Oogenesis

James L. Maller, Maller, James L.

This chapter presents a complex picture of the regulation of oocyte maturation. The events that have been identified can be expressed temporally on a relative time scale, as shown in Figure 8. A major question that lies ahead concerns which of these events is causal for subsequent events and for GVBD. Clearly, the first event at 0.1 GVBD50, a decrease in cAMP, is both necessary and sufficient to induce GVBD. The increase in protein synthesis at 0.3 GVBD50 is apparently necessary, and the activity of MPF appearing at 0.4 GVBD50 is apparently sufficient, as judged by the microinjection assay. It is important to identify further steps after MPF appearance in order to establish the mechanism of action of MPF. The fact that events highly correlated with cell growth (i.e., pHi increase, S6 phosphorylation, and MPF appearance) all occur during amphibian oocyte maturation provides strong support for continued use of the oocyte in studying the general control of cell proliferation, particularly with the technique of microinjection. Investigation of the process of amphibian oocyte maturation has also led to the discovery of a totally new type of steroid hormone mechanism in which a plasma membrane steroid receptor is linked to the adenylate cyclase system. The oocyte system is the first example of a steroid hormone effect mediated by a direct effect on a target enzyme without a requirement for protein or RNA synthesis. The generality of this type of steroid hormone mechanism remains to be established, but its similarity to the P site on adenylate cyclase is suggestive evidence of a general mechanism, since the P site is present in all eukaryotic plasma membranes. Most steroid hormone systems have been investigated with respect to the mechanism of increased protein synthesis after steroid addition, with little attention to any cell division occurring. In the oocyte, the cell division effect of steroids is the predominant one; it is possible that this type of effect is mediated by a different receptor system than the one associated with changes in gene expression in somatic cells. The information described in this chapter hints at the complexity underlying the activation of development during the process of oocyte maturation. The finding that so many of the specific events and general mechanisms operating in oocyte maturation are also used in differentiated tissues strongly indicates that further study of amphibian oocyte maturation will lead to findings of general significance in terms of how information stored in the oocyte is used during development to result in a new member of the species.