Activation of the ribosomal RNA genes in pre-implantation bovine embryos

Pre-implantation bovine development is characterized by a sequential activa tion of genes in the embryo. The first, second and third embryonic cell cyc les, i.e. the one-, two- and four-cell stages, are characterized by a low r ate of transcription, while a burst of transcription is seen during the fou rth cell cycle, i.e. the eight-cell stage. Also during the fourth cell cycl e, nucleolar function is re-established by activation of the rRNA genes. Th e associated process of nucleologenesis is, from an ultrastructural point o f view, a reversal of the nucleolar de-activation seen in the oocyte, which occurs at the completion of the oocyte growth phase. During the first, sec ond and third embryonic cell cycles, attempts are apparently made to re-est ablish nucleolar function as the nucleolar assembly within each of these cy cles proceeds to a certain but still transcriptionally inactive stage.