Primordial Follicle
Within eight to ten weeks of fetal development,
the primordial germ cells undergo multiple mitotic divisions
and differentiate into primordial follicles. The primordial follicle
consists of a primary oocyte surrounded by a layer of spindle-shaped
granulosa cells demarcated by a basal lamina. An oocyte is considered
primary if it has not yet undergone its first meiotic division.
Primordial follicles continue to multiply by mitosis and peak
at a number of approximately 7,000,000 at seven months of fetal
life. Before birth, the primary oocytes enter a final round of
DNA synthesis and are then arrested in prophase of the first
meiotic division until shortly before ovulation. As such, a primary
oocytes may remain in prophase I between 12 years (the onset
of puberty) and 45 years (the onset of menopause).