posterior pituitary

The posterior pituitary develops embryologically from neuroectoderm (downward growth of the diencephalon), distinguishing it from the anterior pituitary which arises from Rathke's pouch (oral ectoderm). It consists of axon terminals of magnocellular neurons whose cell bodies reside in the supraoptic nucleus (mainly ADH) and paraventricular nucleus (mainly oxytocin) of the hypothalamus. Hormones are transported down the hypothalamo-hypophyseal tract via axonal transport and stored in Herring bodies before being released into systemic circulation in response to action potentials.

ADH (vasopressin) acts on V2 receptors in the renal collecting duct to insert aquaporin-2 channels, promoting water reabsorption and concentrating urine; at higher concentrations, it acts on V1 receptors on vascular smooth muscle to cause vasoconstriction. ADH release is stimulated by increased plasma osmolality (sensed by hypothalamic osmoreceptors) and decreased blood volume/pressure (via baroreceptors). Clinically, ADH deficiency or resistance causes diabetes insipidus (central vs nephrogenic), presenting with polyuria and dilute urine; excess ADH causes SIADH, leading to hyponatremia with concentrated urine.

Oxytocin stimulates uterine smooth muscle contraction during labor and milk ejection (via myoepithelial cell contraction) during lactation in response to suckling and cervical stretch (positive feedback loops - Ferguson reflex). It is also implicated in social bonding and orgasm.

Clinical correlations: Pituitary stalk lesions or trauma can cause central diabetes insipidus. Posterior pituitary tumors are rare (most posterior pituitary masses are metastases or gliomas, unlike anterior pituitary adenomas). Understanding the hypothalamic-posterior pituitary axis is essential for interpreting water balance disorders, a common USMLE topic.