MORPHOFUNCTIONAL ASPECTS OF THE HYPOTHALAMUS-PITUITARY-GONADAL AXIS OF ELASMOBRANCH FISHES

The tetrapod hypothalamus-pars distalis axis contains a blood portal s ystem. Contrarily elasmobranchs appear to lack a direct vascular suppl y from the hypothalamus to the ventral lobe of the pituitary where gon adotropic activity resides. The hypothalamus contains GnRH immunoreact ivity and GnRH causes an increase in plasma gonadal steroids, perhaps via ventral lobe stimulation. Therefore, the question arises as to how GnRH reaches the pituitary. We suggest that the general circulation r oute might be practicable. Indeed, in the plasma of the electric ray, Torpedo marmorata, a major early eluting form has been detected using high performance liquid chromatography coupled with region specific ra dioimmunoassay. The presence of GnRH in the blood may allow the molecu le to reach the gonads and to act there by direct mechanisms. Intragon adal levels of steroids may have a paracrine and/or autocrine role in the regulation of steroidogenesis in the testis and in the development of specific germinal cell stages. Particularly, the zonated morpholog y of the testis supports the concept of a diverse environment for diff erent spermatogenic stages. Finally, gonadal steroids ma feed back to affect pituitary activity.