73. Female gametes. Ovarian follicles. Oogenesis. Structure of the mature oocyte.

Female Gametes

• Female gametes are oocytes (immature ova).
• Mature oocytes are haploid cells, containing half the diploid number of chromosomes.
• They are produced through a specialized process called oogenesis.

Oogenesis

• Definition: Oogenesis is the creation of an ovum (egg) and is the female form of gametogenesis.
• Location: The process starts in the germinal epithelium (ovarian surface epithelium), which gives rise to ovarian follicles.
• Early Development (Fetal Life):
• A small population of primordial germ cells migrates from the yolk sac to the gonadal primordia in the first month of embryonic life.
• These cells divide and differentiate as oogonia (primitive oocytes).
• By the fifth month of fetal development, there are over 7 million oogonia.
• Beginning in the third month, oogonia enter the first meiotic division but arrest at Prophase I after completing synapsis and recombination, without progressing to later stages. These arrested cells are called primary oocytes.
• Each primary oocyte becomes surrounded by flattened support cells (follicular cells) to form an ovarian follicle.
• Many primary oocytes are lost through a degenerative process called atresia, which continues throughout a woman's reproductive life. At puberty, ovaries contain about 300,000 oocytes.
• No new oocytes are produced postnatally.
• Post-Puberty (Monthly Cycle):
• Typically, only one primary oocyte per month completes Meiosis I just before ovulation. This division results in a secondary oocyte (which retains almost all the cytoplasm) and a small, non-viable first polar body. The secondary oocyte is a haploid cell (23 chromosomes).
• The nucleus of the secondary oocyte immediately begins Meiosis II but arrests at Metaphase II.
• Meiosis II is only completed if fertilisation occurs, producing a mature ovum and a second polar body. If not fertilised within about 24 hours, the secondary oocyte degenerates.

Hormonal Regulation:

• Follicle-stimulating hormone (FSH) from the pituitary stimulates follicular growth.
• Luteinizing hormone (LH) surge triggers ovulation and the completion of Meiosis I in the primary oocyte.