Ca²⁺ signaling during mammalian fertilization: Requirements, players, and adaptations

Changes in the intracellular concentration of calcium ([Ca²⁺]_i) represent a vital signaling mechanism enabling communication among cells and between cells and the environment. The initiation of embryo development depends on a [Ca²⁺]_i increase(s) in the egg, which is generally induced during fertilization. The [Ca²⁺]_i increase signals egg activation, which is the first stage in embryo development, and that consist of biochemical and structural changes that transform eggs into zygotes. The spatiotemporal patterns of [Ca²⁺]i at fertilization show variability, most likely reflecting adaptations to fertilizing conditions and to the duration of embryonic cell cycles. In mammals, the focus of this review, the fertilization [Ca²⁺]_i signal displays unique properties in that it is initiated after gamete fusion by release of a sperm-derived factor and by periodic and extended [Ca²⁺]_i responses. Here, we will discuss the events of egg activation regulated by increases in [Ca²⁺]_i, the possible downstream targets that effect these egg activation events, and the property and identity of molecules both in sperm and eggs that underpin the initiation and persistence of the [Ca²⁺]_i responses in these species.