Nuclear Reprogramming in Bovine Embryos after Nuclear Transplantation

Abstract

A fundamental and still unsolved problem concerning the developmental process of eukaryotic organisms is whether differentiated cells ration the genomic totipotency of the zygote nucleus (DiBerardino, 1988). There are probably differences among mammalian species in the extent to which nuclei from early embryos are able to support development after transfer to an enucleated egg. Mouse embryos initiate transcription of heterogenous nuclear RNA (hnRNA) at the early 2 cell stage (Flach et al., 1982) and nuclear totipotency in mouse is lost at the time of embryonic genome activitation (McGrath and Solter, 1984). On the other hand, sheep embryos initiate transcription at the 8 to 16 cell stage (Crosby et al., 1988), and blastomere nuclei of these embryonic stages (Willadsen, 1986) and even of ICM stage (Smith and Wilmut, 1989) are able to support development to term, when fused to enucleated oocytes. Similarly, in cattle embryos transcription starts at the 8 cell stage (Camous et al., 1986), and blastomere nuclei of 8 – 16 cell stage (Prather et al., 1987) and even of 32 cell stage (Marx, 1988) are totipotent. The question arises as to what processes on the ultrastructural and molecular level are responsible for reprogramming the mammalian embryonal nucleus.