Book - The Development of the Albino Rat 11
Embryology - 18 Apr 2024    Expand to Translate
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Huber GC. The Development of the Albino Rat (Mus norvegicus albinus). (1915) J Morphol. 26(2).
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Introduction
In the course of my study of the normal development of the albmo rat, from the end of the first to the end of the ninth day after insemination, as recorded in Part I of this series of contributions, there were encountered from time to time ova which appeared to deviate both in rate and type of development from what, as a result of extended study, came to be regarded as the normal developmental cycle of the albino rat. When taken collectively, the number of these abnormal ova is not large, although they embrace nearly all of the developmental stages studied. When taken singly, it may be stated that while it is comparatively easy to record the points of deA'iation from the normal, it must be admitted that the probable fate of the respective stages can only be conjectured. Nevertheless, a record of the abnormal stages met with seems warranted, especially in view of the fact that the literature is very meager in its account of early stages of mammalian ova presenting abnormal development.
The excellent and comprehensive studies of Mall on pathologic
human ova, extending over many years, may be interpreted as
leading to the general conclusion that pathologic ova and monsters "are produced from normal eggs by conditions which either
interfere with their nutrition or poison them." There is evidence to show that defective implantation, using the term in its
broadest sense so as to include relation to the embryotroph or
pabulum, is directly associated with abnormal development.
Comparative experimental teratology so successfully followed by
a number of European and American experimental embryologists warrants the conclusion that all of the abnormalities or
malformations observed in the human embryo may be brought
forth by the application of suitable mechanical interference or
chemical solutions. Experimental teratolog}' possesses the very
great advantage of enabling the observer to follow the pathologic
process from step to step, admitting more readily of their interpretation, than when single stages are obtained from nature.
The evidence appears to be accumulating that the primary causes
which produce pathologic ova lie not in the germ cells, but are
rather to be sought in the environs of the germ cells in the course
of their development.
I am cognizant of the fact that the interpretation of the chance
findings of abnormal stages of mammalian ova is much more
difficult than of abnormal ova produced experimentally. The
fact, however, that nearly all of the abnormal ova observed by
me in my albino rat material were found in tubes and uteri containing normal ova also, tubes and uteri which so far as observable appear in most instances to be normal, and the further fact
that certain of the abnormal ova are of stages prior to what may
be regarded as showing implantation, stages concerning which
we possess no data as far as human ova are concerned, has lead
to the tentative conclusion that certain of the abnormal ova
may be the resultant of abnormal germ cells, perhaps of an
abnormality which may not show a structural expression.
It is my iirimaiy i)urpose to make records of the abnormal ova observed in the material at hand ; and to follow these records with a brief consideration of the observations made. There is no
literature dealing- with the problem, immediately at hand — abnormal rat ova. It is not my purpose at this time to enter into the extensive literature of comparative experimental teratology. This has been critically summarized relatively recently by O. and R. Hertwig, and by Mall, in his several contributions dealing with human pathologic ova.
