A Comparative Study of Daily Sperm Production and Testicular Composition in Humans and Rats

Based on enumeration of maturation-phase spermatids in testicular homogenates from adult men and rats, daily sperm production per gram of testicular parenchyma (DSP/g) was almost seven times greater in rats (21.1 ± 0.9 × 106 vs 3.1 ± 0.5 × 106 ; X̄ ± SEM, n = 10). The relative inefficiency of the human testis was uniformly expressed in cranial, equatorial and caudal regions. Furthermore, DSP/g was highly correlated between paired testes from individual men (r = +0.97, n = 10), and differences between paired testes were not significantly different from zero (P>0.4). Histometric analysis of glutaraldehyde-perfused testes revealed differences (P<0.05) in relative testicular compositions between humans and rats (n = 3) including higher proportions of parenchyma, seminiferous tubules, seminiferous epithelium and germinal cells in the rat testis. Humans exceeded rats only in the proportions of testis occupied by nongerminal components such as tunic, interstitium, tubule boundary tissue and Sertoli cells. Histometric determination of the proportion of the testicular parenchyma occupied by these nuclei and consideration of average volume of round spermatid nuclei yielded DSP/g estimates of 17.4 ± 1.8 × 106 in rats (not significantly different from the result by the homogenization method) and 8.5 ± 1.3 × 106 in humans (almost three times higher than the value obtained by the homogenization method). Although the disparity between values for human DSP/g obtained by the two methods cannot yet be explained, several lines of evidence suggest that the time divisor used for human material processed by the homogenization method is too long, thus yielding an underestimation of daily sperm production. While its absolute accuracy is questionable, the homogenization method remains a rapid and precise technique for estimating DSP/g in humans. Histometric analysis and histometric estimation of DSP/g support the concept that the human is less efficient in sperm production than the rat, but these methods suggest that the rat is only about twice as efficient per gram of testicular parenchyma as the human.