Nuclear and Cytoplasmic Changes During Oocyte Maturation

Abstract

The success or failure of embryonic development is determined by the quality or maturational status of the gametes that participate in the formation of the conceptus. From a phylogenetic point of view, it is clear that while the genetic contribution of each gamete to the zygotic genome is a dominant goal in sexually reproducing animal forms, cytoplasmic contributions from the female gamete are vital to the early livelihood of the embryo (1). Studies of vertebrates and invertebrates on the mechanisms of maternal inheritance now illustrate with great clarity that both nutritional and informational molecules must be expressed and organized during the course of oogenesis in order for normal development to proceed upon fertilization (2). To what extent the developmental blueprint for embryogenesis is coupled with the process of meiosis itself is not at all clear, although insights into this interrelationship on a mechanistic level have derived from recent studies on the determinants of embryonic polarity evident at the terminal phases of oogenesis. One common expression of polarity in oocytes shared in diverse animal species is manifest in the asymmetric cleavage exhibited at the time of polar body extrusion, a process of dualistic function whereby nuclear reductive division occurs concomitant with the conservation of ooplasmic mass.