Incorporated.
Seminiferous Epithelial Cycle and Wave of Spermatogenesis
The cycle of the seminiferous epithelium refers to a cohort of germ cells that progress by clonal expansion within a segment of the seminiferous tubule and change in synchrony with other layers over time through a series of well‐characterized cellular associations referred to as stages [35, 149]. Cross‐sections along the length of the seminiferous tubules will have different morphological appearances, but within a given section the cellular activity will be similar. Successive sections of the seminiferous tubules will have cohorts of germ cells at sequential stages of development with repetition, thus producing a “wave” effect along the seminiferous tubules [35]. Germ cell development progresses from the basal lamina of the seminiferous tubule, and cells migrate toward the tubule lumen and not in a lat eral direction. This synchronous behavior is thought to be associated with biochemical signaling facilitated by the intercellular bridges, but the process is not fully understood. In the bull, it takes about 4.5 cycles of the seminiferous epithelium to complete spermatogenesis, and each cycle is estimated to take approximately 13.5 days, thus requiring 61 days to complete the process [152, 154]. Spermatogenesis continues throughout the natural lifespan of the postpubertal bull. The domestic bull attains puberty at approximately 10–12 months of age when mature sperm begin to appear in the ejaculate. It is estimated that the daily sperm output of the dairy bull is approximately eight billion and approximately seven billion in the beef bull, with the level of production of spermatozoa increasing from puberty to maximum potential at approximately three years of age. The productivity potential of a bull is determined by the size of the testes, which may be breed dependent, and is thought to be related to being a component of Sertoli cell volume within the seminiferous tubules. Hence, breeding soundness examinations of bulls to be used as sires is dependent not only on sperm quality but also on testicular size and volume [155].
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