Ca2+ oscillations and the cell cycle at fertilisation of mammalian and ascidian eggs.

At fertilisation of mammalian and ascidian eggs the sperm induces a series of Ca2+ oscillations. These Ca2+ oscillations are triggered by a sperm-borne Ca2+-releasing factor whose identity is still unresolved. In both mammals and ascidians Ca2+ oscillations in eggs are associated with the period leading up to exit from meiosis and entry into the first embryonic cell cycle. Thus, in mammals Ca2+ oscillations continue for several hours but are complete by within 30 min in the ascidian. In mammals and ascidians Ca2+ oscillations stop at around the time when pronuclei form in the 1-cell embryo. There is evidence to show that cell cycle factors are important in regulating the fertilisation Ca2+ signal. If the formation of pronuclei is blocked either in mammals (by spindle disruption) or in ascidians (by clamping maturation promoting factor levels high) then Ca2+ oscillations continue indefinitely. Here, we explore the nature of the sperm Ca2+-releasing factor and examine the relationship between cell cycle resumption and the control of Ca2+ oscillations at fertilisation.