An ovular cell, more commonly known as an ovum or egg cell, is the female reproductive cell that carries the genetic material necessary to begin a new life. It is the largest cell in the human body, measuring about 100 to 120 micrometers in diameter, making it visible to the naked eye. The ovum’s fundamental function is to wait for fertilization by a sperm cell, contributing half of the offspring’s genetic blueprint to form a zygote.
The Biological Structure and Function of the Egg Cell
The ovum’s anatomy is built around its reproductive purpose. At its core is the haploid nucleus, containing 23 chromosomes, which is half the genetic information found in other body cells. Surrounding this nucleus is a large volume of cytoplasm, rich in nutrients and specialized organelles, particularly mitochondria. These cytoplasmic components provide the energy and initial instructions the embryo needs to sustain its first few days of growth before it can implant in the uterus.
Two protective layers surround the cell membrane to safeguard the ovum and regulate fertilization. The innermost layer is the zona pellucida, a thick, transparent matrix of glycoproteins that acts as a selective barrier. It binds to sperm and helps ensure that only a single sperm cell penetrates and fertilizes the egg, preventing polyspermy. The outermost layer, the corona radiata, is composed of cells that nourish the ovum before its release and help the fallopian tube capture it after ovulation.
The Process of Ovulation
Ovulation is the event where a mature egg is released from the ovary, marking the transition point in the monthly menstrual cycle. This process is triggered by a hormonal signal, specifically a rapid surge in luteinizing hormone (LH) from the pituitary gland. The LH surge causes the dominant fluid-filled sac, or follicle, containing the egg to swell and rupture, ejecting the ovum from the ovary’s surface.
Once released, the egg is swept up by the finger-like projections of the fallopian tube, where it begins its journey toward the uterus. The egg remains viable for fertilization for a short window, typically only 12 to 24 hours after its release. If a sperm cell successfully fertilizes the ovum in the fallopian tube during this brief time, the resulting zygote begins dividing as it continues its path to the uterus. If fertilization does not occur, the egg rapidly deteriorates and is reabsorbed by the body.
Ovarian Reserve and Egg Lifespan
Unlike sperm, which are continuously produced, the ovum supply is finite and established before birth, a quantity referred to as the ovarian reserve. A female fetus reaches a peak of approximately six to seven million immature eggs (oocytes) during gestation, but this number immediately begins to decline. By the time of birth, the reserve has dropped to about one to two million oocytes, and by puberty, only a few hundred thousand remain.
The continuous loss of eggs is primarily due to a natural degenerative process called atresia, where follicles break down and disappear. As a woman ages, this ongoing depletion affects not only the total quantity of eggs but also their quality. Aged eggs are more susceptible to chromosomal abnormalities, which increases the likelihood of fertilization failure or miscarriage. By age 37, the reserve has often declined to about 25,000 eggs. This natural aging process is the primary factor driving the decline in female fertility potential.