Injection of sperm extract mimics spatiotemporal dynamics of Ca2+ responses and progression of meiosis at fertilization of ascidian oocytes.

Sperm extract (SE) of the ascidian, Ciona savignyi, injected into oocytes induced repetitive intracellular Ca2+ increases with kinetics consistent with those at fertilization and caused reinitiation and progression of meiosis as in fertilized oocytes with the formation of polar bodies. The Ca2+ response comprised two sets of Ca2+ oscillations separated by 5 minutes and correlated with the first and second meiotic metaphase. The effects of SE were dose dependent and the critical dose corresponded roughly to a single spermatozoon. In the first Ca2+ transient observed by confocal microscopy, a Ca2+ wave started from the SE injection site at the peripheral region of the oocyte and propagated across the ooplasm. The similar wave was produced by injection at the central region, starting from an arbitrary cortical area after 30 seconds, probably after SE had diffused to the cortex. The sensitivity to SE is thought to be preferentially higher in the cortex. The effective component of SE was heat-unstable, and its molecular weight was estimated as in the range between 10x10(4 )and 3x10(4) using membrane filters. These results suggest that, in ascidian fertilization, a cytosolic sperm protein factor is introduced to the oocyte cortex and induces Ca2+ waves and thereby meiotic resumption, leading to cell-cycle-correlated Ca2+ oscillations.