EARLY CLEAVAGE AND THE MATERNAL ZYGOTIC TRANSITION IN BOVINE EMBRYOS

Early development in bovine embryos is driven by message and proteins stored within the oocyte. Fertilization activates development and cleavage ensues. Cleavage up to the 4-cell stage is synchronous. The first cell cycle is long and contains distinct G1 and G2 phases. The second cell cycle is unique as it fails to demonstrate any appreciable gap phase. The third cell cycle begins to lengthen by the addition of a G2 phase. It is at this point cleavage becomes asynchronous. Embryonic transcription begins at this time as well. A major change in protein patterns is observed in the fourth cell cycle. Protein synthesis declines from the 1-cell stage to the 8-cell stage. Ribosomal RNA synthesis is active by the late 8-cell stage. These points are discussed as they relate to a working hypothesis concerning early cleavage and the maternal zygotic transition in bovine embryos. © 1990.