The posterior pituitary (or neurohypophysis) is the posterior lobe of the pituitary gland which is part of the endocrine system. The posterior pituitary is not glandular as is the anterior pituitary. Instead, it is largely a collection of axonal projections from the

hypothalamus The hypothalamus () is a part of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrine system via the pituitary gland. The hypothalamus ...

that terminate behind the anterior pituitary, and serve as a site for the secretion of neurohypophysial hormones ( oxytocin and

vasopressin Human 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 then ...

) directly into the blood. The hypothalamic–neurohypophyseal system is composed of the hypothalamus (the paraventricular nucleus 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 3,00 ...

), posterior pituitary, and these axonal projections.

Structure

The posterior pituitary consists mainly of

neuron A neuron, neurone, or nerve cell is an membrane potential#Cell excitability, electrically excitable cell (biology), cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous ...

al projections (

axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action ...

s) of magnocellular neurosecretory cells extending from the supraoptic and

paraventricular nuclei The paraventricular nucleus (PVN, PVA, or PVH) is a nucleus in the hypothalamus. Anatomically, it is adjacent to the third ventricle and many of its neurons project to the posterior pituitary. These projecting neurons secrete oxytocin and a smaller ...

of the

. These axons store and release neurohypophysial hormones oxytocin and

into the neurohypophyseal capillaries, from there they get into the systemic circulation (and partly back into the hypophyseal portal system). In addition to axons, the posterior pituitary also contains pituicytes, specialized glial cells resembling

astrocyte Astrocytes (from Ancient Greek , , "star" + , , "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 e ...

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 and pituicytes.

Infundibular stalk

Also known as the infundibulum or pituitary stalk, the infundibular stalk bridges the hypothalamic and hypophyseal systems. The median eminence 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 Pars intermedia is the boundary between the anterior and posterior lobes of the pituitary. It contains colloid-filled cysts and two types of cells - basophils and chromophobes. The cysts are the remainder of Rathke’s pouch. In human fetal life ...

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 Herring bodies before being secreted in the posterior pituitary via the bloodstream.

Clinical significance

Insufficient secretion of

underlies diabetes insipidus, 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 Neuroendocrinology