Relationship of intratesticular testosterone content of stallions to age, spermatogenesis, Sertoli cell distribution and germ cell-Sertoli cell ratios

Testes were obtained from 47 1-20-year-old stallions during the natural breeding season. Total testicular testosterone and testosterone/g testis increased with age (P less than 0.005), and total testicular testosterone was associated with larger testis size (P less than 0.05). Neither testosterone per gram nor per paired testes were related to total Sertoli cell number (P greater than 0.05), but greater testosterone per paired testes was associated with fewer Sertoli cells per unit of seminiferous tubule length (P less than 0.005) or basement membrane area (P less than 0.02) and with a higher number of germ cells supported per Sertoli cell (P less than 0.05). Although values for testosterone per gram and per paired testes were unrelated (P greater than 0.10) to sperm production/g testis or to the yield of spermatids/spermatogonium, testosterone per paired testes was positively related to sperm production per paired testes (P less than 0.05). It is concluded that intratesticular testosterone increases with age, is related in a positive manner to quantitative rates of sperm production, and can account for some of the differences in sperm production among individual stallions within a single breeding season.