Cell-cycle control in oocytes and during early embryonic cleavage cycles in ascidians.

The completely transparent eggs and embryos of the ascidian Phallusia mammillata are well suited for imaging-based studies of how cell cycle control mechanisms have been integrated into the processes of meiosis, fertilization, and embryonic development. Several cell cycle-related issues that pertain to reproduction and development have been addressed using the ascidian model. For example, how are sperm-triggered calcium oscillations controlled by cell cycle kinases? How is chromosome segregation during meiosis regulated? What processes does the Mos/MAPK signaling cascade control in eggs in addition to CSF-mediated cell cycle arrest? Following fertilization ascidians blastomeres display cell cycle asynchrony, oriented cell division, and unequal cleavage resulting in the formation of a distinctive gastrula composed of precisely 112 cells. Here, we shall summarize these topics and where possible show how the mechanisms identified in ascidians compare to those identified in other organisms.