31.16 : Oogenesis

Oogenesis, the process of developing egg cells (female gametes), occurs within the ovaries and is fundamental to female fertility. This sequence begins during fetal development when diploid oogonia in the developing ovaries undergo mitotic divisions to produce primary oocytes. By birth, these primary oocytes enter prophase I of meiosis but become arrested in this stage, remaining suspended until puberty.

Each primary oocyte is surrounded by a layer of pre-granulosa cells, forming what is known as a primordial follicle. When a female reaches puberty, her ovaries contain approximately 400,000 primordial follicles. With the onset of menarche and under the influence of the follicle-stimulating hormone (FSH), a select number of these follicles are recruited each month. Among these, one follicle typically becomes dominant, and its primary oocyte within it resumes meiosis I just before ovulation. This division produces two cells of unequal size: a secondary oocyte, which retains most of the cytoplasm, and a polar body, which eventually degenerates. The secondary oocyte then commences meiosis II but is arrested again at metaphase II and is released from the ovary following a surge in luteinizing hormone (LH).

If fertilization occurs within the fallopian tube, the secondary oocyte completes meiosis II, creating a haploid ovum and another polar body. However, if fertilization does not happen, the secondary oocyte undergoes atresia and does not complete meiosis II. This tightly regulated process, influenced by hormones like FSH, LH, estrogen, and progesterone, ensures proper oocyte maturation and release, which are essential for successful fertilization and embryonic development.