Ontogeny of pituitary gonadotrophin hormone activity and of testicular responsiveness to gonadotrophins in foetal rabbit

Synthesis and secretion of T and DHT by 18, 19 and 20 day old foetal rabbit testes were measured (RIA) in an organ culture system (M199). It was demonstrated that synthesis and secretion in vitro of T and DHT were greater at 19 and 20 days than at 18 days. The T and DHT production could be stimulated by ovine LH at 19 and 20 days but not at 18 days. The pituitaries of 19 and 20 day old male and female foetuses (but not those of foetuses aged 18 days) stimulate synthesis and secretion of T by 20 day old testis. A gonadotrophic factor, active on testicular T production, is present in the pituitary gland of 19 day old foetuses of both sexes. These findings indicate that the testicular responsiveness to LH and to the pituitary appears concomitantly with the pituitary gonadotrophic activity, between 19 and 20 days, just prior to the beginning of male sexual differentiation (20–25 days).