Egg cells, also known as ova (singular ovum), are the female reproductive cells in sexually reproducing organisms. They carry genetic information from the female parent. The egg cell’s primary function is to combine with a sperm cell, leading to fertilization and the development of an embryo. This union forms a single new cell containing the complete genetic blueprint for a new individual.
Origin and Lifespan
Egg cells originate in the ovaries, the female reproductive glands. The formation process, called oogenesis, begins before a female is born. During fetal development, precursor cells multiply and differentiate into primary oocytes. A female fetus at around 20 weeks of gestation may have approximately 6 to 7 million oocytes.
This number significantly decreases, with about 1 to 2 million oocytes present at birth. No new egg cells are generated after birth. By puberty, the egg count further reduces to between 300,000 and 500,000. While only about 400 eggs are typically released over a woman’s reproductive lifetime, many thousands of oocytes degenerate each month. The decline in egg count accelerates in the years leading up to menopause, eventually depleting entirely.
Structure and Purpose
The human egg cell is among the largest cells in the human body, measuring approximately 0.1 to 0.2 millimeters in diameter, roughly the size of a grain of salt. It is significantly larger than a sperm cell, with a volume about 10 million times that of a sperm cell. Its large size allows it to store nutrients and cellular components essential for early embryonic development.
An egg cell contains a central nucleus, which houses half of the genetic material (23 chromosomes) from the female parent. Surrounding the nucleus is the cytoplasm, a jelly-like substance rich in nutrients that provide nourishment for the developing embryo. The egg cell is protected by several outer layers.
The zona pellucida is a thick, transparent layer that surrounds the egg’s plasma membrane. This layer regulates sperm binding and prevents multiple sperm from fertilizing the egg. Outside the zona pellucida lies the corona radiata, which consists of follicular cells that nourish and protect the egg. The egg cell’s purpose is to contribute the female’s genetic material and provide initial nutritional support for a new organism.
The Fertilization Process
The egg cell plays a central role in fertilization, the process that initiates pregnancy. During ovulation, a mature egg is released from the ovary and is viable for only 12 to 24 hours. After release, finger-like projections called fimbriae, located at the end of the fallopian tube, sweep over the ovary to capture the egg. The egg then begins its journey through the fallopian tube, propelled by muscular contractions and microscopic cilia lining the tube.
Fertilization commonly occurs within the fallopian tube if sperm are present. When a sperm cell reaches the egg, it binds to the zona pellucida, triggering a series of events that allow it to penetrate the protective layers. Once a single sperm enters the egg, the membranes of the two cells fuse, and the sperm releases its genetic material into the egg.
This fusion of genetic material forms a single, diploid cell known as a zygote. The zygote contains a complete set of 46 chromosomes, half from each parent. The egg’s outer layers then undergo changes to prevent additional sperm from entering.