Posterior Pituitary Gland

The posterior pituitary (or neurohypophysis) comprises the posterior lobe of the pituitary gland and is part of the endocrine system. Despite its name, the posterior pituitary gland is not a gland; rather, it is largely a collection of axonal projections from the hypothalamus that terminate behind the anterior pituitary gland. 

The posterior pituitary consists mainly of neuronal projections (axons) extending from the supraoptic and paraventricular nuclei of the hypothalamus. These axons release peptide hormones into the capillaries of the hypophyseal circulation. These are then stored in neurosecretory vesicles (Herring bodies) before being secreted by the posterior pituitary into the systemic bloodstream.

Pituitary

Pituitary gland representation.

This is an illustration of a pituitary gland that shows the anterior pituitary and the posterior pituitary, and the hypothalamus above them.

Anatomy of the Posterior Pituitary Gland

The posterior pituitary is derived from the hypothalamus and is distinct from the more fleshy, vascularized anterior lobe. The posterior pituitary is composed of two parts:

• The pars nervosa, also called the neural lobe or posterior lobe, constitutes the majority of the posterior pituitary and is the storage site of oxytocin and vasopressin. 
• The infundibular stalk, also known as the infundibulum or pituitary stalk, bridges the hypothalamic and hypophyseal systems.

Major Hormones Secreted by the Posterior Pituitary Gland

The posterior pituitary stores two hormones secreted by the hypothalamus for later release: 

• Oxytocin, most of which is released from the paraventricular nucleus in the hypothalamus. Oxytocin is one of the few hormones that create a positive feedback loop.
• Antidiuretic hormone (ADH, also known as vasopressin), the majority of which is released from the supraoptic nucleus in the hypothalamus. ADH acts on the collecting ducts of the kidney to facilitate the reabsorption of water into the blood.