Fine structure and cytochemistry of the mammalian pars intermedia

Various cytophysiological aspects of the pars intermedia of the pituitary are discussed. Cells containing melanocyte-stimulating hormone (MSH) have been studied under normal and experimental conditions. They react to variations in ionic equilibrium, but without any clear correlation with natraemia and osmotic blood pressure. The MSH-cell stimulation in hypernatraemic mice, which is not inhibited by bromocriptine, seems more specific than the stimulation in hyponatraemic mice, which is blocked by bromocriptine. The existence of a corticotropic cell system has been clearly demonstrated in the mouse (where it is particularly obvious), in the rat and in the cat but its significance is not clear. Although very poorly vascularized, the pars intermedia is rapidly invaded by tracer protein (horseradish peroxidase) injected either intravenously or intracerebroventricularly. The hypophysial cleft rapidly stores the tracer which can be resorbed by macrophagic epithelial cells lying free in the colloid contained in the cleft. Horseradish peroxidase lingers in the pars intermedia but is rapidly eliminated from the other hypophysial lobes after intraventricular (third ventricle) injection. Diffuse innervation of the pars intermedia applies to both glandular (MSH and ACTH) and non-glandular (epithelial and stellate) cells. While aminergic innervation of the pars intermedia is obvious, cholinergic innervation has not been demonstrated ultrastructurally or histochemically. Peptidergic fibres only occasionally penetrate marginal areas of the pars intermedia and seldom establish synaptic contacts with glandular cells. A specific relationship might exist between the pars intermedia and oxytocin fibres in view of the marginal distribution of the latter in the neural lobe. Numerous stellate cells of the pars intermedia react intensely with antiserum to gliofibrillar acid protein, indicating their astrocyte nature, which reinforces the idea of an analogy between the folliculo-stellate system of the hypophysis and the glial cells.