- Calcium Signals for Egg Activation in Mammals

Intracellular calcium ion is a key second messenger that regulates a wide variety of cellular functions. Egg activation at fertilization is one of the important Ca2+- dependent biological phenomena. In fertilization, the sperm not only provides one half of the genomes to the egg but also awakes the egg that is arrested at a certain stage of meiotic cell division in a species-specific manner. The release from the meiotic arrest is referred to as “egg activation” characterized by formation of the polar body and male and female pronuclei. Egg activation is caused by a dramatic increase in intracellular Ca2+ concentration ([Ca2+]i) common to every animal species ever examined (1, 2). The [Ca2+]i rise is mainly due to Ca2+ release from the endoplasmic reticulum (ER) and forms a “Ca2+ wave” that starts from the site of sperm-egg fusion and propagates the Ca2+ signal over the whole egg. Besides the spatial Ca2+ signal, repetitive [Ca2+]i rises designated as “Ca2+ oscillations” occur as temporal Ca2+ signals in various species including mammals (1, 2). In regard to the cell signaling in fertilization, there are two essential subjects upstream and downstream of the [Ca2+]i rise: how the sperm induces the [Ca2+]i rise and how the [Ca2+]i rise leads to egg activation. Here we review advances in the research on Ca2+ signals at fertilization with special attention to mammals.

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