After their initial development in the embryonic gonads, the immature male germ cells, or spermatogonia, are mostly dormant in the testes until the onset of puberty. At this time, the production of the sperm, or spermatogenesis, takes place when the spermatogonia begin to mature. The spermatogonia, located in numerous tiny tubules contained in the testes, undergo a period of proliferation by mitosis, and then some of the spermatogonia differentiate into the primary spermatocytes. Each diploid primary spermatocyte produces two daughter cells, the secondary spermatocytes. In turn, each of the two secondary spermatocytes produces two haploid spermatids. A lengthy period of maturation then takes place during which the spermatids are transformed from spherical cells into the characteristic streamlined shape of the sperm cells.
Two special features that characterize human spermatogenesis are the distribution of the chromosomes and the incomplete division of the cytoplasm. In spermatogenesis, however, the daughter cells are not all identical in chromosomal makeup. One important difference in the chromosomal makeup of the resulting cells involves the distribution of the "sex chromosome." Of the 46 chromosomes (23 matching or homologous pairs) in each human cell, one pair, the sex chromosomes, is specially involved in sex determination. Since there are two types of sex chromosomes, they are not always a homologous pair. One is a medium-sized chromosome called the X chromosome; the other is a smaller chromosome called the Y chromosome. Normally, a female possesses two X chromosomes and a male possesses one X and one Y chromosome. The other 22 chromosome pairs are similar in both sexes. Since pairs of homologous chromosomes are separated during meiosis, of the four spermatids produced during spermatogenesis, two contain the haploid X chromosome and two contain the haploid Y chromosome.
Another characteristic of spermatogenesis is that the cytoplasm of the developing germ cells does not separate completely to produce individual cells during the stages of mitosis and meiosis. Due to this incomplete separation, the daughter cells are connected by cytoplasmic bridges that remain until the final stage of sperm differentiation. During this procedure, a great deal of the mass of the cell is discarded and only a small amount of cytoplasm is incorporated into the functional sperm cell, with the chromosome tightly packed inside. It is not until the sperm cells are separated from the residual bodies that they become mature sperm, obtaining the streamlined shape needed to move in the female reproductive tract.