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.
