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THE HUMAN REPRODUCTIVE SYSTEM: GENERAL PRINCIPLES OF SEXUAL REPRODUCTION

During sexual reproduction, specialized reproductive cells called gametes combine to form a zygote destined to develop into a new individual. The characteristics of these two gametes, the ovum and the sperm, enable them to execute the functions that are unique to them as well as those that they share in common. In many ways, the reproductive work that the sperm and the ovum do together represents a highly successful division of labor.

Sexual reproduction begins when the primordial gametes migrate to the developing zygote's gonads (ovaries in women, testes in men). There the production of the mature gametes, the egg and the sperm, begins. In tracing the development of the egg (oogenesis) and sperm (spermatogenesis), we will notice that they develop in distinctive ways that permit them to communicate the correct amount of genetic information to the zygote. When somatic (or body) cells divide, each new cell must have all the genetic information of the parent cell, information that is transmitted through the process of mitosis (see the appendix). But when gametes, or germ cells, divide, each new cell must have only half the information of the parent cell, information that is transmitted through meiosis (see the appendix). In humans, the diploid (or parent) cells contain 46 chromosomes composed of 23 homologous pairs of chromosomes, one individual of each pair coming from the mother and the other from the father. In contrast, the haploid cells contain 23 chromosomes (that is, 23 individual chromosomes, not 23 pairs of homologous chromosomes), obtained neither exclusively from the mother nor exclusively from the father. Rather, due to so-called genetic shuffling, these 23 individual chromosomes are selected in random fashion partly from the maternal diploid cell and partly from the paternal diploid cell. Since new combinations of genes are constantly being created first through meiosis and then through the fusion of the egg cell and sperm cell, human individuality is not at risk. No one of us will look entirely like his or her mother or father.

Thus far we have been discussing human sexual reproduction in general terms, focusing neither on the sperm nor on the eggs. If we are to appreciate the complexity of this process, however, it is necessary to understand fully the nature and production of the sperm and the egg separately. For each of these two gametes, we will first describe the cell, then discuss the mechanism and hormonal control of its production, and finally describe the reproductive tracts (the male tract and the female tract) through which the gametes are transported and in which both the gametes and the developing offspring are housed and nourished.

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