Development of calcium signalling mechanisms during maturation of human oocytes

Sperm-induced Ca2+ signals mediate the events of oocyte activation at fertilization. In this study, the development of mechanisms involved in the generation of Ca2+ signals in human oocytes was investigated. The thiol reagent, thimerosal, which induces oscillations of intracellular Ca2+ ([Ca2+]i) similar to those seen during fertilization, was used to mobilize Ca2+ in in-vivo matured, immature and in-vitro matured human oocytes. There was an increase in the sensitivity to thimerosal during maturation of human oocytes, with oocytes from small antral follicles being relatively insensitive, compared with those from luteinized follicles, which displayed a large spike followed by sustained oscillations in [Ca2+]i. These oscillations were inhibited by caffeine which suggests that they were mediated by the inositol trisphosphate receptor Ca2+ release system. When immature oocytes were cultured in vitro they acquired the capacity to undergo a single large spike in [Ca2+]i, however, subsequent sustained oscillations were not observed, indicating that these oocytes failed to develop fully competent Ca2+ signalling mechanisms during culture in vitro. This finding may be a key factor in the poor developmental competence of in-vitro matured human oocytes.