. This ovum and that shown in B of the same figure w^as obtained from the uterus of rat No. 90, 6 days, 17 hours, after insemination. In the uterus of this rat there are found six ova, only one of which was developed to a stage comparable to that shown in figure 24 (Part I) of about the same age. Three other vesicles present a slightly younger stage and may be compared with vesicles shown in D and E of figure 23, Part I. None of these four vesicles is favorably cut, but so far as may be determined, are of normal structure for the respective stages represented. A of figure 6 is also cut slightly obliquely, not sufficiently so, however, to make difficult its interpretation. The figure drawn is that of the third of a series of seven sections having 10 ^ thickness, and depicts what is regarded as representing an o\'um with abnormal segmentation cavity formation. In this ovum, the segmentation cavity is slightly more eccentric than is that shown in figure 5, and contains a granular detritus which in the preparations is distinctly stained with Congo red. The roof of this vesicle is composed almost throughout of more than one layer of cells. There is no differentiation of ectoplacental cone and ectodermal node, nor of yolk entoderm. Two cells regarded as phagocytic leucocytes, staining much more deeply in Congo red than do the cells of the ovum, have, in the section figured, penetrated the egg-mass, indicating early degenerative changes.
The vesicle shown in B of figure 6, obtained from the same rat, is favorably cut, and is readily followed through the series. The structural appearance presented by this vesicle is not explained by supposing it due to very oblique plane of section of a normal vesicle, a plane of section which might include the roof of the vesicle while avoiding its floor. The vesicle is abnormal in that it presents a want of development of the thickened germ disc, and a hyperdevelopment of the yolk entoderm. In none of the sections of the series which inchides (his vesicle, whicli is cut in very favorable longitudinal direction, and is thus readily oriented with reference to mesometrial and aiitiincsometrial j)()rtion, is there seen any thickeninp; of the outei- hiyer of cells, to form the part known as the floor of the vesicle, which at this stajic of development is uniformly directed toward the niesonict ri.il border. In A and B of figure 23, Part I, are shown vesicles with which the ovum here discussed may be compared. In the preparation under discussion, the yolk and parietal cnlodcrni form almost a continuous layer, one of the detached cells showing a mitotic phase. In the normal \'esicles of this stage of development the parietal entoderm is represented by a few scattered cells, as may be observed by a study of the figures to which reference is above made. Whether this vesicle is to be regarded as showing a later stage of an ovum in which there was irregularity in the formation of the segmentation cavity, I must for the present, leave as problematic. It has occurred to me that by enlargement of the segmentation cavity of an ovum such as shown in figure 5, with centrally placed segmentation cavity, there might result in further development the formation of a vesicle such as shown in B of figure 6.
It is freely admitted that the deductions here made, relative to irregularity in the formation of segmentation cavity, are not supported by conclusive evidence. It has seemed to me, however, that the interpretations given to the appearances presented are less open to criticism than others that might be suggested. These abnormal ova also suggest an inherent defect in the ova, leading to abnormal development, rather than abnormal development resulting from defective environment.
Degeneration of Ova as a Result of Pathologic Uterine Mucosa
In figure 7 are reproduced two ova which seem to me to show the primary stages of degeneration owing to pathologic condition of the uterine mucosa. Vesicle A w-as taken from the uterus of rat No. 91, 5 days, 16 hours, after insemination. In the uterus of this rat there were found only two ova. Vesicle B w^as taken from rat No. 104, 6 days after insemination. In the uterus of this rat there were found six ova. In both of these rats, the ova present essentially the same stage of development, comparable to that shown in A and B of figure 23, Part I. As may be observed from the text of Part I (page 301) the stages obtained at the end of the sixth day and early hours of the seventh day, were found very difficult to fix. At this stage the ovum consists of a relatively large, thin walled vesicle, very prone to fixation shrinkage. All of the ova or vesicles obtained from rats Nos. 91 and 104, are very badly folded in their roof portion. Those shown
Fig. 7 'J\vo ova of (lie alhiiui lat partly .surrouiulcd l)y niatt-inal blood with many jjliagocytie leucocytes. The folding of the roof of the vesicles is due to fixation shrinkage. X 200. A, rat No. 91, 5 days, 16 hours, after the beginning of insemination. U, rat Xo. 104, 6 days after the beginning of insemination.
