DEVELOPMENT OF CALCIUM SIGNALING 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 i n the generation of Ca2+ signals in human oocytes was investigated. Th e thiol reagent, thimerosal, which induces oscillations of intracellul ar Ca2+ ([Ca2+](i)) similar to those seen during fertilization, was us ed 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 fol licles being relatively insensitive, compared with those from luteiniz ed follicles, which displayed a large spike followed by sustained osci llations in [Ca2+](i). These oscillations were inhibited by caffeine w hich suggests that they were mediated by the inositol trisphosphate re ceptor Ca2+ release system. When immature oocytes were cultured in vit ro they acquired the capacity to undergo a single large spike in [Ca2](i), however, subsequent sustained oscillations were not observed, in dicating that these oocytes failed to develop fully competent Ca2+ sig nalling mechanisms during culture in vitro. This finding may be a key factor in the poor developmental competence of in-vitro matured human oocytes.