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The Production of Eggs
Before a baby girl is born, all of the cells that will later grow into eggs are already formed in her ovaries. In their primitive beginnings, the cells are called oogonia. These oogonia first turn into primary oocytes and then into secondary oocytes, some of which eventually give rise to mature eggs (ova).
The primary oocytes - a large, but gradually dwindling number
The process of egg production, called oogenesis, begins in the female fetus, but soon comes to a halt at birth. Thus, every girl is born with nearly 500,000 primary oocytes which remain in their state of suspended development until she reaches puberty. During this time most of them gradually die. By the time a girl reaches puberty, there may be no more than 30,000 primary oocytes left that are capable of further development. At the age of thirty, this number has dwindled even further to about 10,000, and when the woman reaches her menopause, all primary oocytes are gone.
The mature eggs (ova) - a small number produced monthly until menopause
Once the process of egg production has resumed during puberty, one or several mature eggs are produced each month by either one of her ovaries until both of them cease functioning following menopause. In the course of her fertile life, a woman may produce some 400 mature eggs. Of course, only a very small fraction of these can ever contribute to conceptions. All of this provides a striking contrast to the way sperm cells are produced in the male (continuous production of millions of sperm daily, beginning with puberty).
The monthly process of egg maturation
The development of a mature egg proceeds in several steps:
- Each primary oocyte is contained in a cluster of supporting cells. These clusters lie beneath the outer layer of the ovary. Each month, under the influence of certain hormones, one of the clusters grows to a point where it appears as a rather large blister on the surface of the ovary. This blister is called a Graafian follicle (after the 17th century Dutch anatomist Regnier de Graaf). During the period of follicle growth, the primary oocyte, which like any other female body cell contains 46 chromosomes (including two X chromosomes), divides into two new cells of very unequal size: a relatively large secondary oocyte and a minute socalled polar body. In this division, the 46 chromosomes are split apart, and half of them are allotted to each of the new cells. Thus, the secondary oocyte as well as the polar body each contain only 23 chromosomes (including one X chromosome). The polar body dies and disintegrates.
- Only the secondary oocyte is destined for further maturation. First, it floats freely inside the growing follicle which contains fluid. Eventually, the follicle bursts, releasing the secondary oocyte into the abdominal cavity. This release is known as ovulation. The secondary oocyte then enters the nearest Fallopian tube.
- The scondary oocyte now divides again into two new cells of unequal size: a relatively large ootid (mature egg cell) and a minute second polar body. However, this time the division reproduces rather than splits the number of chromosomes. Thus, both of the new cells retain 23 chromosomes (in each case including one X chromosome). This last division and the expulsion of the second polar body occur only after fertilization. While the second polar body dies just as the first one, the 23 chromosomes of the mature egg cell unite with the 23 chromosomes of the sperm cell, thus forming a new cell (the zygote) which again contains 46 chromosomes like all other cells of the body.
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