Half Embryos in Mammalia
The first preparation to which attention is called is one taken from the oviduct of rat No. 60, 1 day, 18 hours, after insemination. The two oviducts of this rat contained seven ova in the 2cell stage, to one of which especial attention w^as drawn in Part I (page 271). As there recorded, in one of the 2-cell stages, the first two blastomeres were separated by an appreciable distance. There is loss of oolemma. The possibility of half emjjryos in Mammalia was suggested. The preparation under consideration is figured in figure 1, A and B. In A of this figure there is presented a portion of the wall of the oviduct, its epithelial lining and the immediately adjacent mucosa, including the fourth of a series of six sections (10 ju) passing through the two blastomeres. In this region, the cilia of the epithelium are clearly observable, as may be seen from the figure. In B of this figure there are sketched in approximately relative position the several sections of the series passing through the tw^o blastomeres, the relative position of which, with reference to the walls of the tube, is shown in A of the figure. The six drawings were made from a well ribboned series; the slide was moved from section to section by means of a mechanical stage, and the perpendicular indicated on each drawing as made. The relative position of the several drawings, therefore, is quite correct. It may be observed that throughout the series the two blastomeres are separated by an appreciable space, and that one of the cells has rotated slightly on its axis. If these two blastomeres had remained in close apposition, they w^ould present the appearance of a normal 2-cell stage as shown in B and C of figure 1, Part I. There is here clearly a separation of the first two blastomeres and not a close approximation of two unfertilized ova. In all of the unfertilized ova met with in the oviducts in the series at my disposal, these present the second maturation spindle and oolemma and are not to be confused with the blastomeres of the 2-cell stage, either as to size or structure. Both of the blastomeres in the preparation under consideration present normal protoplasmic structure, having a finely granular i)rotoplasm. Their nuclei, as may be seen from the figures, are of normal size and structure. They present regular form, are distinctly membranated, have large chromatoid nucleoli, and chromatin scattered in fine granules and threads. However, attention needs to be drawn to the presence of two micro-nuclei, one in each of the two blastomeres, showing in the third and fourth section of the series respectively (B, fig. 1). These micro-nuclei are nearly free from chromatin, each presenting a small chromatoid nucleolus. They are not to be regarded as cell inclusions, as perhaps representing phagocytic leucocytes. It may be conjectured that they were formed by amitotic division, by budding and constriction from the parent nuclei, perhaps indicating altered metaholism in llic I wo hlnstomeres. I am inclined to think that hoth of these cells would have degenerated in the course of i'uitiier development ; however, their fate can only he guessed and not predicted. The possibility of their developing into half (Mnbryos is suggested. Half embryos developing as a result of a separation of the first two blastomeres has not been observed in the Mammalia, and an experimental test of the question is for the present not a probability.
Fig 1 Oviduct and ovum of albino rat, in 2-ccll stage, with first two blastomeres separated. Rat No. 60, 1 day, 18 hours, after the beginning of insemination. X 200. A, epithelial wall of oviduct with adjacent mucosa, and the fourth of a series of six sections of the 2-ccll stage with separated blastomeres, showing them in their relation to the epithelium. B, the series of six sections which pass through the separated blastomeres, the fourth of which is shown in A. The series reads fi'om right to left.
As a result of experimental embryology it has been cleai-ly shown that through mechanical interference polysomatous monsters may be produced from normal ova. The first two blastomeres are totipotent as expressed by Driesch. Driesch was able to produce polysomatous forms by mechanical separation of the first two blastomeres in sea urchin eggs; Wilson, by separating through shaking of 2- and 4-cell stages in Ami)hi()xus; O. Hertwig, Herlitzka and Spemann, by separating the first two cells in amphibian eggs; O. Schultze and others, by use of gravity and compression; and Loeb and others by use of chemical agents. By various means, then, when suitably applied and at the right time, hemiembryos have been produced by separating or potentially separating the first tw^o blastomeres in certain forms. O. Hertwig states:
- Bei den kleinen, niit geringen Mengen von Dotter ausgestattcten Eiern der Wirbeltiere sind spontan eiitstandene, das heisst, ohne experimentelle Eingriffe veranlasste Mehrfachl^ildungen ausseror(k^ntlich selten, bei manchen Klassen iiberhaupt noch nie beohachtet wordon, dagegen sind sie relativ haufi.ge Befunde bei manchen untcrsuchten Arten von Knochenfischen und Vogeln, besonders bei der Foi-elle und beim Hiihnchen.
So far as I am aware, the possibility of hemiembryos in Mammalia has not been shown. In the albino rat, the oolemma may be lost as early as the 2-cell stage. In forms with early loss of oolemma, the separation of the two first blastomeres does not appear to me as an impossibility. The probable fate of separated mammalian blastomeres can only be conjectured, since it is manifestly impossible, for the present, to follow them in further development.
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Cite this page: Hill, M.A. (2024, April 18) Embryology Book - The Development of the Albino Rat 11. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_The_Development_of_the_Albino_Rat_11
- © Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G
