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* To whom correspondence should be addressed. E-mail: rpalra63{at}msn.com.
Understanding dynamics of spermatogenesis is central to clinical andrology or probing environmental impact on human testes. This review considers what is known about renewal and proliferation of spermatogonia, how germ cells are organized in cellular associations constituting the cycle of the seminiferous epithelium, relative frequencies of cellular associations, durations of the cycle of the seminiferous epithelium and spermatogenesis, and measurement of daily sperm production. Daily sperm production (DSP) per testis tends to decline with advancing age. Regardless of age, there is substantial loss of potential sperm from degeneration of spermatocytes, but not spermatids. DSP/g testis parenchyma or DSP/testis can not be predicted based on testis size or age of a man. The review shows why our 1960s data base is neither robust nor precise, and suggests how deficiencies might be rectified. New cellular associations should be defined, with none representing >15% of the cycle of the seminiferous epithelium. Then determine when Apale-spermatogonia become committed to proliferate or how many mitotic divisions occur thereafter. Restudy the duration of spermatogenesis, because the accepted value might be in error by ~6 days. Restudy of human spermatogenesis will benefit clinicians, toxicologists and epidemiologists probing testis function by direct evaluations or indirectly via evaluations of quantity and quality of sperm ejaculated. It also will benefit scientists interested in renewal and proliferation of spermatogonia, or a spermatogonium as a prototype stem cell.
Key words: Spermatogenesis •
Testis •
cellular associations •
duration of spermatogenesis •
kinetics of spermatogenesis
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