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 number of serious mitochondrial diseases--is refreshed and purified as it passes, via the oocyte's cytoplasm, from one generation to the next; and (2), endeavouring to discover what cytoplasmic factor(s) it is that permits some eggs but not others to produce viable embryos and ongoing pregnancies. We review here in detail the passage of mitochondria through the female germ cell line. For mitochondria, the processes of oogenesis, follicle formation and 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 such a strong evolutionary imperative that we should expect at least some features of ovarian follicular formation, function and loss to be primarily adapted to this specific purpose. We predict, moreover, that to prevent accumulation 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 oöpause.