The posterior pituitary (or neurohypophysis) is the
posterior lobe of the
pituitary gland 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 that terminate behind the anterior pituitary, and serve as a site for the secretion of
neurohypophysial hormones (
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) directly into the blood.
The hypothalamic–neurohypophyseal system is composed of the hypothalamus (the
paraventricular nucleus and
supraoptic nucleus), 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 cells extending from the
supraoptic and
paraventricular nuclei of the
hypothalamus. These axons store and release neurohypophysial hormones
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 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). In addition to axons, the posterior pituitary also contains
pituicytes, specialized
glial cells resembling
astrocytes 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 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 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
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 ...
before being secreted in the posterior pituitary via the bloodstream.
Clinical significance
Insufficient secretion of
vasopressin 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).
See also
*
Anterior pituitary
*
Hypothalamic–pituitary–adrenal axis
The hypothalamic–pituitary–adrenal axis (HPA axis or HTPA axis) is a complex set of direct influences and feedback interactions among three components: the hypothalamus (a part of the brain located below the thalamus), the pituitary gland ( ...
*
Hypothalamic–pituitary–thyroid axis
*
Hypothalamic–pituitary–gonadal axis
References
Additional images
Image:Gray1180.png, The posterior pituitary comprises the posterior lobe of the pituitary gland.
External links
www.pituitary.orgnbsp;— The Pituitary Network Association
{{DEFAULTSORT:Posterior Pituitary
Endocrine system anatomy
Neuroendocrinology