Oogenesis, the biological process of egg formation, begins long before a female is born and continues throughout her reproductive years. This process creates an ovum, or egg, the female gamete essential for human reproduction. The ovum is the largest cell in the human body, uniquely equipped to contribute half of the genetic material needed to form a new individual.
Formation Before Birth
Egg cell formation begins early, while a female fetus develops. This initial stage involves primordial germ cells, precursors to both sperm and eggs. These cells originate in an extra-embryonic region, like the yolk sac, and migrate to the embryonic ovaries. There, primordial germ cells differentiate into oogonia.
The oogonia then undergo rapid mitotic divisions, significantly increasing their numbers. By around 20 weeks of gestation, a female fetus may have approximately 6 to 7 million germ cells in her ovaries. These oogonia then transform into primary oocytes.
These primary oocytes then initiate meiosis I but arrest at prophase I. A female is born with her lifetime supply of these primary oocytes, encased within primordial follicles. Though a female fetus starts with millions, this number significantly decreases by birth to 1 to 2 million, and further declines to between 300,000 and 500,000 by puberty.
Maturation and Release
Egg maturation and release resumes after birth, beginning at puberty with menstrual cycles. Each cycle involves recruiting a cohort of primary oocytes from the resting pool. These oocytes are housed within ovarian follicles, fluid-filled sacs that provide a nurturing environment for the developing egg.
Follicle development progresses through several stages: primordial, primary, secondary, and finally to a mature Graafian follicle. Hormones, particularly follicle-stimulating hormone (FSH), promote the growth of these follicles. As a follicle matures, the primary oocyte within it completes meiosis I, resulting in two cells of unequal size: a large secondary oocyte and a smaller first polar body.
The secondary oocyte then begins meiosis II but pauses at metaphase II. A surge in luteinizing hormone (LH) triggers ovulation, when the mature Graafian follicle ruptures, releasing the secondary oocyte from the ovary. This released secondary oocyte, still in metaphase II, is ready for fertilization. Meiosis II will only complete if a sperm fertilizes the egg.
The Egg’s Journey and Uniqueness
After ovulation, the released secondary oocyte travels through the fallopian tube. It remains viable for fertilization for a short window, between 12 to 24 hours. If fertilization does not occur, the egg degenerates and is reabsorbed by the body.
A distinguishing feature of oogenesis, compared to sperm production, is the finite number of eggs a female possesses from birth, as no new egg cells are generated during a woman’s lifetime. The quality of these eggs can also decline with age, partly due to the extended period they remain in a suspended meiotic state, increasing the chance of chromosomal errors during later division.
Another unique aspect of egg formation is unequal cytokinesis, the uneven distribution of cytoplasm during cell division. During meiosis, the dividing egg cell ensures almost all cytoplasm and its contents are concentrated into one large secondary oocyte. This process results in smaller polar bodies, which contain minimal cytoplasm and degenerate, allowing the future embryo to receive a rich supply of nutrients and cellular machinery for early development.