ONTOGENESIS OF PITUITARY-GONADAL AXIS IN CHICK EMBRYO

The results of the present investigation constitute the first direct evidence of a difference in steroid hormone synthesis between gonads of intact and hypophysectomized chick embryos, as indicated by differences in the activity of histochemically demonstrable Δ5-3β-hydroxysteroid dehydrogenase (Δ5-3β-HSD), a key enzyme in steroidogenesis. This change is first observed on Day 13.5 of incubation, and is interpreted as demonstrating that the pituitary-gonadal axis first becomes functional at this time in the embryonic development of the chick. It has also been shown in the present study that Δ5-3β-HSD is histochemically demonstrable in the cells of the genital ridge of the chick embryo as early as Day 2.0 of incubation. The presence of Δ5-3β-HSD activity in the undifferentiated gonad would appear to indicate that this organ has the enzymatic capacity to synthesize sex hormones prior to its morphological differentiation in either the male or female direction. Thus, steroid hormones produced at this early age might possibly be responsible for the process of gonadal differentiation itself. © 1969.