In humans, as all other mammalian species studied so far, normal fertilization is characterized by a particular type of calcium signals consisting of calcium oscillations and calcium waves (refer to previously discussed section "Sperm-Oocyte Fusion and Oocyte Activation"). The same form of calcium signals was observed after conventional IVF (4), subzonal insemination (18), ICSI (5,18), and round spermatid injection (19). Animal experiments have suggested that the quality of the signal encoded by the frequency and amplitude of calcium oscillations is important not only for the immediate success of fertilization, but that it conditions further post-fertilization development during a relatively extended time period, up to the blastocyst stage (20-22).
It is reasonable to suspect that calcium waves and oscillations are beneficial for normal embryonic development in humans, too. It was suggested that truncated calcium signals in human oocytes at fertilization compromise the completion of meiotic division (23) and may cause chromosome nondisjunction leading to aneuploidy (24). Abnormal patterns of fertilization-associated calcium signals can be produced experimentally by exposing oocytes to activators (Fig. 2B) and inhibitors (Fig. 2C and D) of PKC (6). It was noted that experimental modifications of the temporal and spatial patterns of sperm-induced calcium signals in human oocytes often lead to abnormalities of subsequent developmental events, namely the formation and internal structural differentiation of the male and the female pronucleus (11). Similar abnormalities occasionally occur spontaneously after conventional IVF or ICSI and are particularly frequent in some cases, suggesting the existence of sperm- or oocyte-derived activation abnormalities as the underlying cause.
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