The role of steroid signals in the maturation of mammalian oocytes.

The resumption of meiosis in mammalian oocytes is associated with sequential changes in follicular steroidogenesis, the principal features of which are an initial stimulation and then suppression of the secretion of oestrogens and androgens, followed by a steady increase in the secretion of progesterone. The addition of steroid enzyme inhibitors to isolated follicles in vitro alters the normal profile of steroids secreted during maturation and induces intracellular changes in the oocytes which are expressed as nuclear abnormalities at fertilization. Different abnormalities are induced by selectively modifying steroid biosynthesis during maturation, the maturing oocyte being more sensitive to an imbalance in the steroid profile rather than the total inhibition of steroid secretion. The cause of the developmental aberrations may either result from or be associated with the abnormal patterns of proteins synthesized by oocytes denied the support of the correct balance or sequence of steroids during maturation.