Neurohypophysis

At a glance

English

Neurohypophysis

Latin (TA2)

Neurohypophysis

System

Endocrine gland

Region

Whole body

Side

Single, on the midline

About the neurohypophysis

The neurohypophysis comprises the hind lobe of the pituitary plus the stalk that joins it to the floor of the hypothalamus. It sits in the back of the hypophysial fossa of the sphenoid bone, behind the glandular adenohypophysis and separated from it by the thin pars intermedia. Unlike the anterior lobe, it is not made of secretory epithelium. It is a downgrowth of the embryonic diencephalon and so is really an extension of the brain.

Two regions are usually recognised. The pars nervosa, or neural lobe, forms the rounded bulk in the fossa. The infundibular stalk rises from it through the diaphragma sellae to the median eminence at the base of the hypothalamus, wrapped in front by the pars tuberalis of the anterior lobe. Whether the median eminence itself belongs to the neurohypophysis varies between accounts.

Most of the tissue consists of unmyelinated axons whose cell bodies sit in two hypothalamic cell groups, the supraoptic and the paraventricular nuclei. These large neurosecretory cells make two peptide hormones, vasopressin (antidiuretic hormone) and oxytocin, and ship them down the axons in granules. The hormones collect in swollen axon endings, visible under the microscope as Herring bodies, beside a dense network of fenestrated capillaries, and are released into the blood when the neurons fire. Specialised glial cells, the pituicytes, surround the axon endings and help regulate this release. Together the nuclei, axons and lobe form the hypothalamo-neurohypophysial system.

Vasopressin makes the kidney collecting ducts permeable to water and so concentrates the urine; at higher levels it also constricts blood vessels. If too little is released, as after head injury, surgery or tumours near the stalk, the result is central diabetes insipidus, with large volumes of dilute urine and thirst. Excess release gives SIADH (inappropriate ADH secretion), with water retention and a low blood sodium. On MRI the normal posterior lobe often gives a bright spot that is lost in diabetes insipidus.

Function and supply

Function

Stores and releases vasopressin, which controls water retention by the kidney and vessel tone, and oxytocin, into the systemic circulation.

Nerve supply

Built from hypothalamic axons of the supraoptic and paraventricular cell groups (the hypothalamo-hypophysial tract), whose firing triggers hormone release.

Blood supply

The inferior hypophysial arteries from the internal carotid arteries supply the posterior lobe, the superior hypophysial arteries the stalk; veins drain to the cavernous sinus.