Future Eggs and Sperm

Dive Deeper
Did you know that all of a woman’s eggs formed before she was born?

In fact, she had more egg cells as a fetus than at any other point in her life. Both the egg cells in a female, and the sperm-producing cells in a male come from primary germ cells that originate in the yolk sac. During the fifth week of pregnancy, 1000 to 2000 primary germ cells migrate from the yolk sac through the developing umbilical cord into the developing ovary or testes 1 These cells will provide the entire pool of future egg and sperm cells.

The picture below shows how eggs form in a process known as oogenesis. In females, the primary germ cells immediately continue dividing via mitosis. During mitosis, cells make an exact copy of all of their genetic information. In fact, there are special mechanisms to make sure that there are no copying mistakes in the primary germ cells when the DNA replicates. These cells have a full set of 46 chromosomes.2

This diagram depicts oogenesis. Oogenesis is the process by which a primary germ cell becomes an egg that can be released during ovulation and fertilized. Each primary oocyte undergoes meiosis so that only half of the genetic information from the mother gets passed to her child. (Image Credit: Science Source)

By four months after conception, the primary germ cells start dividing to create egg cells via meiosis. Meiosis creates two new cells, each with half the original number of chromosomes. The primary germ cells enter meiosis, but they get stuck at different points in the process. For example, starting around 4 months after conception, the primary germ cells enter a part of the cell cycle where they get ready to divide. This is called prophase. The resulting cells are called primary oocytes, or egg cells. The primary oocytes stay stuck in prophase until puberty.3

All oocytes form before birth. Interestingly, a female fetus will have produced over 7 million oocytes by the 5th month after conception, but many of these are selectively eliminated so that she only has 2 million oocytes when she is born.4 By puberty, only about 300,000 eggs remain. During a woman’s reproductive lifetime, she will only ovulate 300 to 400 total eggs.5

At puberty, a woman’s body starts selecting between 6 and 20 primary oocytes each month. The selected cells continue dividing via meiosis so that each resulting cell contains 23 of the original 46 chromosomes. One cell becomes the secondary oocyte and the other cell becomes a polar body, which cannot be fertilized.6

Each month, one primary oocyte fully matures. It gets released into the fallopian tube during ovulation. The egg remains stuck in another part of the cell cycle called metaphase and only finishes meiosis after a sperm enters the egg.  Now the genetic material from the sperm and egg can combine to create a zygote – a unique human individual.7

When does sperm form?

In males, the primary germ cells travel to the developing testes five weeks after conception and lie dormant until puberty. After sexual maturity, these primary germ cells, now called spermatagonia, both divide to replace themselves and divide to form sperm cells. Spermatagonia divide via meiosis to form sperm cells. Meiosis creates new haploid cells with half of the original number of chromosomes. The young sperm cells then get a cap, called the acrosome, which helps the sperm penetrate the egg. They also form a tail to help propel the sperm toward the egg. The process of growing a sperm takes about 74 days.8