TABLE 2

Duration of spermatogenesis and the seminiferous epithelial cycle in rodents and men

The duration of spermatogenesis (in days) is the time it takes for one type A spermatogonium to form 4096 mature spermatozoa (in theory; see Fig. 4) with half of the original number of chromosomes (see Fig. 4) in rodents, so that sperm will be released from the seminiferous epithelium into the tubule lumen at spermiation. This can be estimated by using [³H]thymidine incorporation. Because of germ cell apoptosis and/or degeneration as a result of the limited number of Sertoli cells in the rat testis (∼5 × 10⁶ Sertoli cells/mouse testis and ∼30–40 × 10⁶ Sertoli cells/rat testis)(Orth, 1982; Berndtson and Thompson, 1990; França et al., 1998), only ∼25% of the germ cells become spermatozoa (Shaha, 2008; Shaha et al., 2010) and are released to the tubule lumen at spermiation (O'Donnell et al., 2011). The duration of the seminiferous epithelial cycle (or one cycle of the seminiferous epithelium) is the time it takes to complete the series of morphological changes between two appearances of the same developmental stage, such as stage VIII, in a given area of the seminiferous epithelium in a tubule under stereomicroscopy (Clermont, 1972; Parvinen, 1982; França et al., 1998; Hess and de França, 2008). Thus, the duration of spermatogenesis takes ∼4.2 (∼68 days) to ∼4.5 cycles (∼58 days) in humans (Amann, 2008) and rodents (Amann and Schanbacher, 1983), respectively, to complete to allow type A spermatogonia to differentiate to sperm. The stages of spermatogenesis are divided according to the specific cellular associations using cross-sections of the seminiferous tubules originally based on the use of Periodic acid-Schiff'reaction (PAS) of the acrosome in developing spermatids (Clermont, 1972; Hess and de FrançaZ, 2008), which can easily be defined with the use of hematoxylin and eosin staining using paraffin sections of testes (see Fig. 2).