Ca2+ cascade and meiotic resumption of the caprine primary oocyte

In this study, we investigated the fluctuations of concentration of intracytoplasmic free Ca2+ during in vitro maturation of caprine primary oocytes and its role in meiotic resumption. Oocytes that were extracted from caprine ovaries were cultured and allowed to mature in vitro to determine their developmental stages including germinal vesicle (GV), germinal vesicle breakdown (GVBD), metaphase of the first meiotic division (MI) and metaphase of the second meiotic division (MII). Intracytoplasmic free Ca2+ turnovers of caprine oocytes at these different developmental stages were measured using the calcium fluorescent probe Fura-2/AM (C44H47N3O24) to investigate the dynamics of cytosolic free Ca2+ during in vitro maturation of oocytes and the role of Ca2+ in inducing the initiation of meiotic resumption of oocytes. Moreover, the oocytes were cultured in Ca2+ culture medium and Ca2+-free culture medium to examine the effect of extracellular Ca2+ on the oocyte maturation. The results indicated that Ca2+ concentrations at GV, GVBD, MI and MII stages were 78.06, 147.41, 126.97 and 97.73 nmol/l, respectively, and that 86.30% of oocytes remained at the GV stage and no oocyte developed to MII in Ca2+-free culture medium, and 1.1% of oocytes stayed at the GV stage and 83.5% of oocytes developed to MII in Ca2+ culture medium. These results suggest that the occurrence of GVBD and cell cycle progression to MI and MII stages are closely related to Ca2+, and that extracellular Ca2+ performs a specific function for the initiation of meiotic resumption in caprine oocytes.