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, six stages or cell associations are defined in the cycle of the seminiferous epithelium (Fig. 22.13). 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 biopsies 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.
Waves of the Seminiferous Epithelium
As indicated above, the cycle of the seminiferous epithelium describes changes that occur with time at any given site in the tubule. In addition, the wave of the seminiferous epithelium describes the distribution of patterns of cellular association (stages) along the length of the tubule. In rodents and other mammals that have been studied, including subhuman primates, each stage occupies a significant length of the seminiferous tubule, and the stages appear to occur sequentially along the length of the tubule. In the rat, there are approximately 12 waves in each tubule. A transverse section through the tubule usually reveals only one pattern of cell associations. There are no waves in the human seminiferous epithelium. Each pattern of cellular associations (stage of the cycle) has a patch-like distribution in the human seminiferous tubule (Fig. 22.14). Patches do not extend around the circumference of the tubule, nor are they in sequence. Therefore, a transverse section through a human seminiferous tubule may reveal as many as six different stages of the cycle arranged in a pie-wedge fashion around the circumference of the tubule.
FIGURE 22.13 • Schematic drawing of the stages of the human seminiferous epithelium. This diagram shows each of the six recognizable cell associations that occur in the cycle of the human seminiferous epithelium. Sa, Sb, Sc, and Sd indicate spermatids in various steps of differentiation. (Based on Clermont Y. The cycle of the seminiferous epithelium in man. Am J Anat 1963;112:50.)