Under arrest: cytostatic factor (CSF)-mediated metaphase arrest in vertebrate eggs.

In most animals, the development of the immature oocyte into a fertilizable gamete, a process known as oocyte maturation, involves an arrest in the meiotic cell cycle while awaiting fertilization. Depending on the organism, this arrest can occur at the beginning of meiosis I, in metaphase of meiosis I, in metaphase of meiosis II, or, after the completion of meiosis altogether, in the pronuclear stage. In the case of the vertebrate oocyte, maturation begins at the G2/M-phase border of meiosis I, and the arrest at the end of oocyte maturation occurs at metaphase of meiosis II. In vertebrates, premature arrest during oocyte maturation, as well as parthenogenetic release from the meiosis II arrest, is often the cause of infertility (Winston et al. 1991; Levran et al. 2002). In addition to gaining insight into the process of infertility, elucidation of the mechanism of meiotic arrest may increase our understanding of embryonic development, the molecular signal transduction pathways that operate in cell division, and cell cycle controls that may be altered in cancer cells. Over 30 years ago, YoshioMasui and ClementMarkert (1971) published an historic paper describing cytoplasmic control over the behavior of nuclei of both meiotic and mitotic cells. This paper described an activity in the cytoplasm of eggs from the leopard frog Rana pipiens that was able to initiate oocyte maturation when injected into immature G2-arrested oocytes, an activity they termed maturation-promoting factor (MPF). In the same paper, Masui and Markert discovered that microinjection of the same egg cytoplasm into one blastomere of a two-cell embryo produced a cleavage arrest in the injected blastomere, whereas the uninjected blastomere continued to divide normally. They observed that the “mitotic apparatus” of the injected blastomere was “arrested at metaphase. ” As a control, cytoplasm taken from immature oocytes or early embryos did not inhibit cleavage of the injected blastomere. This led to the supposition that a “specific cytoplasmic factor or factors is responsible for the inhibition of mitosis and cleavage. This hypothetical factor from the cytoplasm of maturing oocytes can be tentatively labeled ‘cytostatic factor. ’” Accordingly, the arrest of vertebrate eggs in meiosis II has since been renamed cytostatic factor (CSF) arrest, and the terms mature oocyte, metaphase-arrested oocyte, meiosis II-arrested oocyte, and CSF-arrested oocyte are synonymous with the term unfertilized egg. CSF, by definition, does not describe a single molecule or protein, but rather an activity found in the egg. This cell division inhibitor must accumulate during oocyte maturation, must be capable of functioning in meiosis II, and must be inactivated on fertilization or parthenogenetic activation. Since 1971, numerous attempts have been made to identify CSF(s) in vertebrate eggs and to elucidate the molecular mechanism of the meiotic metaphase arrest. Much of the progress in our understanding of the biochemistry and cell biology of CSF arrest since then has come from studies using the denuded oocytes, embryos, and cell-free extracts of the South African clawed frog Xenopus laevis. Although not as extensively studied, the oocytes of higher vertebrates, such as mice, recapitulate most features of Xenopus oocyte maturation, and contain components of CSF activity homologous to those that have been identified in the Xenopus system (Masui 2000; Kubiak and Ciemerych 2001). To understand the genesis of CSF activity and the mechanism of its action, it is important to consider the biology and biochemistry of oocyte maturation.