Role of Hormones in Oocyte Maturation

In order for normal fertilization and embryonic development to occur, oocytes, during the course of meiosis, undergo a strict sequence of morphological and physiological transformations in the cytoplasm, nucleus, and at the cell surface. The nature of these maturational changes at various stages of meiosis and the role hormones play in them have been discusssed primarily on the basis of in vitro data. Gametogenic and endocrine functions are performed by the gonads of most species, and evidence presented here leads to the general conclusion that certain aspects of these two processes occur as a result of an interaction between germinal and somatic cells. Typically, gametic and somatic endocrine cells are in physical juxtaposition within an ovarian follicle, and structural and functional changes which occur in these cells during the course of gametogenesis appear to be important for allowing oocyte maturation to proceed as well as for regulating the process. A central event in the oocyte maturational process is interruption of oocyte arrest as characterized by the disintegration of the nucleus or germinal vesicle. Oocytes, prior to this event, differentiate through prophase I of meiosis and undergo the major portion of cytoplasmic and nuclear growth. Subsequent to nuclear breakdown, chromosomes proceed through the remaining meiotic events and differentiation of new functions occurs in the cytoplasm. Considerable evidence in nonmammalian species indicates that particular steroids or other low molecular weight substances mediate certain aspects of the cytoplasmic and nuclear maturation processes in oocytes. In mammals, a direct involvement of hormones (pituitary and/or ovarian) in initiating nuclear or cytoplasmic maturation has not been established, although the data are suggestive. Spontaneous oocyte maturation is a complicating factor in many species, and its relevance is discussed in terms of processes of follicular and oocyte differentiation. In amphibians, the nature of the steroids, the time at which they function, and the functions they perform vary in relation to particular stages of meiosis. Thus, ovarian estrogenic steroids are required (indirectly) for oocyte growth, their function being to stimulate hepatic synthesis of vitellogenin, the major cytoplasmic yolk platelet precursor. Subsequent to its release into the circulation, vitellogenin is sequestered into the oocyte by a micropinocytotic process at the oocyte surface and undergoes transformati