ABSTRACT
Meiosis is a key process in spermatogenesis, with the final goal of production of gametes with haploid genetic content. A significant number of exhaustive revisions of this process have been published in the specialized literature [1-14]. In meiosis, DNA replication is followed by two consecutive cell divisions that reduce the number of chromosomes by half. The first meiotic division is reductional and involves the segregation of homologous chromosomes to opposite poles, while in the second meiotic division sister chromatid segregation occurs (Figure 2.1). Prophase of the first meiotic division includes highly complex events and consists of different substages (leptotene, zygotene, pachytene, diplotene, and diakinesis). During prophase I, homologous chromosomes pair; synaptonemal complexes are assembled and disassembled; and reciprocal exchanges between homologous chromatids take place, a phenomenon called meiotic recombination. In male meiosis, specifically at the zygotene and pachytene stages, chromosomes X and Y form a peculiar structure called a sex vesicle. At the end of prophase I, chromosome bivalents show chiasmata, which should be interpreted as the visible manifestation of recombination. Next, at metaphase I, bivalents reach maximum condensation, chiasmata are still visible, and bivalents appear arranged at the equatorial plate to segregate the homologous chromosomes to the opposite poles during anaphase I. As a result of the first meiotic division, two haploid cells are produced (each chromosome with two sister chromatids). During the second meiotic division, which starts without previous DNA replication, the cohesion between sister chromatids is lost, and the segregation results in four haploid cells (each chromosome with one chromatid).
