Posterior pituitary

The posterior pituitary gland at the base of the brain is the source of the hormones vasopressin and oxytocin. The gland, which is also called the "neurohypophysis" or "neural lobe", is connected to the brain by a stalk (the "neural stalk"). It contains the nerve endings of magnocellular neurosecretory cells, whose cell bodies lie in the anterior hypothalamus, notably in the supraoptic nucleus and paraventricular nucleus. In the rat, there are about 18,000 magnocellular neurons is all, about half of which produce oxytocin and half vasopressin.

In the posterior pituitary gland, the axons of the magnocellular neurons each give rise to about 2,000 nerve endings and a few hundred larger swellings, all of which are densely packed with the large, membrane bound vesicles that contain oxytocin or vasopressin. Typically each nerve ending contains a few hundred of these vesicles, and each swelling contains several thousand of them. Each vesicle contains about 85,000 molecules of the hormone together with the precursor molecule from which it is derived. In all, the posterior pituitary gland of the rat contains about 1 microgram of vasopressin and 1 microgram of oxytocin; this is enough to maintain basal concentrations of the hormones in the circulation for over 30 days. Thus the posterior pituitary gland is a massive store of these hormones. However under conditions of high demand, even this large store can be depleted quite rapidly - within a few days.

Oxytocin and vasopressin can be released from the nerve endings and also from most of these axonal swellings. However the gland also contains some exceptionally large swellings, called "Herring bodies", which apparently are the site of degradation of aged vesicles that have not been secreted.

These vesicles are released from the nerve endings and swellings in response to electrical signals (action potentials) conducted along the axons. When an action potential invades a nerve ending, the membrane depolarisation results in the opening of voltage-dependent calcium channels, and the resulting calcium influx into the terminal increases the probability that one or more of the vesicles might fuse with the terminal membrane (a process called "exocytosis"), releasing its contents into the extracellular space. Because the blood vessels in the posterior pituitary gland are fenestrated, the peptide released into the extracellular space can diffuse freely into the circulation.

The major cell type in the posterior pituitary gland is a class of glial cells - a species of specialised astrocytes, called "pituicytes". The pituicytes have extensive filamenyous processes that surround the nerve terminals. These cells help to maintain the electrolyte composition of the extracellular fluid, important for maintaining normal electrical excitability of the axons, and probably have other roles as well.

Role in disease

Insufficient secretion of vasopressin is central diabetes insipidus, in which the body loses the capacity to concentrate urine. Affected individuals excrete as much as 20 L of dilute urine per day. Oversecretion of vasopressin causes the syndrome of inappropriate antidiuretic hormone.