posterior pituitary

The posterior pituitary is best understood not as a gland that makes hormones, but as a storage-and-release dock for hormones made elsewhere. Imagine a warehouse with loading bays: the goods (hormones) are manufactured in the hypothalamus and shipped down nerve fibers to be held here until needed.

Physically, the posterior lobe is an extension of the brain itself — a downward growth of neural tissue. It is composed mainly of the axon terminals of neurons whose cell bodies sit up in the hypothalamus, along with supporting glial cells called pituicytes.

Two hormones are released from the posterior pituitary: oxytocin, which drives uterine contractions in labor and milk let-down during breastfeeding, and vasopressin (antidiuretic hormone), which tells the kidneys to conserve water. Both are made in hypothalamic neurons, packaged into granules, carried down the axons, and stored at the nerve endings until an electrical signal triggers their release directly into the bloodstream.

Because the posterior lobe is just a release site, its disorders usually reflect problems upstream in the hypothalamus or the connecting stalk. Loss of vasopressin function, for example, causes central diabetes insipidus, with the production of large volumes of dilute urine.