Mechanisms of Ca2+ signalling in oocytes Basic properties of Ca2+ stores and channels in mouse oocytes

Fertilization marks the end of oogenesis and the beginning of embryogenesis. The legacy of oogenesis is, however, maintained by virtue of the stored nutritional and regulatory molecules that fuel, and in many cases direct, early development. The signal that stimulates the transition from oocyte to embryo is a sperm-induced increase in the concentration of intracellular free Ca2+. Thus successful initiation of embryogenesis hinges on the ability of the oocyte to respond to the fertilizing spermatozoa with a suitable increase in intracellular Ca2+. How the signal transduction systems differentiate during oogenesis to ensure a successful transition from oocyte to embryo is the subject of this review. Normally fertilization occurs at the end of the final stage of oogenesis, that is, oocyte maturation (see Fig. 1). Oocyte maturation in mammals consists of nuclear (cell cycle) progression from the first meiotic prophase through to metaphase of the second meiotic division where the oocyte arrests until fertilization. In addition, there are cytoplasmic changes that are necessary to support early developmental events (Fig. 1). The process of oocyte maturation in vivo is stimulated by the preovulatory surge of gonadotrophins, while in vitro it occurs spontaneously when the fully grown oocyte is removed from the inhibitory follicular environment to a suitable culture medium. Fertilization triggers two key events that signal the oocyte has been activated and has entered the programme of embryonic development. First, exocytosis of the cortical granules, which prevents the penetration of supernumerary spermatozoa and, second, the completion of meiosis and entry into the first embryonic mitosis. The explosive increase of intracellular Ca2+ at fertilization is responsible for triggering both of these events (Kline and Kline, 1992; Whitaker and Swann, 1993). Before considering how the Ca2+ release systems develop during oocyte maturation, we will discuss how intracellular Ca2+ is regulated in mature oocytes.