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 mecha-nisms 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 pro-ceed 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. Thus, the coupling of meiosis with unequal cytoplasmic partitioning necessarily sets the stage for the beginnings of embryogenesis. In conven-tional parlance, this terminal phase of oogenesis is referred to as meiotic maturation and must implicitly constitute a rather remarkable integration of both nuclear and cytoplasmic events in order to produce a viable and developmentally competent ovum. This chapter seeks to define aspects of nuclear and cytoplasmic matura-tion that occur prior to and during the course of meiotic maturation by means of a series of studies that take advantage of advances in three areas of recent research on mammalian oocytes: (i) the developmental com-3 B. D. Bavister (ed.), Preimplantation Embryo Development © Springer-Verlag New York, Inc. 1993 4 D.F. Albertini et al. petence of oocytes grown and matured in culture (3), (ii) the applica-tion of fluorescence staining techniques that permit precise definition of nuclear and cytoplasmic components (4), and (iii) new findings on the mechanisms of cell cycle control (5). As a background to an analysis of meiotic maturation in the mammalian oocyte, we first review seminal findings on mitotic control mechanisms in somatic cells with respect to cytoplasmic and nuclear events.