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human eggs

Number of eggs

A woman has her maximum number of potential eggs (primary oocytes) while still a fetus, more than 7 million. By birth the number has fallen to 1 or 2 million, and by puberty to about 300,000.¹ Only 300 to 400 reach maturity.

For many years biologists assumed that at birth female mammals of most species had all the eggs they would ever have.² Then in 2004 researchers reported³ discovering germline stem cells in mouse ovaries. These cells enable mice to continue producing new oocytes and follicles after birth. In 2012, researchers reported that the ovaries of reproductive-age women also possess small numbers of germline stem cells similar to those of the mouse. Thus, women may be able to produce new eggs after they are born.

1. T. G. Baker.
A quantitative and cytological study of germ cells in human ovaries.
Proceedings of the Royal Society of London. Series B, Biological Sciences, vol. 158, no. 972 (October 22, 1963).

2. S. Zuckerman.
The number of oocytes in the mature ovary.
Recent Progress in Hormone Research, vol 6, pages 63-108 (1951).

3. Joshua Johnson, Jacqueline Canning, Tomoko Kaneko, James K. Pru & Jonathan L. Tilly.
Germline stem cells and follicular renewal in the postnatal mammalian ovary.
Nature, vol. 428, pages 145-150 (11 March 2004).
doi:10.1038/nature02316;

4. Yvonne A. R. White, Dori C. Woods, Yasushi Takai, Osamu Ishihara, Hiroyuki Seki & Jonathan L. Tilly.
Oocyte formation by mitotically active germ cells purified from ovaries of reproductive-age women.
Nature Medicine, 2012 (published online 26 February 2012)
doi:10.1038/nm.2669.

Size of eggs

A human oocyte is about 100 micrometers in diameter, which is large for a cell. If it is not fertilized within a day of its ejection from the ovary, it dies.