Reproductive biology of elasmobranchs with emphasis on endocrines

The endocrinology of reproduction in elasmobranch fish has received a good deal of attention from comparative anatomists, histologists, and endocrine physiologists. This review deals with the gonadal morphology and the control of gonadal functions exerted via the hypothalamus-hypophysis axis and with how the gonads utilize or respond to putative local stimuli. It is well known that, in the majority of vertebrate classes, gonadal functions are under hypothalamic control, whose secretory activity regulates the hypophysis. In elasmobranchs, a capillary system joining the hypothalamus to particular regions of the pituitary (rostral, median, and neurointermediate lobes) has been described, suggesting hypothalamic control of these lobes. Conversely, the ventral lobe, which produces gonadotropin, has no vascular contact with the hypothalamus. Recent papers argue for the existence of substances able to reach the pituitary ventral lobe via the general circulation, acting as gonadotropin-releasing factors. As for pituitary-gonadal control, in elasmobranchs, steroidogenesis and spermatogenesis are not strictly dependent on pituitary control. It has been demonstrated that a partial removal of the gland causes the appearance of a “zone of breakdown,” coincident with the degeneration of late spermatogonial ampullae. To date, only one gonadotropin has been isolated from the ventral lobe. As in other vertebrates, it has been hypothesized that in elasmobranchs the gonads have internal mechanisms of regulation involving substances probably produced by different compartments of the gonad.