Spatiotemporal dynamics of the [Ca2+](i) rise induced by microinjection ofsperm extract into mouse eggs: Preferential induction of a Ca2+ wave from the cortex mediated by the inositol 1,4,5-triphosphate receptor

Hamster sperm extract (SE) possessing Ca2+ oscillation-inducing activity wa s microinjected into the peripheral or central region of mouse eggs, and th e first increase in intracellular Ca2+ concentration ([Ca2+](i)), together with the spread of fluorescence-labeled SE in the ooplasm, was investigated by imaging with confocal microscopy. Injection into the periphery always i nduced a Ca2+ wave that started from the injection site after a delay of 5 to 30 s depending on the concentration of SE. The diluted SE caused a wave of two-step [Ca2+](i) rises, which was always observed at fertilization. In jection into the center could induce a radial Ca2+ wave with relatively hig h dose of SE, but lower dose of SE caused a [Ca2+](i) rise after a longer d elay which was initiated synchronously over the ooplasm or was preceded in a peripheral area. Injection of diluted SE remarkably prolonged the delay t ime and reduced the rate of [Ca2+](i) rise. The critical concentration of S E needed to induce [Ca2+](i) rise was significantly lower in the periphery. These results indicate that the sensitivity to SE is higher in the cortex. SE-induced [Ca2+](i) rises were blocked by an antibody against the type 1 inositol 1,4,5-trisphosphate receptor (InsP(3)R). The cortex was substantia lly more sensitive to injected InsP(3) induction of Ca2+ release than the c enter. It is suggested that the cortex of mouse eggs may involve a function ally specialized organization of InsP(3)Rs and Ca2+ pools in which a cytoso lic sperm factor(s) could act upon sperm-egg fusion to cause Ca2+ release, leading to the Ca2+ wave at fertilization. (C) 1999 Academic Press.