An age-dependent thymic secretion modulates testicular function.

The acetone extract obtained from the thymuses of 14-day-old rats contains a factor that interacts with hCG in the adult testis cells and inhibits testosterone production. Experiments were designed to investigate the possible secretion of this factor. The media from the incubation of thymuses from 14-day-old rats were processed by molecular sieve chromatography and the fractions assayed using a bioassay with testicular cells in vitro. A fraction of approx. 30 kDa was found to inhibit the hCG-stimulated production of testosterone. In addition, the influence of age on the release of the active fraction was investigated. The inhibitory effect of this thymus product was greatest in the neonatal period (1-14 days) and declined thereafter towards the onset of puberty. The age-related decline of the inhibitory activity correlated with relative thymus weight and also with the amount of protein released to the incubation media. Thymic fraction activity is, however, present in the adult gland. These results suggest that the thymus secretes active agents that are able to modulate the response of testicular cells to hCG and that their release seems to be age-related.