ABSTRACT
Life cycles typically are defi ned as sequential processes whereby the birth of an organism is followed by development to and through a phase of reproductive competence, and fi nally as it approaches death, it has either lost its reproductive potential or retained it for varying durations (1). Death before the end of the reproductive life span is the norm in nature for most mammals. Death after the demise of the reproductive competence is the norm for Homo sapiens. Mammals, not surprisingly then, vary widely in their duration of life span and in relation to the fraction of their overall life span that is dedicated to acquiring and sustaining their fecundity or reproductive potential (1). The life cycle paradigm can be applied at a cellular and molecular level to the natural history of the female germ line. For example, there is a period of time either during fetal development (primates, ungulates) or postnatally (rodents, lagomorphs) when primordial germ cells initiate oogonial mitotic proliferation in anticipation of their formal entry into meiosis and become arrested in the diplotene stage of meiotic prophase 1 (2). This is defi ned as the birth of the oocyte. The next stage of the oocyte life cycle is represented by the storage phase during which the germ cells remain in a state of meiotic arrest confi ned within the primordial follicle. Oocytes proceed through a growth phase during which the conditions of hypertrophy are satisfi ed by the accumulation of organellar and molecular determinants that support embryogenesis (3). Finally, the oocyte undergoes a defi nitive period of maturation around the time of ovulation in most mammalian species (4).
