Differentiating spermatogenic cells are not arranged at random in the seminiferous epithelium; specific cell types are grouped together. These groupings or associations occur because intercellular bridges are present between the progeny of each pair of type Ap spermatogonia and because the synchronized cells spend specific times in each stage of maturation. All phases of differentiation occur sequentially at any given site in a seminiferous tubule as the progeny of stem cells remain connected by cytoplasmic bridges and undergo synchronous mitotic and meiotic divisions and maturation (see Fig. 21.10).
Each recognizable grouping, or cell association, is considered a stage in a cyclic process. The series of stages that appears between two successive occurrences of the same cell association pattern at any given site in the seminiferous tubule constitutes a cycle of the seminiferous epithelium. The cycle of the seminiferous epithelium has been most thoroughly studied in rats, in which 14 successive stages occur in linear sequence along the tubule. In man, 6 stages or cell associations are defined in the cycle of the seminiferous epithelium (Fig. 21.13). These stages are not as clearly delineated as those in rodents because in man the cellular associations occur in irregular patches that form a mosaic pattern.
Duration of spermatogenesis in humans is approximately 74 days
After injecting a pulse of tritiated thymidine, a specific generation of cells can be followed by sequential biop sies of the seminiferous tubules. In this way, the time required for the labeled cells to go through the various stages can be determined. Several generations of developing cells may be present in the thickness of the seminiferous epithelium at any given site and at any given time, which produces the characteristic cell associations. Autoradiographic studies have revealed that the duration of the cycle of the seminiferous epithelium is constant, lasting about 16 days in humans. In humans it would require about 4.6 cycles (each 16 days long), or approximately 74 days, for a spermatogonium produced by a stem cell to complete the process of spermatogenesis. It would then require approximately 12 days for the spermatozoon to pass through the epididymis. Approximately 300 million sperm cells are produced daily in the human testis. The length of the cycle and the time required for spermatogenesis are constant and specific in each species. Therefore, in any pharmacologic intervention (e.g., therapy for male infertility), if a drug is given that affects the initial phases of spermatogenesis, approximately 86 days are required to see the effect of that compound on sperm production.
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