The vascularization and the neuroendocrine pathways of the pituitary gland in the Atlantic salmon, Salmo salar

Abstract

The pituitary gland of the Atlantic salmon, Salmo salar, is supplied by two arterial systems. The hypophysial arteries irrigate the rostral neurohypophysis and the pars distalis. The caudal hypothalamic arteries enter the neurointermediate lobe. In contrast with the capillary plexa in the neurohypophysis, the dorsal arteriol loops lack neurocrine terminals. An elaborate system of branches of the perivascular spaces extends into the neurohypophysis and it is continuous with the basement membrane in the neuroadenointerface and the intercellular channels of the pars distalis. This system is assumed to be an efficient conveyor of neurocrine factors and hypophysial hormones and an important vascular component. In the salmon the brain mediates its information to the pituitary through neurovascular links and the cells of the adenohypophysis are not innervated.The rostral neurohypophysis fulfills all criteria of being a median eminence although it lacks a well-defined portal system and it represents a secondary development from an original type seen in gnathostome vertebrates. It is suggested that the direct innervation of the endocrine cells in the pars distalis is a specialization and appeared late in the evolution of the teleost pituitary.