Pituicytes, glia and control of terminal secretion

Once thought to be hormone-synthesizing cells, the pituicytes are now known to be the resident astroglia of the neurohypophysis (also referred to here as the posterior pituitary). Early investigators interpreted light microscopic observations as demonstrating pituicyte secretion, since pituicytes appeared to contain neuro-secretory material when hormone demand was low and not when it was increased. Ultrastructural studies have shown that pituicytes actually engulf or completely surround neurosecretory axons and axonal endings under basal conditions, and release these neural processes when conditions require increased hormone output. Thus, the pituicytes appeared to the early workers to contain and release hormone when they actually contained and released axons and terminals in which the hormone was, in fact, contained. Dynamic interactions of pituicytes with various of the other elements in the gland have also been demonstrated. When hormone demand is low, the pituicytes not only engulf the neurosecretory processes but also interpose their own processes between the secretory endings and the basal lamina. Since any hormone that is secreted must pass through the basal lamina and into the perivascular spaces in order to enter the fenestrated capillaries, pituicyte interpositions form physical, and perhaps chemical, barriers to hormone entering the circulation. Increasing hormone demand results in retraction of pituicyte processes from the basal lamina, permitting increased neural contact. Studies of isolated neurohypophysis and of cultured adult rat pituicytes have shown that these glia undergo appropriate morphological changes in response to osmotic stimuli or to receptor-mediated activation of adenylate cyclase. Both these events are thought to be effectors of the alterations seen in vivo. Some possible mechanisms by which pituicytes may participate in the control of secretory events are discussed.