The bottleneck: mitochondrial imperatives in oogenesis and ovarian follicular fate

Molecular geneticists and ovarian physiologists today face the challenge of defining and reconciling two major biological imperatives that each center on oogenesis, folliculogenesis and competition between ovarian follicles: (1), defining how the mitochondrial genome-important in both aging and a nu mber of serious mitochondrial diseases-is refreshed and purified as it pass es, via the oocyte's cytoplasm, from one generation to the next; and (2), e ndeavouring to discover what cytoplasmic factor(s) it is that permits some eggs but not others to produce viable embryos and ongoing pregnancies. We r eview here in detail the passage of mitochondria through the female germ ce ll line. For mitochondria, the processes of oogenesis, follicle formation a nd loss constitute a restriction/amplification/constraint event of the kind predicted by L. Chao for purification and refinement of a haploid genome. We argue that maintaining the integrity of mitochondrial inheritance is suc h a strong evolutionary imperative that we should expect at least some feat ures of ovarian follicular formation, function and loss to be primarily ada pted to this specific purpose. We predict, moreover, that to prevent accumu lation of mild mitochondrial genomes in the population there is a need for physiological female sterility prior to total depletion of ovarian oocytes, a phenomenon for which there is empirical evidence and which we term the o opause. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.