Somatic control of protein synthesis in mammalian oocytes during maturation

The extent to which signals from the follicular compartment regulate protein synthesis in sheep oocytes is considered. Evidence is derived from experiments on the control of actin synthesis and also from a quantitative analysis of protein synthesis in oocytes matured in follicles in which specific somatic signals have been altered. Actin represents about 10% of the total protein synthesized by oocytes enclosed by follicle cells but under 2% in denuded oocytes. Although follicle cells synthesize and release actin, no evidence of its uptake into oocytes has been obtained. Instead, the results suggest that regulatory signals of somatic origin are transmitted directly into the oocyte by intercellular transport. A variety of cell types in addition to those of follicular origin support actin synthesis in oocytes, provided only that functional communication between the somatic and germinal compartment is maintained. Finally, the results on precursor uptake, actin turnover and RNA inhibition, and those of Dawn Giebelhaus on actin mRNA measurements, indicate that the somatic signals act on post-transcriptional events, possibly by influencing the mobilization of stored actin message. The selective inhibition of 17 alpha-hydroxylase in follicles depresses oestrogen and androgen and enhances progesterone biosynthesis. These steroid alterations, if induced during the first 8 h of maturation, lead to developmental abnormalities at fertilization. In addition, these steroidogenic changes alter the synthesis of a small number of acidic proteins in the maturing oocyte. It is postulated that steroid signals regulate the synthesis of certain proteins associated specifically with maturation.