To track the lineage of both blastomeres of 2-cell embryos during mouse preimplantation development, each cell was injected with dextran solutions conjugated with different fluorochromes. The fate of the progeny of the first two blastomeres was followed with confocal microscopy during cleavage and during the formation of the blastocyst. We observed that in most of cleaving embryos the cells derived from the two first blastomeres intermingled in both the trophectoderm and the inner cell mass (ICM) and did not form two discrete groups. We conclude that embryonic parts of blastocysts contain descendants of both blastomeres of 2-cell embryo.

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CD9 is a member of the tetraspanin superfamily proteins and is the only protein on the mouse oocyte which is known to be indispensable in sperm-egg fusion. Here, using indirect immunofluorescence we show that CD9 appears on the oolemma during the early stages of the growth of the oocyte, when it measures 13-22 microm in diameter. When the oocyte reaches a diameter of 17-22 microm, the density of CD9 in its oolemma is similar to the density of this protein in the cell membrane of the fully grown secondary oocyte. The appearance of CD9 in growing oocytes correlates with the previously reported time of the acquisition of fusibility between the spermatozoon and the egg. Accordingly we propose that during oogenesis the development of the ability of the oolemma to fuse with sperm may be regulated by synthesis of CD9 by the oocyte.

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