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Posterior Pituitary (Neurohypophysis)
The posterior pituitary, or neurohypophysis, is the neural portion of the pituitary gland that stores and releases oxytocin and antidiuretic hormone synthesized by hypothalamic neurons. It consists primarily of the pars nervosa and infundibulum and is directly connected to the hypothalamus by the hypothalamo-hypophyseal tract.
Region-
SystemEndocrine System
The posterior pituitary, also called the neurohypophysis, is the neural component of the pituitary gland. It forms a direct anatomical and functional extension of the hypothalamus and serves primarily as the site where antidiuretic hormone (ADH, or vasopressin) and oxytocin are stored and released into the systemic circulation.
Unlike the anterior pituitary, the posterior pituitary does not contain a major population of hormone-producing endocrine cells. ADH and oxytocin are synthesized within neurosecretory neurons whose cell bodies lie in the hypothalamus. Their axons descend through the pituitary stalk and terminate within the posterior pituitary, where the hormones are released into a dense capillary network.
The neurohypophysis is therefore best understood as a specialized neurovascular interface. It converts electrical activity generated by hypothalamic neurons into systemic endocrine signals.
Location
The posterior pituitary occupies the posterior portion of the pituitary gland within the hypophyseal fossa of the sella turcica of the sphenoid bone.
It lies posterior to the anterior pituitary and remains connected superiorly to the hypothalamus by the infundibulum, or pituitary stalk.
Relationship to the Hypothalamus
The neurohypophysis is directly continuous with the hypothalamus. Axons arising from hypothalamic neurosecretory neurons descend through the infundibulum and terminate in the pars nervosa.
This direct neural connection distinguishes the posterior pituitary from the anterior pituitary, which is regulated predominantly through the hypothalamo-hypophyseal portal vascular system.
Components of the Neurohypophysis
The neurohypophysis includes structures derived from the neural ectoderm of the developing diencephalon.
| Component | Description |
|---|---|
| Pars nervosa | Major posterior pituitary region containing neurosecretory axons and terminals |
| Infundibulum | Neural stalk connecting the hypothalamus with the pituitary |
| Median eminence | Basal hypothalamic region associated with neuroendocrine communication |
Pars Nervosa
The pars nervosa forms the largest portion of the posterior pituitary. It contains numerous unmyelinated axons originating from magnocellular neurosecretory neurons in the hypothalamus.
These axons terminate close to fenestrated capillaries, allowing neurohormones released from their terminals to rapidly enter the bloodstream.
Infundibulum
The infundibulum, commonly called the pituitary stalk, connects the pituitary gland to the floor of the hypothalamus.
It contains descending axons of hypothalamic neurosecretory neurons as well as vascular structures associated with hypothalamic-pituitary communication.
Hypothalamo-Hypophyseal Tract
The hypothalamo-hypophyseal tract is formed by axons extending from magnocellular neurosecretory neurons of the hypothalamus to the posterior pituitary.
These axons provide the pathway by which ADH and oxytocin are transported from their sites of synthesis to their sites of storage and release.
Hypothalamic Nuclei
The principal neurons supplying the posterior pituitary are located within the supraoptic nucleus and paraventricular nucleus of the hypothalamus.
Both nuclei contain neurons capable of producing ADH and oxytocin, although ADH-producing neurons are particularly prominent in the supraoptic nucleus and oxytocin-producing neurons are particularly prominent in the paraventricular nucleus.
Supraoptic Nucleus
The supraoptic nucleus lies in the anterior hypothalamus near the optic chiasm.
Its magnocellular neurons contribute extensively to the hypothalamo-hypophyseal tract and are an important source of vasopressin released from the posterior pituitary.
Paraventricular Nucleus
The paraventricular nucleus lies adjacent to the third ventricle. It contains several populations of neurosecretory neurons with different projections and functions.
