Injection of sperm cytosolic factor into mouse metaphase II oocytes induces different developmental fates according to the frequency of [Ca2+]i oscillations and oocyte age

Intracellular calcium ([Ca2+](i)) rises are a hallmark of mammalian fertilization and are associated with normal activation of embryonic development. Injection of mammalian sperm cytosolic factor (SCF) into oocytes has been shown to trigger [Ca2+](i) rises similar to those observed during fertilization, and to initiate normal embryonic development. However, Ca2+ release has also been shown to be associated with cell death, but the mechanisms of the detrimental effects of Ca2+ stimulation on development have not yet been investigated. Thus, studies were undertaken using SCF to test the effects of [Ca2+](i) oscillations on oocyte activation in freshly ovulated and aged oocytes. Injections of 1 mg/ml SCF into freshly ovulated mouse metaphase II oocytes, which evoked Ca2+ responses with low frequency and short duration, induced normal activation and cleavage to the two-cell stage. Conversely, injection of 15 mg/ml SCF, which triggered high-frequency and persistent Ca2+ responses, induced abnormal activation that was characterized by abnormal chromatin configurations, inhibition of DNA synthesis, and lack of first mitotic spindle assembly. More importantly, fertilization-like Ca2+ responses induced by injection of 1 mg/ml SCF triggered cell death, rather than activation, in in vitro-aged oocytes. These oocytes exhibited extensive cytoplasmic and DNA fragmentation that was accompanied by activation of protein caspases, all of which are signs of apoptotic cell death. Fewer similarly aged oocytes that were either unstimulated or activated with 7% ethanol underwent fragmentation. Together, these results suggest that [Ca2+](i) oscillations are required to activate freshly ovulated oocytes, but if initiated at abnormally high frequency and duration or if induced in aged oocytes, the [Ca2+](i) oscillations may trigger premature termination of embryonic development.