Calcium and other ion dynamics during gamete maturation and fertilization

Ion currents and cytosolic free calcium ([Ca2+]i) elevations are crucial events in triggering the complex machinery involved in both gamete maturation and fertilization. Oocyte maturation is triggered by hormone signaling which causes ion currents and [Ca2+]i increase. Extracellular calcium seems to be required for meiosis progression since: (i) calcium depletion in the maturation medium severely affects oocyte developmental competence; (ii) the activity of plasma membrane L-type Ca2+ currents decreases during maturation; (iii) the exposure to verapamil, a specific Ca2+ channel blocker, decreases in vitro maturation efficiency. In spermatozoa, maturation initiates inside the epididymis and ends in the female genital tract. During their journey through the female reproductive tract, sperm undergo a dramatic selection and capacitation achieving fertilization competence. Adhesion to the tubal epithelium extends sperm life through depression of [Ca2+]i until capacitation signals trigger an [Ca2+]i elevation followed by sperm release. At fertilization, egg-sperm interaction evokes well-described transient and almost simultaneous events: i.e., fertilization current, a change in resting potential, and an increase in free [Ca2+]i concentration. These events, termed oocyte activation, are the direct consequence of sperm interaction via either activation of a receptor or entry of a sperm factor. The latter hypothesis has been recently supported by the discovery of PCLζ, a sperm-specific isozyme triggering a dramatic [Ca2+]i increase via inositol 1,4,5-trisphosphate (IP3) production. The course of ion currents and [Ca2+]i transients during maturation and fertilization plays a pivotal role in correct embryo development.