POSTERIOR PITUITARY GLAND
The posterior pituitary is neural tissue and is formed by the distal axons of the supraoptic nucleus (SON) and the paraventricular nucleus (PVN) of the hypothalamus. The axon terminals store neurosecretory granules that contain vasopressin and oxytocin—both are nonapeptides consisting of a six–amino acid ring with a cysteine-to-cysteine bridge and a three–amino acid tail. In embryogenesis, neuroepithelial cells of the lining of the third ventricle migrate laterally to and above the optic chiasm to form the SON and to the walls of the third ventricle to form the PVN. The blood supply for the posterior pituitary is from the inferior hypophysial arteries, and the venous drainage is into the cavernous sinus and internal jugular vein.
The posterior pituitary serves to store and release vasopressin and oxytocin. The posterior pituitary stores enough vasopressin to sustain basal release for approximately 30 days and to sustain maximum release for approximately 5 days. Whereas approximately 90% of the SON neurons produce vasopressin, and all its axons end in the posterior pituitary, the PVN has ﬁve subnuclei that synthesize other peptides in addition to vasopressin (e.g., somatostatin, corticotropin-releasing hormone, thyrotropin-releasing hormone, and opioids). The neurons of the PVN subnuclei project to the median eminence, brainstem, and spinal cord. The major stimulatory input for vasopressin and oxytocin secretion is glutamate, and the major inhibitory input is -aminobutyric acid (GABA). When a stimulus for secretion of vasopressin or oxytocin acts on the SON or PVN, an action potential is generated that propagates down the long axon to the posterior pituitary. The action potential triggers an inﬂux of calcium that causes the neurosecretory granules to fuse with the cell membrane and release the contents of the neurosecretory granule into the perivascular space and subsequently into the fenestrated capillary system of the posterior pituitary.
The stored vasopressin in neurosecretory granules in the posterior pituitary produces a bright signal on T1-weighted magnetic resonance imaging (MRI)—the “posterior pituitary bright spot.” The posterior pituitary bright spot is present in most healthy individuals and is absent in individuals with central diabetes insipidus. In addition, this bright spot may be located elsewhere in individuals with congenital abnormalities such that the posterior pituitary is undescended—it may appear at the base of the hypothalamus or along the pituitary stalk. Although posterior pituitary function is usually intact, this “ectopic posterior pituitary” may be associated with a hypoplastic anterior pituitary gland and with varying degrees of anterior pituitary dysfunction.