Human Reproduction

There is a tremendous difference between men and women in the way they form their reproductive cells. First, let us begin with a short description of spermatogenesis, the process of making sperm. Spermatogonia (sperm stem cells) are the first cells in the lineage that leads to sperm. They have the ability to continually divide by mitosis. At puberty, some of these spermatogonia begin the process of meiosis, and each produces four haploid cells containing twenty-three chromosomes. These haploid cells also begin the process of compacting the DNA in their nucleus, forming the sperm head, and gradually developing the sperm tail, with a concomitant loss of much of their cellular material. This process, from beginning to end, takes almost fifty days. It is a continuous process, so once it begins, the testes contain reproductive cells at every stage of development. Mature sperm cells accumulate, and a single ejaculate contains hundreds of millions of sperm.

In quite a contrast, the process of making ova or eggs in the female begins before birth. While the young female embryo is still barely recognized as a pregnancy, the egg stem cells divide to make oocytes. By the eighth week of fetal development, the total number of oocytes that a female will ever carry is present, about half a million. These oocytes then begin the process of meiosis. Unlike sperm, however, oocytes do not divide equally in meiosis. Only one out of the four cells with a haploid chromosome number becomes a potential egg. But even further removed from spermatogenesis, meiosis in oocytes stops after the DNA replicates. This happens well before the female baby is born. Thus, all half-million potential egg cells in a female are in "suspended animation" at this early stage of meiosis. At puberty, one or two eggs are released each month in ovulation. Upon ovulation, the first of two divisions of meiosis occurs in the released egg. This means that some eggs are in this suspended state of animation for over fifty years! Then, only if a sperm fertilizes this egg does the egg quickly complete the second division of meiosis to make the haploid egg. In this way, the new individual is result of the joining of a haploid egg and a hap-loid sperm.

The above process of forming our gametic cells is important to understand, especially in light of the action of mutagens and teratogens. When we are exposed to mutagens or teratogens, our potential progeny, our reproductive cells, are also getting exposed. In the case of males, the cells that go on to make sperm are vulnerable to mutagenic agents. Because sperm cells are continually being made, sperm cells were thought to be less vulnerable than female gametes, which are never renewed. However there are now documented cases of male exposure to mutagens that result in higher incidence of leukemia among their children. Of course, for females, the window of vulnerability begins even before birth. Thus, exposing unborn females to radioactive fallout has had long-term consequences for girls born to exposed mothers.

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