The Frequency of Calcium Oscillations in Mouse Eggs at Fertilization Is Modulated by the Number of Fused Sperm

AbstractIn a variety of calcium signaling systems, the frequency of intracellular calcium oscillations is physiologically important. Probably multiple factors control the frequency of calcium oscillations in the egg after fertilization and many of these remain to be identified. In this study, we present the first rigorous set of data showing that monospermic fertilization is important for setting the physiological calcium oscillation frequency. Recordings in 152 zona-free eggs show that the general pattern of the calcium oscillations is identical in monospermic and polyspermic eggs; however, the oscillation frequency is higher in polyspermic eggs (P < 10−6). The frequency of the late oscillations increases with the number of sperm heads incorporated: 5.2 ± 0.3 spikes per hour (mean ± SEM; n = 55) in monospermic eggs, 6.6 ± 0.3 (n = 62) in dispermic eggs, 8.7 ± 0.7 (n = 23) in trispermic eggs, and 8.9 ± 0.9 (n = 12) in eggs with four or more sperm heads. The frequency of the early oscillations is also increased in polyspermic eggs. Seventy-eight additional eggs were divided into two groups and inseminated with two different sperm concentrations (“low” and “high”) to obtain one group mainly monospermic and the other mainly polyspermic. The two groups of eggs oscillated at different frequencies (P < 10−5). These data rule out the possibility of an egg effect in which some eggs would have the dual properties of oscillating faster and of being able to fuse with several sperm cells. These data instead suggest that the sperm modulates the frequency of the oscillations in a dose-dependent manner