Oogenesis Near the time of bih, all primary oocytes have staed prophase of meiosis I, but instead of proceeding into metaphase, they enter the diplotene stage, a resting stage during prophase that is characterized by a lacy network of chromatin Primary oocytes remain in prophase and do not finish their first meiotic division before pubey is reached, apparently because of oocyte maturation inhibitor (OMI), a substance secreted by follicular cells. The total number of primary oocytes at bih is estimated to vary from 700,000 to 2 million. During childhood most oocytes become atretic; only approximately 400,000 are present by the beginning of pubey, and fewer than 500 will be ovulated. Some oocytes that reach maturity late in life have been dormant in the diplotene stage of the first meiotic division for 40 years or more before ovulation. Whether the diplotene stage is the most suitable phase to protect the oocyte against environmental influences is unknown. The fact that the risk of having children with chromosomal abnormalities increases with maternal age indicates that primary oocytes are vulnerable to damage as they age. (Refer: Langman's Medical Embryology, 9th Edition, Chapter. 1, Pg 21 - 22)