Cell cycle progression of parthenogenetically activated mouse oocytes to interphase is dependent on the level of internal calcium.

Nuclear maturation of the mouse oocyte becomes arrested in metaphase of the second meiotic division (MII). Fertilization or parthenogenetic activation induces meiotic completion, chromosomal decondensation and formation of a pronucleus. This completion of meiosis is probably triggered by a transient increase in cytosolic calcium ions. When activated just after ovulation by a low concentration of the calcium ionophore A23187, the majority of the mouse oocytes go through a metaphase to anaphase transition and extrude their second polar body but they do not proceed into interphase; instead their chromatids remain condensed and a microtubular metaphase spindle reforms (metaphase III). However, a high percentage of these oocytes will undergo a true parthenogenetic activation assessed by the formation of a pronucleus, when exposed to a higher concentration of the calcium ionophore. The capacity of the mouse oocyte to pass into metaphase III is lost with increasing time post-ovulation. Direct measurement of intracellular calcium with Fura-2 reveals higher levels of cytosolic calcium in aged oocytes and/or using higher concentrations of calcium ionophore for activation. It is concluded that the internal free calcium level determines the transition to interphase.