Oogenezis

Oogenesis is the process by which female gametes, or ova (eggs), are produced. This complex process begins during fetal development and continues throughout a woman's reproductive life. In the early stages of fetal development, primordial germ cells differentiate into oogonia, which are diploid cells. These oogonia then undergo mitosis, increasing their numbers, and eventually develop into primary oocytes. Primary oocytes then enter meiosis I but arrest in prophase I. This arrest persists until puberty.

Upon reaching puberty, hormonal signals trigger the completion of meiosis I in a select number of primary oocytes each menstrual cycle. Meiosis I results in two haploid cells: a secondary oocyte and a smaller polar body. The secondary oocyte contains most of the cytoplasm, while the polar body is a non-functional byproduct. The secondary oocyte then begins meiosis II but arrests again, this time in metaphase II. Ovulation occurs at this stage, releasing the secondary oocyte from the ovary.

Fertilization is required to complete meiosis II. If sperm penetrates the secondary oocyte, it stimulates the final meiotic division. This division produces a mature ovum and a second polar body. The ovum, now a mature haploid gamete, contains the necessary genetic material and cellular components to form a zygote upon fusion with a sperm. The polar bodies degenerate. Oogenesis results in the production of a single functional ovum and polar bodies, which is a contrast to spermatogenesis, where four functional sperm are produced from a single precursor cell. This unequal cytoplasmic division ensures the ovum is well-provisioned for early embryonic development.