Cleavage is the first stage of embryogenesis.
This lecture first completes the discussion of embryonic cell cycle control and the role of MPF, primarily in Xenopus, and then reviews the above aspects of cleavage and blastula formation, including control of cleavage plane orientations, patterns of cleavage, and form of the resulting blastula in several different embryos. This relatively simple system, particularly in the large and accessible early embryos of amphibia, was used for some of the most fruitful early research on the cell cycle. It identified MPF as a key component, in all mitotic as well as meiotic divisions of the basic cell-cycle oscillator, with which you should be familiar from Cell Biology. Destruction of the cyclin subunit inactivates MPF, allowing the cell to exit from mitosis and enter interphase. During interphase, the dephosphorylated cdk-1 complexes with newly synthesized cyclin, which prepares it for phosphorylation of first Y and then T by separate kinases.