Abstract
The possession of a scrotum to contain the male gonads is a characteristic feature of almost all mammals, and appears to have evolved to allow the testes and epididymis to be exposed to a temperature a few degrees below that of core body temperature. Analysis of cryptorchid patients, and those with varicocele suggest that mild scrotal warming can be detrimental to sperm production, partly by effects on the stem cell population, and partly by effects on later stages of spermatogenesis and sperm maturation. Recent studies on the effects of clothing and lifestyle emphasize that these can also lead to chronically elevated scrotal temperatures. In particular, the wearing of nappies by infants is a cause for concern in this regard. Together all of the evidence indirectly supports the view that lifestyle factors in addition to other genetic and environmental influences could be contributing to the secular trend in declining male reproductive parameters. The challenge will be to provide relevant and targeted experimental results to support or refute the currently circumstantial evidence.
Highlights
The exteriorization of the male gonads in a special sac called the scrotum is a uniquely mammalian feature, and one that at first glance requires some explanation
The most plausible evolutionary explanation relates to the requirement of spermatogenesis for an optimum temperature lower than core abdominal temperature [1]
Testicular descent into a scrotum evolved probably more than 150 million years ago concurrently with the acquisition by ancestral mammals of a regulated hyperthermia [1], which provided the abdominal organs with a controlled, stable body temperature of ca. 36–38°C
Summary
The exteriorization of the male gonads in a special sac called the scrotum is a uniquely mammalian feature, and one that at first glance requires some explanation. A large number of studies in animals with induced cryptorchidism, or local testicular heating as adults, show that the process of spermatogenesis is exquisitely sensitive to temperature (Fig. 2), with increased germ cell apoptosis (mostly spermatocytes and spermatids) a consequence of only relatively short periods of mild testicular heating (reviewed in [2]). This time is required to allow positive effects on all stages of spermatogenesis, including stem cells to become manifest in the ejaculate Taken together, these studies all indicate that a more sedentary lifestyle can and most likely does cause a significant impairment in sperm production and quality. This experiment showed that the temperature sensing mechanisms regulating epididymal gene expression are exquisitely sensitive, and that probably artificial elevation of scrotal temperature is likely to have negative effects on spermatogenesis in the testis and on sperm maturation and storage processes within the epididymis. There is no information about effects of scrotal temperature on the early development of the epididymis or its functions, or whether long-term exposure to higher temperatures early in life could elicit adaptive processes of benefit in adulthood
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