Investigations into the degree of cell mixing that occurs between the 8‐cell stage and the blastocyst stage of mouse development

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Abstract
This study was designed to assess the degree of cell mixing that occurs during the early development of the mouse embryo, and thus provide information which is important in relation to the current theories of differentiation. Previous studies of this nature have involved either chimeric composites, or have only followed a very limited number of cells in the embryo. Here the products of one of the 4‐cell stage blastomeres have been labeled with tritiated thymidine, at a level which allows their descendants to be identified three or four cell divisions later, and recombined with the remaining blastomeres of the same embryo. After fixing and sectioning of the embryos at the blastocyst stage the locations of the labelled cells have been analyzed to assess the degree of clumping that they display. A significant tendency for the products of this one 4‐cell stage blastomere to be confined to a single area in the blastocyst is demonstrated. This indicates that there is little marked cell movement during the observation period. The relevance of these results to current knowledge of blastocyst development is discussed.