Single Ca transients vs oscillatory Ca signaling for assisted oocyte activation: limitations and benefits.

Oocyte activation is a calcium (Ca)-dependent process that has been investigated in depth, in particular, regarding its impact on assisted reproduction technology (ART). Following a standard model of signal transduction, Ca drives the meiotic progression upon fertilization in all species studied to date. However, Ca changes during oocyte activation are species specific, and they can be classified in two modalities based on the pattern defined by the Ca signature: a single Ca transient (e.g. amphibians) or repetitive Ca transients called Ca oscillations (e.g. mammals). Interestingly, assisted oocyte activation (AOA) methods have highlighted the ability of mammalian oocytes to respond to single Ca transients with normal embryonic development. In this regard, there is evidence supporting that cellular events during the process of oocyte activation are initiated by different number of Ca oscillations. Moreover, it was proposed that oocyte activation and subsequent embryonic development are dependent on the total summation of the Ca peaks, rather than to a specific frequency pattern of Ca oscillations. The present review aims to demonstrate the complexity of mammalian oocyte activation by describing the series of Ca-linked physiological events involved in mediating the egg-to-embryo transition. Furthermore, mechanisms of AOA and the limitations and benefits associated with the application of different activation agents are discussed.