THE MOLECULAR AND CELLULAR BASIS OF PREIMPLANTATION MOUSE DEVELOPMENT

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Summary: The formation of a mouse blastocyst involves the use of genetic information derived from the expression both of maternal genes in the preovulatory oocyte and of embryonic genes from the mid‐2‐cell stage onwards. Evidence is reviewed suggesting that maternal RNA transcripts made up to 20 days prior to ovulation are available for use after fertilization, that no significant KNA synthesis occurs between germinal vesicle breakdown and the mid‐2‐cell stage and that during this interval of transcriptional inactivity production of protein is regulated at a post‐transcriptional level. Embryonic genes first become active at the mid‐2‐cell stage and at, or shortly after, this time most maternal mRNA is inactivated. Thereafter, the available evidence suggests that although embryonic mRNA has considerable stability, much of it is translated immediately after it is transcribed. This tight linkage of mRNA transcription and utilization appears to be crucial for certain key events in development.The cellular interactions operating during early development are described. It is suggested that asymmetry of cell contacts in the 8‐cell embryo imparts a mosaic structure to the embryo, and that this mosaicism is conserved and elaborated with division to the 16‐ and 32‐cell stages. The mechanisms are discussed by which mosaicism is induced and through which the two cell lineages of the blastocyst are founded.Temporal events in preimplantation development show a periodicity of about 27 h. It is proposed that within each period differentiative events are cumulative but reversible, and that the boundaries between each developmental period constitute points of commitment at which reversal of development becomes no longer possible.

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