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 fertili zation and are associated with normal activation of embryonic development. Injection of mammalian sperm cytosolic factor (SCF) into oocytes has been s hown to trigger [Ca2+](i) rises similar to those observed during fertilizat ion, and to initiate normal embryonic development. However, Ca2+ release ha s also been shown to be associated with cell death, but the mechanisms of t he 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 I I oocytes, which evoked Ca2+ responses with low frequency and short duratio n, 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 abnor mal chromatin configurations, inhibition of DNA synthesis, and lack of firs t mitotic spindle assembly. More importantly, fertilization-like Ca2+ respo nses induced by injection of 1 mg/ml SCF triggered cell death, rather than activation, in in vitro-aged oocytes. These oocytes exhibited extensive cyt oplasmic and DNA fragmentation that was accompanied by activation of protei n caspases, all of which are signs of apoptotic cell death. Fewer similarly aged oocytes that were either unstimulated or activated with 7% ethanol un derwent fragmentation. Together, these results suggest that [Ca2+](i) oscil lations are required to activate freshly ovulated oocytes, but if initiated at abnormally high frequency and duration or if induced in aged oocytes, t he [Ca2+](i) oscillations may trigger premature termination of embryonic de velopment.