Oogenesis

During fetal development, diploid oogonia in the developing ovaries undergo mitotic divisions to produce primary oocytes. These primary oocytes then enter prophase I of meiosis but become arrested in this stage, remaining suspended until puberty. This arrested state is crucial for preserving the oocyte pool throughout a woman's reproductive years.

A primordial follicle consists of a primary oocyte surrounded by a layer of pre-granulosa cells. By puberty, the ovaries contain approximately 400,000 primordial follicles, representing the total egg supply a woman will have throughout her reproductive life. These follicles remain dormant until recruited by hormonal signals during the menstrual cycle.

Following menarche, follicle-stimulating hormone (FSH) recruits a select number of primordial follicles each month. Among these recruited follicles, one typically becomes dominant, and its primary oocyte resumes meiosis I just before ovulation. This hormonal regulation ensures that only one mature oocyte is typically released per cycle.

When a primary oocyte completes meiosis I, it produces two cells of unequal size: a secondary oocyte, which retains most of the cytoplasm and nutrients, and a polar body, which eventually degenerates. This unequal division ensures the secondary oocyte has sufficient resources for early embryonic development if fertilization occurs.

The secondary oocyte is arrested at metaphase II of meiosis and is released from the ovary following a surge in luteinizing hormone (LH). This LH surge triggers ovulation, propelling the secondary oocyte into the fallopian tube where it may encounter sperm and become fertilized.

If fertilization does not occur in the fallopian tube, the secondary oocyte undergoes atresia and degenerates without completing meiosis II. This prevents the formation of a haploid ovum and ensures that only fertilized oocytes progress to complete meiosis and support embryonic development.

When a secondary oocyte completes meiosis II following fertilization, it produces a haploid ovum and a second polar body. The ovum contains the female genetic material and cytoplasm necessary for early embryonic development, while the polar body degenerates and plays no role in reproduction.