Nuclear Transfer in Mouse Embryos: Activation of the Embryonic Genome

Following fertilization, normal development requires a strictly defined inventory of cellular components. These components, the products of male and female gametogenesis, reflect two distinct processes of differentiation. It is apparent that the transcriptional activity of the respective gametic nuclei must meet different morphogenetic and functional requirements. In the male, transcriptional activity persists into the haploid phase of spermatogenesis (Distel et al. 1984), whereas synthesis of poly(A)+ RNA in oocytes in the female continues beyond the completion of oocyte growth (Wassarman 1983). Thus, the male and female genomes at the moment of fertilization reflect functionally different germ cell lineages that must undergo a coordinated transition for the specification of embryo-specific gene products. This transition requires complex interactions among the pronuclei, the ovum cytoplasm, and their environment. The nature of these interactions and the individual contributions of each cellular component may be assessed by nuclear and cytoplasmic transfer. We present experiments in...