An ovum, or egg, is merely the size of a pinhead, but it’s the only cell in the entire human body that can be seen with the naked eye. At birth, a girl has about 2 million immature egg cells stored in two ovaries, but less than a quarter of the cells exist by the time she reaches puberty. Of those, only 300 to 400 are released during her reproductive years, from about ages 12 to 50.

Inside the ovary each immature egg lies dormant for years in its own fluid-filled cavity called a follicle. About once a month—in the ovulation phase of the menstrual cycle—a handful of these follicles arise from their slumber and begin maturing into fully formed eggs. During the process, a large cell is created. Its nucleus holds 23 chromosomes, or half of the number that comprises a human’s DNA, which is the genetic blueprint for each individual. Surrounding the nucleus is cytoplasm, as well as the zona pellucida (clear zone), a membrane that encloses the cytoplasm and must be penetrated by a sperm to fertilize the egg. Usually only this one large cell is released per menstrual cycle.

That precious cell is coated with three protective layers when released in the process of ovulation. The inner one is a thin cellular membrane that is covered with the second, the microscopic zona pellucida. Finally, there’s the corona radiata (radiating crown) of cells. After the egg is released from the ovary into one of the fallopian tubes, it lives for about 3 days before it begins deteriorating. The sperm usually have about 72 hours to fertilize the egg.

ADDITIONAL FACTS

  1. The word ovum (plural ova) is Latin for “egg.”
  2. Eggs are released from either ovary at random; the ovaries don’t alternate months.
  3. Human sperm can survive for up to 7 days in the female reproductive system.