in A and B, figure 7, are representative. This folding, a result of imperfect fixation, is present in all of the vesicles of this stage, even though the respective vesicles ])resent normal structure. The ova here figured may be regarded as having fairly normal structure, both as to rate of development and as to arrangement, form, and structure of constituent cells. All of the eight vesicles obtained from these two rats (No, 91, 2 ova; No, 104, 6 ova) are in part surrounded by exudated maternal blood, containing numerous leucocytes. Small masses of blood with leucocytes are found here and there in different parts of the uterine lumen of both rats, lodged in mucosal folds other than the characteristic decidual crypts enclosing the respective ova. These decidual c'ryi)ts are relatively shallow when compared with those of normal uteri of similar stages with normal ova. The uterine mucosa of the two rats under discussion does not appear to have reacted in a normal manner. In these preparations, attention is especially drawn to the presence of maternal })loo(l with numerous phagocytic leucocytes found in relation with the ova, a condition never observed in normal devel()i)ment of ova and utei-ine nuicosa. In A and B, figure 7, the red and white blood cells with granular detritus may be observed as found in relation with the respective vesicles, these presenting essentially the same appearances as do the other six ova obtained from these two rats; the one figured having been more favorably cut than any of the others. The appearances presented in these two i-ats ai'c interpreted as showing a probable degeneration of th(> eight o\'a, and probably complete dissolution and removal. The vesicles appear to have developed normally to the stage at which they were obtained. As a result, however, of pathologic condition of the uterine mucosa, maternal blood, especially leucocytes, have entered the lumen of the uterus, the leucocytes being destined to play the role of phagocytes. In normal de\'(^l()pment of the albino rat, maternal blood does not enter the lumen of the uterus — decidual crypts — until after the uterine epithelium has become detached from the mucosa of the wall of the decidual crj^pt, in the region of lodgment of the enclosed ovum. Normally, very few leucocytes are met with in the lumen of the uterus, even in later stages of de\'elopnient, stages in which maternal red blood cells are met with in the decidual crypts. After experience had accumulated, uterine tubes supposed to contain developmental stages aging from the fourth to the sixth day, which on examination revealed blood and especially leucocytes in the lumen of the uterus, were regarded as not favorable specimens for finding ova. In a number of such uteri, cut completely in serial sections, no ova were found. It is possible that, owing to phagocytic action of the leucocytes present, the ova may have been completely removed prior to killing and fixing the tissues. In such condition, it would seem to me as pertinent to speak of faulty implantation, due to abnormal uterine mucosa. It seems to me significant that in the two rats in which the pathologic condition affects primarily the maternal tissue, the uterine mjacosa, all of the contained ova are prone to degeneration. In the abnormal ova previously described, for which it was suggested that the causes for the abnormality were to be sought in the ova themselves, in the great majority of instances, only one abnormal ovum was found in each uterus along with a variable number of ova which are to be regarded as normal for the respective stage.
Imperfect Development of the Ectodermal Vesicle
The series contains two ova, very favorably cut, ova in which the ectodermal vesicle with the antimesometrial portion of the proamniotic cavity does not seem to have developed normally. Stages showing the differentiation of the egg-cylinder, the formation of the ectodermal vesicle with the antimesometrial portion of the proamniotic cavity, the formation of the mesometrial portion of the proamniotic cavity in the extraembryonic ectoderm, the union of the two primary proamniotic cavities to form a single space, are clearly shown in figures 26 and 27, Part I, in the series of closely approximated stages there portrayed. From a study of these figures, it will be observed that the antimesometrial portion of the proamniotic cavity develops within the ectodermal node before the mesometrial portion of this cavity develops in the extraembryonic ectodermal portion of the egg-cylinder. In the egg-cylinder shown in figure 8, rat No. 94, 8 days after the beginning of insemination, such is not the case. In the uterus of this rat there were found seven egg-cylinders, one of which, very favorably cut, is shown in C, figure 27, Part I. The other egg-cylinders obtained from this uterus, except the abnormally developed one to be discussed, though not favorably cut, present essentially the same form and structure as that figured under C of the figure above referred to. The egg-cylinder portrayed in figure 8 compares in size and form with those regarded as normal and taken from the same uterus. For the greater part it presents normal structure and normal relations of cells. The ectoplacental cone, only in part included in the figure, and the parietal
Fig. 8 Egg-cylinder of albino rat showing retarded development of ectodermal node and of the formation of the antimesometrial portion of the proamniotic cavity. X 200. Rat No. 94, S days after the beginning of insemination. ect.pl., ectoplacental cone or Trager; v.ent., visceral entoderm; met.},!-., mesometrial portion of the proamniotic cavity; p.ect., parietal or transitory ectoderm; pr.emb.e7it., primary embryonic entoderm; ecl.n., ectodermal node; a.met.pr., imperfectly developed antimesometrial portion of proamniotic cavity; ex.ect., extraembryonic ectoderm.
