Age-Related Changes in the Function of the Pituitary-Gonadal Axis in a Sterile Male Rat Mutant (hd/hd)1

Testicular growth is depressed in the genetically sterile male rat (bd/hd) relative to its LE phenotype littermates (by 50% and 73% at 27 and 90 days of age, respectively). Within the bd/hd testis, both the tubular and seminiferous tubule tissues are affected by the mutation. In addition, there is significantly less germ cell production from the primary spermatocyte stage of spermatogenesis onwards and the total number of Sertoli cells observed is less. In the intertubular tissue, the total volume and the total number of Leydig cells per testis is significantly less, but the mean volume of an average Leydig cell is not modified. The serum gonadotropin levels are higher in the bd/hd rat, whereas from 40 days of age onwards the level of testosterone is lower. The FSH and LH binding affinity constants are unchanged by the mutation; however, the total number of FSH binding sites per 106 Sertoli cells is lower while that of LH per 106 Leydig cells is greater. Indeed, it is likely that the lesser concentration of serum testosterone in the bd/hd rat is a result of a smaller number of Leydig cells since their individual function is not modified. The testicular androgen binding protein (ABP) content and the ABP output towards the epididymis are lower as a consequence of both a lesser number and an altered function of the Sertoli cells. At the present time it is difficult to provide a clear explanation for the spermatogenic failure in the bd/bd rat, but it is tempting to postulate that the genetic malfunction is effective during the phase of rapid testicular growth (20–40 days), implying, first, an effect on the proliferation of Leydig cells and, consequently, on Sertoli cell function and spermatogenesis.