Oogenesis is the process of female gamete or egg cell production that occurs in the ovaries. It begins during fetal development and continues throughout the reproductive life of a woman until menopause. The process involves the formation and development of immature egg cells or oocytes, which undergo meiosis to produce haploid gametes or mature eggs.
During fetal development, oogonia or germ cells multiply by mitosis and differentiate into primary oocytes. These primary oocytes are arrested in prophase I of meiosis I and remain dormant until puberty. At puberty, one or more primary oocytes are stimulated to resume meiosis I by follicle-stimulating hormone (FSH) and develop into a Graafian follicle, which eventually leads to ovulation.
The primary oocyte within the Graafian follicle undergoes the first meiotic division, producing two haploid cells: a secondary oocyte and the first polar body. The secondary oocyte, which contains most of the cytoplasm and organelles, is released during ovulation and enters the oviduct. If fertilization occurs, the secondary oocyte completes meiosis II to form a mature haploid ovum and a second polar body. The ovum then fuses with the sperm to form a zygote, which develops into an embryo.
If fertilization does not occur, the secondary oocyte degenerates and the corpus luteum forms from the remnants of the Graafian follicle. The corpus luteum secretes progesterone and prepares the endometrium for implantation. If implantation does not occur, the corpus luteum degenerates, progesterone levels drop, and the endometrium is shed during menstruation.
Overall, oogenesis is a complex process that involves multiple stages of meiosis, follicular development, and hormonal regulation. It ensures the production of genetically diverse and functionally competent eggs for reproduction.