15.7D: The Posterior Pituitary

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The posterior pituitary secretes two important endocrine hormones—oxytocin and antidiuretic hormone.

Learning Objectives
• Identify the location of the posterior pituitary and the hormones associated with it

Key Points

• The posterior pituitary (or neurohypophysis) comprises the posterior lobe of the pituitary gland and is part of the endocrine system.
• Hormones known as posterior pituitary hormones are synthesized by the hypothalamus, and include oxytocin and antidiuretic hormone.
• The hormones are then stored in neurosecretory vesicles (Herring bodies) before being secreted by the posterior pituitary into the bloodstream.

Key Terms

• oxytocin: A hormone that stimulates contractions during labor.
• posterior pituitary: 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 true gland; rather, it is largely a collection of axonal projections from the hypothalamus that terminate behind the anterior pituitary gland.
• Antidiuretic hormone: A hormone that stimulates water re-absorption in the kidneys.

Posterior Pituitary Gland

Pituitary: Pituitary gland representation.

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.

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.

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• Pituitary gland. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/File:Pituitary_gland.png. License: CC BY-SA: Attribution-ShareAlike
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