Oögenese

Oögenese, commonly known as oogenesis, is the developmental process by which female gametes, or ova, are formed and mature. It occurs in the ovaries and involves both mitotic proliferation of germ cells and subsequent meiotic divisions that produce a mature ovum along with polar bodies.

Origin and early development: Primordial germ cells migrate to the gonads and differentiate into oogonia, which proliferate by mitosis during fetal life. In most species, oogonia enter meiosis and become primary oocytes, which initiate meiosis I but become arrested in prophase I until puberty or birth, depending on the species.

Maturation and completion: During each reproductive cycle, selected primary oocytes resume meiosis I to form a secondary oocyte and the first polar body. Meiosis II begins but is typically arrested at metaphase II and only completes after fertilization, yielding a haploid ovum and a second polar body.

Cytoplasmic asymmetry and yield: Oogenesis produces one large, cytoplasm-rich ovum and typically one or more small polar bodies that eventually degenerate. This unequal cytokinesis ensures most cytoplasm is retained in the ovum to support early embryonic development.

Regulation and variation: The process is regulated by the hypothalamic-pituitary-gonadal axis, with follicle-stimulating hormone promoting follicle development and luteinizing hormone triggering ovulation; estrogens and progesterone coordinate maturation and the uterine lining. Oogenesis shows variation among taxa; in placental mammals it relies on a finite pool of oocytes formed prenatally, while other species may exhibit different timings. Oocytes can remain arrested for extended periods, and maternal age is associated with increased risk of meiotic errors and aneuploidies.