The posterior pituitary

Abstract

The posterior pituitary is comprised of the end terminals of specialized hypothalamic neurons called neurohypophyseal neurons. The two hormones of the posterior pituitary, arginine vasopressin (AVP) and oxytocin (OT), are synthesized in the cell bodies of the neurohypophyseal neurons. The pre-prohormones are cleaved from the signal peptide in the endoplasmic reticulum, and the prohormones, provasopressin and pro-oxytocin, are packaged along with processing enzymes into neurosecretory granules. The neurosecretory granules are transported out of the perikaryon of the neurohypophyseal neurons via microtubules down the long axons that form the supraopticohypophyseal tract to terminate in axon terminals in the posterior pituitary. The entire unit including the magnocellular neurons in the SON and PVN, the supraopticohypophyseal tract, and the axon terminals in the posterior pituitary is called the neurohypophysis. The primary physiologic action of AVP is its function as a water-retaining hormone via activation of vasopressin V2 receptors in the principal cells of the kidney collecting duct. Disorders of AVP synthesis or secretion can lead to excessive water excretion (diabetes insipidus) or inappropriate water retention (syndrome of inappropriate antidiuretic hormone secretion). The primary physiologic action of OT is its function to stimulate milk secretion during suckling via activation of OT receptors in the breast with subsequent contraction of myoepithelial cells around the breast alveoli and ductules to eject milk. Disorders of OT synthesis or secretion can lead to inability to eject breast milk postpartum, though clinical cases have not been described in humans. Both AVP and OT are also synthesized in parvocellular hypothalamic neurons that project within the brain and regulate a variety of central nervous system functions, not all of which have been characterized.