Cell cycle regulation in early mouse embryos.

For a long time it has been thought that the cell cycles of the early mouse embryo do not differ from the somatic cell cycle. They are long and are composed of classical G1, S, G2 and M phases and have functional checkpoint controls. However, a few characteristics observed during the earliest mitotic cleavage divisions suggest that the embryonic cell cycle could differ significantly from the somatic ones. Understanding these differences could have an important impact on our understanding of both general cell cycle mechanisms as well as the developmental programme of the early mouse embryo. Over the last few years our laboratories have undertaken a project focused on describing the differences in the first two cell cycles of the mouse embryo. We discuss here the results concerning (1) the way mouse oocytes switch from the meiotic to the mitotic cell cycle upon activation of development (inactivation of the cytostatic factor, CSF); (2) how the entry into the first and the second mitotic M phase is regulated (nucleus-independent activation of M phase-promoting factor, MPF); and (3) how the duration of the early embryonic mitoses is regulated. These data show that developmentally regulated phenomena are superimposed on and highly coordinated with the cell cycle machinery.