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Mitotic division provides a continuous supply of spermatogonia, while meiotic division is a process whereby B spermatogonia undergo reduction division to form haploid spermatocytes [35]. This meiotic process not only halves the number of chromosomes in spermatocytes, but also ensures genetic diversity by DNA replication and crossing‐over, random events that produce genetically unique individual spermatozoa. This replication and crossing‐over of DNA material occurs during the first meiotic division, while the second meiotic division results in the formation of the haploid spermatocytes. Spermatocytes may be found at all stages of development in seminiferous epithelium due to the prolonged nature of the meiotic period of spermatogenesis, which ranges from 18 to 21 days in the bull (Figure 2.4a). At the completion of this meiotic process in bulls, each B spermatogonium will result in the production of four haploid spermatids, with a total of 64 spermatids emerging from a single active (A3) spermatogonium [152–154].

Figure 2.4 Schematic representations of the germinal epithelium of the bull testis during stage IV and stage VIII of the seminiferous epithelium cycle. Stage IV of the cycle (a) is characterized by the very active generation of primary spermatocytes into secondary spermatocytes during meiosis I and progress on with the completion of the meiotic division of meiosis II and the formation of spermatids. Stage VIII of the cycle (b) is characterized by the emerging free spermatozoa that have undergone differentiation from the round spermatids.

Source: From [151], © 2003, Germinal Dimensions Incorporated.