“…In all the mammalian species studied, there is an oversupply of germ cells within the late fetal ovary (Coucouvanis et al 1993, Vaskivuo et al 2001, Vaskivuo & Tapanainen 2002. Depending on the species, many oocytes are eliminated by apoptosis prior to and at the time of birth (Ratts et al 1995, Reynaud & Driancourt 2000, Shirota et al 2003, De Felici 2004, and those that remain in the neonatal mouse ovary acquire and are surrounded by granulosa cells, establishing a pool of diplotene-arrested oocytes identified as primordial follicles (Pedersen 1969, Peters 1969. The enclosure of oocytes by granulosa cells during the first postnatal week of ovarian development exhibits temporal and spatial heterogeneity within the ovary, since larger diplotene-arrested oocytes that form primordial follicles first appear in the deep cortex and medulla (Byskov & Nielson 2003).…”