Summary. The response of rat Leydig cells to hCG in vitro was examined using decapsulated testes and enriched Leydig cell suspensions of a known purity. The secretion of testosterone and 5α-androstan-3α,17β-diol was used as the response and medium extracts were assayed before and after celite chromatography. At least 4 important phases in the development of steroidogenic function of the Leydig cells were detected. (1) A period after birth up to 10 days of age when the Leydig cell preferentially secretes testosterone. The cells may constitute the remnants of a fetal Leydig cell population. (2) A period which begins between 10 and 15 days of age and continues until 35–40 days of age, when the major products of the Leydig cell are 5α-reduced metabolites of testosterone, especially 5α-androstan-3α,17β-diol. (3) A phase at 20–30 days of age when responsiveness of the Leydig cell to hCG in vitro undergoes a distinct increase (the quantity of androgen produced with maximum hCG stimulation) and attains adult levels, although the sensitivity of the Leydig cells to hCG (the dose of hCG eliciting a half-maximal response, ED50) is gradually decreased after Day 20. (4) A period from 35–40 days of age into adulthood when testosterone increasingly becomes the major secretory product of the Leydig cell.
Hypophysectomy for 8 days decreased the quantity of androgens produced per 106 Leydig cells in response to hCG to an extent dependent on age. At 28 days of age steroidogenic function was impaired to a much greater extent (reduced by some 80–90%) than at 38 or 58 days (reduced by 40–50%). At all ages the sensitivity of the Leydig cells to hCG was increased after hypophysectomy.