ectoderm, in structure and relation to decidual crypt, are to be regarded as . of normal development. The visceral entoderm, surrounding the extraembryonic ectodermal portion of the egg-cylinder, is of normal structure, showing the three zones evidencing its absorptive function. The extraembryonic ectoderm, enclosing the mesometrial portion of the proamniotic cavity, presents normal structure and relations of cells. The only abnoi'mality observed is in the region of the ectodermal node, the anlage of the ectodermal vesicle with the enclosed antimesometrial portion of the proamniotic cavity. With this stage of development of the egg~cyhnder (see figs. 26 and 27, Part I) the ectodermal node presents a well formed cavity, surrounded by the cells of the primary embryonic ectoderm, radially arranged. In the egg-cyhnder under discussion (fig. 8) there is distinctly a retardation in the development of the ectodermal vesicle with full differentiation of the primary embrj^onic ectoderm. An imperfectly developed antimesometrial portion of the proamniotic cavity is evident. This small cavity, indistinctly boimded, extends obliquely through several sections of the ectodermal node, and contains amorplious granular detritus, which in the preparations is stained by Congo red. The cells destined to form the primary embryonic ectoderm show no definite arrangement, especially as concerns the more centrally placed cells of the node. Since the primary embrj^onic ectoderm is the anlage for the ectoderm of the embryo, an arrest in its differentiation would of necessity profoundly affect further development of the embryo. Antimesometrial to the ectodermal node (just above it in the figure) there is found a small vesicle the walls of which are not distinctly delimited and composed of extraembryonic ectodermal cells, surrounding a small, completely bounded cavity. I am not prepared to say whether this small vesicle is to be regarded as developing from cells of the extraembryonic ectoderm, or from a displaced, accessory ectodermal node, in which a discrete portion of the proamniotic cavity has developed. If the latter, the possibility of a double anlage for the embryonic ectoderm is to be considered. jNIy interpretation of this eggcyhnder as showing a retardation of the development of the ectodermal node and differentiation of the primary embryonic ectoderm, is confirmed from a study of a slightly older stage showing essentially the same condition. This ovum is presented in figure 9, and is taken from rat No. 41, 8 days, 16 hours, after the
Fig. 9 Egg-cylinder of albino rat, in which the antiniesomctrial and mesometrial portions of the proamniotic cavity have failed to unite to form a single or definite proamniotic cavity. X 200. Rat No. 41, 8 days, IG hovirs, after the beginning of insemination. ccLpL, ectoplaccntal cone or Triiger; p.crt.. parietal or transitory ectoderm; r.ent., visceral entoderm; met.pr., mesometrial portion of the proamniotic cavity; ex.eci., extraembryonic ectoderm; ajnet.pr., antimesometrial portion of the proamniotic cavity; pr.emh.cct., primary embryonic ectoderm; +, region at which, in normal development, by the end of the eighth and beginning of the ninth day, the two portions of the proamniotic cavity would have united to form a single space, the definite proamniotic cavity.
beginning of insemination. The uterus of this rat contains eight egg-cyhnders, all of which, except the one here figured, show normal structure, though presenting quite different stages of development. One of these, cut serially in cross-section, is figured in C, figure 32, Part I, as showing anlage of mesoderm with primitive streak and groove. Two of the other egg-cylinders show the anlage of the mesoderm, two others show late premesoderm stages of the egg-cylinder, the remaining egg-cylinders are less fully developed, one showing a development which may be compared to B of figure 26, Part I, thus a much younger stage. By the end of the eighth day and with the early hours of the ninth day after the beginning of insemination in the albino rat, the two parts of the proamniotic cavity, which develop discretely, have joined to form a single space (C, fig. 27, Part I). The egg-cylinder shown in figure 9, presents normal development in all parts, except that there is as yet no union of the two parts of the proamniotic cavity. This egg-cylinder is most favorably cut, in longitudinal direction; the plane of section being almost parallel to the mid-sagittal plane. This eggcylinder, therefore, is easily followed through the several sections of the series into which it was cut. The irregularity of outline of the ectodermal vesicle, lower right of figure, it is believed, is not due to fixation shrinkage. Judging from size and structural differentiation of this egg-cjdinder, union of the antimesometrial and mesometrial portions of the proamniotic cavity should have been completed before this stage of development was reached, with the primary embrj^onic ectoderm and the extraembryonic ectoderm forming a continuous layer, as shown in figure 29, Part I. The folding of the wall of the antimesometrial portion of the egg-cylinder, lower right of figure, evident in nearly all of the sections of the series, is regarded as indicating an abnormal growth of the primary embryonic ectodermal cells composing the wall of the ectodermal vesicle, as a result of retarded extension of the antimesometrial portion of the proamniotic cavity, perhaps an adjustment to meet the altered mechanical stress resulting from abnormal development. The condition here seen, it would seem, is foreshadowed in the egg-cylinder shown in figure S.
Fig. 10 Two egg cylinders of the albino rat found within the same decidual crypt, with in part common ectoplacental cone. X 150. Rat No. 87, 9 days after the beginning of insemination, eci.pl., ectoplacental cone or Triiger; p.cct., parietal or transitory ectoderm; r.ent., visceral entoderm; ex.ect., extraembryonic ectoderm; /».(., proamniotic cavity; pr.emh.ect.. primary embryonic ectoderm; pr.emh.etit., primary embryonic entoderm; nies., mesoderm.
The causes operative in this retardation of development and differentiation of the ectodermal A'esicle and primary embryonic ectoderm, I have been unable to determine. They would appear to be inherent in the egg-cylinder, since ectoplacental cone and visceral entoderm, so far as may be determined from a study of sections, appear to have functioned normally, in furnishing the necessary embryotroph in the form of maternal hemoglobin, as is normal for egg-cylinders of the albino rat of this stage of development.
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