Spatiotemporal dynamics of intracellular calcium in the mouse egg injected with a spermatozoon

Oscillatory rises in intracellular Ca2+ concentration ([Ca2+]i) are the pivotal signal in the fertilization of mammalian eggs. The spatiotemporal dynamics of [Ca2+]i rises in mouse eggs subjected to intracytoplasmic sperm injection (ICSI) were analysed by Ca2+ imaging and compared with those subjected to in-vitro fertilization (IVF). The first Ca2+ transient occurred 15-30 min after ICSI in most eggs, and was followed by Ca2+ oscillations which lasted for at least 6 h at intervals of approximately 10 min. The pattern of Ca2+ oscillations, an initial relatively larger Ca2+ transient followed by smaller Ca2+ transients, was similar to that at fertilization. Confocal Ca2+ imaging during early Ca2+ transients showed that, in fertilized eggs, [Ca2+]i increased in a wave which started from the sperm attachment site and propagated across the egg cytoplasm. In eggs subjected to ICSI, [Ca2+]i increased gradually and then a Ca2+ spike was generated when [Ca2+]i reached a certain level. The [Ca2+]i rise occurred in the whole egg, associated with neither a wave nor significant heterogeneity between the cortical and central regions. It is suggested that cytosolic factor(s) may leak from the injected spermatozoon, diffuse slowly in the egg cytoplasm, and then cause a synchronous Ca2+ release from intracellular Ca2+ stores.