Seasonal testosterone levels in leydig and sertoli cells of the snapping turtle (Chelydra serpentina) in natural populations

Leydig cells were enzymatically separated from the seminiferous tubules and were then purified by isoosmotic Percoll gradient centrifugation so that steroidogenic activity in the two sites could be measured separately. After purification, the cells in fractions 29–31 were identified as Leydig cells since these tested positively for 3β hydroxysteroid dehydrogenase activity (3β HSD), had steroidogenic ultrastructural features (smooth endoplasmic reticulum), and were capable of synthesizing testosterone (T) in vitro. The rest of the fractions tested negatively for the 3β HSD activity and were incapable of synthesizing detectable amounts of T. Sertoli cells from seminiferous tubules also showed the above steroidogenic properties. The T concentration of intratubular origin (Sertoli cells) was low prior to spermatogenesis but began to rise at the onset of the spermatogenic phase (late May to early June). The level of Sertoli T increased steadily during the summer. In late August to early September intratubular T reached a peak that coincided with spermiation and maximal testicular development. Leydig cells (interstitial origin) were more active prior to spermatogenesis, producing large amounts of T at a time of minimal testicular mass. The sharply rising T concentrations peaked in May or early June (mating period). After the onset of spermatogenesis, T of interstitial origin began to drop until early August to early September when, like T of intratubular origin, it peaked in synchrony with spermiation and maximal testes development. During September the testes regressed rapidly and T from both sources declined significantly. Throughout the cycle the Leydig cells appeared to be the major site of testosterone synthesis. © 1993 Wiley‐Liss, Inc.