The female body possesses a system for housing reproductive cells, or eggs. Understanding where these eggs are stored and how they develop is central to female reproductive biology, ensuring their availability throughout a woman’s reproductive years.
The Ovaries: Primary Egg Repositories
The ovaries are the primary organs for storing eggs. These two small, almond-sized, oval glands are located in the pelvic cavity, one on each side of the uterus. They house egg cells and produce hormones that regulate the menstrual cycle and support pregnancy.
The ovaries attach to the uterus near the opening of the fallopian tubes. Within them, they contain female gamete cells, known as oocytes or eggs. This placement allows for efficient egg collection after release.
Follicular Protection and Development
Eggs are not stored loosely within the ovaries; instead, each immature egg, or oocyte, is encased within a tiny fluid-filled sac called an ovarian follicle. These follicles protect and nourish the eggs as they mature, and also secrete hormones that regulate the menstrual cycle.
There are different stages of follicular development, starting from primordial follicles that house immature eggs. As an egg prepares for release, its surrounding follicle progresses through stages such as primary, secondary, and tertiary (Graafian) follicles, growing in size and complexity. This encapsulation ensures the egg’s viability and provides the environment for its eventual maturation.
Egg Count and Lifespan
Females are born with their entire lifetime supply of eggs; no new eggs are produced after birth. A female fetus at 20 weeks of gestation typically has between 6 to 7 million eggs, which decreases to 1 to 2 million by birth. This number continues to decline before puberty.
By puberty, a female usually has around 300,000 to 500,000 eggs remaining. The continuous reduction in egg count occurs due to a natural process called atresia, which is the degeneration and reabsorption of follicles. Only about 300 to 500 of these eggs will mature and be released during a woman’s reproductive lifespan. The quantity and quality of eggs decline with age, with a more rapid decrease in fertility observed after age 35, eventually leading to menopause when virtually no eggs remain.
From Storage to Release
After being stored and maturing within a follicle, a mature egg is eventually released from the ovary in a process called ovulation. This occurs around day 14 of an average 28-day menstrual cycle. A surge in luteinizing hormone (LH) triggers the follicle to rupture, expelling the egg.
Once released, the egg is swept into the nearby fallopian tube by small, finger-like projections called fimbriae. Within the fallopian tube, the egg can be fertilized by sperm. If fertilization does not occur within 12 to 24 hours, the egg will break down and be reabsorbed by the body, and a new cycle of egg maturation will begin.