Posterior pituitary

The posterior pituitary (also called the neurohypophysis) comprises the posterior lobe of the pituitary gland and is part of the endocrine system. It consists mainly of neuronal projections (axons) extending from the supraoptic and paraventricular nuclei of the hypothalamus that secrete peptide hormones into the capillaries of the hypophyseal circulation. The neurohypophysis also contains a specialised type of astrocytic glial cell - "pituicytes".

Anatomy

Despite its name, the posterior pituitary gland is not a gland, per se; rather, it is largely a collection of axonal projections from the hypothalamus that terminate behind the anterior pituitary gland. It is derived embryonically from downward-growth of the diencephalon. This tissue receives its blood supply from the inferior hypophyseal artery, and secretes hormones into the hypophyseal vein. These hormones are actually made in the hypothalamus and transported to the posterior pituitary gland, where they are stored.

The posterior pituitary has three main components:

• pars nervosa
• infundibular stalk
• median eminence

Major hormones secreted

Hormones known classically as posterior pituitary hormones are synthesized by the hypothalamus. They are stored and secreted by the posterior pituitary into the bloodstream.

Hormone	Target	Effect
Oxytocin	Uterus, mammary glands	Uterine contractions; lactation
Vasopressin (antidiuretic hormone; ADH)	Kidneys or Arterioles	Stimulate water retention or raise blood pressure by contracting arterioles

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.