The last generation of type B spermatogonia divide and produce the primary sper-matocytes. These are the last diploid germ cells in the production of spermatozoa. Historically, they were called resting spermatocytes, but this is a misnomer, because these cells are not quiescent (13). The primary spermatocytes are the most mature germ cells to synthesize DNA in preparation for meiosis I and II. Moreover, these germ cells synthesize four times more DNA than is found in the haploid state. As the primary spermatocytes prepare for and enter the long prophase of meiosis I, they move from the basement membrane of the seminiferous tubule toward its lumen. At this point in the spermatogenesis process, germ cells bind to the Sertoli cells through specialized cellular junctions and remain so until the immature spermatozoa are released from the seminiferous epithelium. During this transition, the spermatocytes progress from leptonema, characterized by the formation of long, thin chromosomes, to zygonema, characterized by homologous chromosome pairing. As spermatocytes form the next cellular layer, the cytological event "crossing over" occurs. Because the chromosome pairs are thick, this phase of meiosis I is called pachynema. The duration of prophase of meiosis I is 2-3 wk, depending on the species. In rapid fashion after the completion of prophase I, the spermatocytes complete Meiosis I, forming haploid secondary spermatocytes that, in turn, complete meiosis II, resulting theoretically in four haploid spermatids.
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