Future Eggs and Sperm

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 gestational week 7, 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

By four months, the primary germ cells start dividing to create egg cells. Egg cells will undergo two rounds of a special form of cell division known as “meiosis,” where the genetic information within the cell will be reduced to half of the normal amount. Primary germ cells begin the first round of meiosis, and then pause half-way in a stage of the process known as “prophase,” where they will remain until puberty. During prophase genetic information is exchanged between the chromosomes originating from the woman’s mother and her father, producing chromosomes with unique combinations of genetic information that will be inherited by her children. This process is called “crossing over.” These cells, called primary oocytes, remain in prophase until puberty.³

All oocytes form before birth. Interestingly, a female fetus will have produced over 7 million oocytes by five months gestation, 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, each month, between 6 and 20 primary oocytes begin to mature.  The maturing cells complete the first round of meiosis, eliminating half of their original DNA in a small structure known as a polar body.  The egg cells are now known as secondary oocytes.  They contain 23 pairs of chromosomes, but because of the exchange of genetic information during prophase of meiosis I, each chromosome of the pair contains a new arrangement of information—allowing any child produced by fertilization of the oocyte to have its own unique genome.⁶

Each month, one secondary oocyte typically gets released into the fallopian tube during ovulation.  If more than one oocyte matures and is released, a woman can become pregnant with fraternal (non-identical) twins.  After ovulation, the secondary oocyte can be fertilized, but it remains in a stage of the second meiotic cycle known as metaphase.  The second meiotic cycle (meiosis II) is only completed after sperm-egg fusion.  Sperm-egg fusion produces the one-cell embryo, or zygote, with a unique genome with each chromosome being different from either parent and from all of the baby’s four grandparents.⁷

In males, the primary germ cells travel to the developing testes around week 7 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