The mitotic division of type B spermatogonia produces primary spermatocytes. They replicate their DNA shortly after they form and before meiosis begins, so that each primary spermatocyte contains the normal chromosomal number (2n) and double the amount of DNA (4d ). Each chromosome consists of two sister chromatids; hence, 4d amount of DNA.
Meiosis I results in reduction of both the number of chromosomes (from 2n to 1n) and the amount of DNA to the haploid condition (from 4d to 2d); thus, secondary spermatocyte is characterized by haploid number of chromosomes (1n) and 2d amount of DNA. Because no DNA replication precedes meiosis II, after this division each spermatid has the haploid (1n) number of chromosomes, each containing a single chromatid (1d). Meiosis is described in detail in Chapter 3 (see page 89); a brief description of spermatocyte meiosis follows.
Prophase of the first meiotic division, during which the chromatin condenses into visible chromosomes, lasts up to 22 days in human primary spermatocytes. At the end of prophase, 44 autosomes and an X and a Y chromosome, each having two chromatin strands (chromatids), can be identified. Homologous chromosomes are paired as they line up on the metaphase plate.
The paired homologous chromosomes, called tetrads because they consist of four chromatids, exchange genetic material in a process called crossing-over. During this exchange, the four chromatids rearrange into a tripartite structure called a synaptonemal complex. This process ensures genetic diversity. Through genetic exchange, the four spermatids produced from each spermatocyte differ from each other and from every other spermatid. After crossing-over is complete, the homologous chromosomes separate and move to the opposite poles of the meiotic spindle. Thus, the tetrads, which have been modified by crossing-over, separate and become dyads again. The two chromatids of each original chromosome (although modified by crossing-over) remain together. This is just the opposite of what happens in mitosis, in which the paired chromatids—one representing the “template” and the other, newly synthesized DNA—separate.
FIGURE 22.9 • Schematic diagram illus trating the generations of spermato genic cells. This diagram shows the clonal nature of the successive generations of spermatogenic cells. Cytoplasmic division is complete only in the primitive type A dark spermatogonia that serve as stem cells. All other spermatogenic cells remain connected by intercellular bridges as they undergo mitotic and meiotic division and differentiation of the spermatids. The cells separate into individual spermatozoa as they are released from the seminiferous epithelium. The residual bodies remain connected and are phagocytosed by the Sertoli cells. (Reprinted with permission from Dym M, Fawcett DW. Further observations on the numbers of spermatogonia, spermatocytes, and spermatids connected by intercellular bridges in the mammalian testis. Biol Reprod 1971;4:195–215.)
The movement of a particular chromosome of a homologous pair to either pole of the spindle is random (i.e., maternally derived chromosomes and paternally derived chromosomes do not sort themselves out at the metaphase plate). This random sorting is another source of genetic diversity in the resulting sperm.