Oogeneesi

Oogenesis is the developmental process by which female gametes, or ova, are produced within the ovaries. It involves the generation and maturation of haploid oocytes through meiosis, beginning with primordial germ cells that give rise to oogonia during fetal life and subsequently to primary oocytes that arrest in meiosis I.

In humans, oogonia proliferate by mitosis during fetal development, then enter meiosis I to become primary oocytes. By birth most individuals have a finite pool of primordial follicles, each containing a primary oocyte that is arrested in prophase I. Most of these oocytes will undergo atresia before adulthood.

At puberty, circulating hormones trigger cycles of follicle growth. Usually one follicle becomes dominant each cycle; its primary oocyte resumes meiosis I, completing it to form a secondary oocyte and a first polar body. Meiosis II begins but arrests at metaphase II and only completes if fertilization occurs.

Upon fertilization, meiosis II finishes, yielding a mature ovum and a second polar body. If fertilization does not occur, the oocyte and the follicle are shed during menstruation. Across a typical reproductive lifetime, about 400–500 oocytes are ovulated; the rest disappear through atresia.

Regulation involves the hypothalamic-pituitary-gonadal axis, with follicle-stimulating hormone promoting follicle development and luteinizing hormone triggering ovulation and luteinization. Oogenesis contrasts with spermatogenesis in producing one viable gamete per meiotic event and in having a prolonged, often decades-long arrest of meiosis I and II.