The posterior pituitary (or neurohypophysis) is the posterior lobe of the

pituitary gland The pituitary gland or hypophysis is an endocrine gland in vertebrates. In humans, the pituitary gland is located at the base of the human brain, brain, protruding off the bottom of the hypothalamus. The pituitary gland and the hypothalamus contr ...

which is part of the

endocrine system The endocrine system is a messenger system in an organism comprising feedback loops of hormones that are released by internal glands directly into the circulatory system and that target and regulate distant Organ (biology), organs. In vertebrat ...

. Unlike the anterior pituitary, the posterior pituitary is not glandular, but largely a collection of axonal projections from the

hypothalamus The hypothalamus (: hypothalami; ) is a small part of the vertebrate brain that contains a number of nucleus (neuroanatomy), nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrin ...

that terminate behind the anterior pituitary, and serve as a site for the secretion of

neurohypophysial hormone The neurohypophysial hormones form a family of structurally and functionally related peptide hormones. Their representatives in humans are oxytocin and vasopressin. They are named after the location of their release into the blood, the neurohy ...

s (

oxytocin Oxytocin is a peptide hormone and neuropeptide normally produced in the hypothalamus and released by the posterior pituitary. Present in animals since early stages of evolution, in humans it plays roles in behavior that include Human bonding, ...

and

vasopressin Mammalian vasopressin, also called antidiuretic hormone (ADH), arginine vasopressin (AVP) or argipressin, is a hormone synthesized from the ''AVP'' gene as a peptide prohormone in neurons in the hypothalamus, and is converted to AVP. It ...

) directly into the blood. The hypothalamic–neurohypophyseal system is composed of the hypothalamus (the

paraventricular nucleus The paraventricular nucleus (PVN) is a nucleus in the hypothalamus, located next to the third ventricle. Many of its neurons project to the posterior pituitary where they secrete oxytocin, and a smaller amount of vasopressin. Other secretions ...

and

supraoptic nucleus The supraoptic nucleus (SON) is a nucleus of magnocellular neurosecretory cells in the hypothalamus of the mammalian brain. The nucleus is situated at the base of the brain, adjacent to the optic chiasm. In humans, the SON contains about 3,000 ...

), posterior pituitary, and these axonal projections.

Structure

The posterior pituitary consists mainly of

neuron A neuron (American English), neurone (British English), or nerve cell, is an membrane potential#Cell excitability, excitable cell (biology), cell that fires electric signals called action potentials across a neural network (biology), neural net ...

al projections (

axon An axon (from Greek ἄξων ''áxōn'', axis) or nerve fiber (or nerve fibre: see American and British English spelling differences#-re, -er, spelling differences) is a long, slender cellular extensions, projection of a nerve cell, or neuron, ...

s) of

magnocellular neurosecretory cell Magnocellular neurosecretory cells are large neuroendocrine cells within the supraoptic nucleus and paraventricular nucleus of the hypothalamus. They are also found in smaller numbers in accessory cell groups between these two nuclei, the larges ...

s extending from the supraoptic and paraventricular nuclei of the

. These axons store and release neurohypophysial hormones

and

into the neurohypophyseal capillaries, from there they get into the

systemic circulation In vertebrates, the circulatory system is a organ system, system of organs that includes the heart, blood vessels, and blood which is circulated throughout the body. It includes the cardiovascular system, or vascular system, that consists of ...

(and partly back into the

hypophyseal portal system The hypophyseal portal system is a system of blood vessels in the microcirculation at the base of the brain, connecting the hypothalamus with the anterior pituitary. Its main function is to quickly transport and exchange hormones between the hyp ...

). In addition to axons, the posterior pituitary also contains pituicytes, specialized

glial cell Glia, also called glial cells (gliocytes) or neuroglia, are non-neuronal cells in the central nervous system (the brain and the spinal cord) and in the peripheral nervous system that do not produce electrical impulses. The neuroglia make up ...

s resembling

astrocyte Astrocytes (from Ancient Greek , , "star" and , , "cavity", "cell"), also known collectively as astroglia, are characteristic star-shaped glial cells in the brain and spinal cord. They perform many functions, including biochemical control of en ...

s assisting in the storage and release of the hormones. Classification of the posterior pituitary varies, but most sources include the two regions below:

Pars nervosa

Also called the neural lobe or posterior lobe, this region constitutes the majority of the posterior pituitary and is the storage site of oxytocin and vasopressin. Sometimes (incorrectly) considered synonymous with the posterior pituitary, the pars nervosa includes

Herring bodies Herring bodies or neurosecretory bodies are structures found in the posterior pituitary. They represent the terminal end of the axons from the hypothalamus, and hormones are temporarily stored in these locations. They are neurosecretory termin ...

and pituicytes.

Infundibular stalk

Also known as the infundibulum or pituitary stalk, the infundibular stalk bridges the hypothalamic and hypophyseal systems. The

median eminence The median eminence is generally defined as the portion of the ventral hypothalamus from which the portal vessels arise. The median eminence is a small swelling on the tuber cinereum, posterior to and on top of the pituitary stalk; it lies in th ...

is only occasionally included as part of the posterior pituitary. Other sources specifically exclude it from the pituitary. A few sources include the

pars intermedia The pars intermedia is one of the three parts of the anterior pituitary. It is a section of tissue sometimes called a middle or intermediate lobe, between the pars distalis, and the posterior pituitary. It is a small region that is largely with ...

as part of the posterior lobe, but this is a minority view. It is based upon the gross anatomical separation of the posterior and anterior pituitary along the cystic remnants of Rathke's pouch, causing the pars intermedia to remain attached to the neurohypophysis.

Function

Hormone secretion

Two hormones are classically considered as being related to the posterior pituitary: oxytocin and vasopressin. These hormones are created in the hypothalamus and released in the posterior pituitary. After creation, they are stored in neurosecretory vesicles regrouped into

before being secreted in the posterior pituitary via the bloodstream.

Clinical significance

Insufficient secretion of

underlies

diabetes insipidus Diabetes insipidus (DI) is a condition characterized by large amounts of dilute urine and increased thirst. The amount of urine produced can be nearly 20 liters per day. Reduction of fluid has little effect on the concentration of the urine. ...

, a condition in which the body loses the capacity to concentrate urine. Affected individuals excrete as much as 20 liters of dilute urine per day. Oversecretion of vasopressin causes the syndrome of inappropriate antidiuretic hormone (SIADH).

References

Additional images

Image:Gray1180.png, The posterior pituitary comprises the posterior lobe of the pituitary gland.

External links

www.pituitary.org
nbsp;— The Pituitary Network Association {{DEFAULTSORT:Posterior Pituitary Endocrine system anatomy Neuroendocrinology