Temporal regulation of embryonic M-phases

Early embryonic mitoses are regulated differentially The duration of the first two embryonic mitoses differs significantly in various species we studied so far. Curi-ously, the first mitosis lasts always longer than the sec-ond one. In nematode Caenorhabditis elegans the first one lasts about 4.5 min, while the second one 3.5-4 min. In sea urchin Sphaerechinus granularis the dif-ferences become more pronounced and the first mito-sis takes about 20 min while the second one 15 min. In frog Xenopus laevis we estimated the mitotic duration for the first embryonic mitosis on 25 min, vs. 15 min for the second one [1,2]. The most dramatic difference was observed in the mouse Mus musculus embryos. The first mitosis takes about 120 min, while the second one approximately 70 min [3-5]. These differences in mitotic timings are not trivial since they are also observed in parthenogenetic embryos and in embryos with reduced volume of cyto-plasm [3]. Even cytoplasmic fragments of mouse zygotes persisted in a mitotic stage for longer time than similar cytoplasmic fragments obtained from 2-cell stage dividing blastomeres, as judged by the behaviour of microtubules and the measurement of the activity of the major M-phase factor (MPF or M-phase Promoting Factor) [3,6]. This means that at least in the mouse embryo an internal, cytoplasmic, clock regu-lates differentially the duration of the first two mitoses. The first two mitoses are most strikingly different in their duration. However, also the next embryoni