Pituicyte

Pituicytes are the principal cell type intrinsic to the posterior pituitary gland, making up 25-30% of the volume of the gland. Most express glial fibrillary acidic protein, which identifies them as a subpopulation of astrocytic glial cells. Like other astrocytes, they are extensively interconnected by prominent gap junctions, via which they are electrotonically coupled. The cytoplasm contains abundant Golgi bodies, free ribosomes and lipid bodies. The pituicytes have long processes that extend among the neurosecretory axons that innervate the lobe and onto the basement lamina of the perivascular space. The morphology of the pituicytes changes dramatically in different physiological states.

Because of this morphological plasticity, pituicytes have long been suspected to have a role in the regulation of neurohypophysial hormone secretion. These morphological changes are apparent in physiological states such as parturition, lactation, and dehydration. In pituicytes cultured in vitro, morphological changes ("stellation") that appear similar to those that occur physiologically involve actin depolymerization can be induced by beta-adrenergic or A1-adenosine receptor activation, and appear to result from inhibition of the small GTPase RhoA. The neurohypophysial hormones vasopressin and oxytocin can both reverse stellation and return pituicytes to their basal shape by activating Cdc42, another small GTPase that reorganizes the actin cytoskeleton. Adenosine and neurohormones also have opposite actions on the efflux of taurine, a local messenger that is released by pituicytes in hypotonic conditions and which can inhibits vasopressin secretion from axon terminals.