Oogenesis

Oogenesis is the production of eggs, or ova, the female gametes. In humans it begins before birth: in the fetal ovary, germ cells called oogonia multiply by mitosis and then enter meiosis as primary oocytes. These halt partway through the first meiotic division, in prophase I, and stay arrested inside follicles for years or decades. A newborn has one to two million of them, the number falls to a few hundred thousand by puberty, and no new ones are made, so the supply runs out around menopause.

From puberty, the surge of luteinizing hormone in each menstrual cycle lets one primary oocyte finish meiosis I. The division is unequal: nearly all the cytoplasm goes to the secondary oocyte, while a tiny first polar body receives the other set of chromosomes. The secondary oocyte is released at ovulation and pauses again at metaphase of meiosis II. It completes meiosis only if a sperm enters it, giving off a second polar body and leaving a mature ovum whose nucleus joins the sperm's to form a zygote.

Unlike spermatogenesis, oogenesis yields one large gamete rather than four small ones, keeping nutrients for the early embryo. The long arrest may help explain why errors in chromosome separation, such as those behind Down syndrome, become more common with maternal age.