Its magnocellular neurons contribute to posterior pituitary secretion, particularly of oxytocin, while other paraventricular neurons participate in additional hypothalamic endocrine and autonomic pathways.
Magnocellular Neurosecretory Neurons
Magnocellular neurosecretory neurons are large hypothalamic neurons specialized for neuroendocrine secretion.
Their cell bodies synthesize peptide hormones, while their long axons transport secretory vesicles through the pituitary stalk to terminals in the pars nervosa.
Neurosecretory Pathway
| Step | Event |
|---|---|
| 1 | ADH or oxytocin is synthesized in hypothalamic neuronal cell bodies |
| 2 | Hormone is packaged into neurosecretory vesicles |
| 3 | Vesicles undergo axonal transport through the hypothalamo-hypophyseal tract |
| 4 | Hormone-containing vesicles accumulate within posterior pituitary axons and terminals |
| 5 | Action potentials travel from the hypothalamus toward the nerve terminals |
| 6 | Calcium-dependent exocytosis releases hormone |
| 7 | Hormone enters posterior pituitary capillaries and systemic circulation |
Histological Organization
Histologically, the posterior pituitary differs markedly from the densely cellular anterior pituitary. The pars nervosa contains numerous axons and nerve terminals interspersed with supporting glial cells and capillaries.
Its lighter microscopic appearance reflects the predominance of neural processes rather than densely packed endocrine epithelial cells.
Unmyelinated Axons
Most axons within the pars nervosa are unmyelinated. They arise from hypothalamic magnocellular neurons and terminate near capillaries within the posterior pituitary.
The axons contain neurosecretory vesicles carrying ADH or oxytocin together with associated carrier proteins.
Pituicytes
Pituicytes are specialized glial cells forming an important supporting cellular population of the posterior pituitary.
They resemble astrocytes functionally and structurally and interact with neurosecretory axons and terminals. Pituicytes do not serve as the principal source of ADH or oxytocin.
Functions of Pituicytes
Pituicytes provide structural support within the pars nervosa and participate in the organization of neurosecretory terminals around the capillary network.
Changes in pituicyte morphology may influence access of neurosecretory terminals to the perivascular space during periods of increased hormonal release.
Herring Bodies
Herring bodies are focal dilations of neurosecretory axons within the posterior pituitary.
They contain accumulations of secretory granules carrying ADH or oxytocin and their associated neurophysins. They are characteristic histological features of the neurohypophysis.
Neurophysins
Neurophysins are carrier proteins associated with posterior pituitary hormones during synthesis and axonal transport.
| Hormone | Associated Carrier Protein |
|---|---|
| Oxytocin | Neurophysin I |
| ADH | Neurophysin II |
Posterior Pituitary Hormones
The two principal hormones released from the posterior pituitary are ADH and oxytocin.
| Hormone | Major Peripheral Targets | Major Functions |
|---|---|---|
| ADH | Kidney collecting ducts and vascular smooth muscle | Water conservation and regulation of plasma osmolality, with vasoconstrictor effects at higher concentrations |
| Oxytocin | Uterine myometrium and mammary myoepithelial cells | Uterine contraction and milk ejection |
Antidiuretic Hormone
Antidiuretic hormone (ADH), also called arginine vasopressin, is synthesized by magnocellular hypothalamic neurons and released from the posterior pituitary.
Its principal physiological function is regulation of body water balance through control of renal water reabsorption.
ADH and the Kidney
ADH acts primarily on V2 receptors expressed by principal cells of the renal collecting duct.
V2 receptor signaling promotes insertion of aquaporin-2 water channels into the apical membrane, increasing water permeability and allowing greater water reabsorption.
ADH and Osmolality
ADH secretion is strongly regulated by plasma osmolality. Specialized osmoregulatory mechanisms within the hypothalamic region detect changes in extracellular fluid concentration.
Increasing effective plasma osmolality stimulates vasopressin release and thirst, promoting conservation and replacement of water.
ADH and Blood Volume
Changes in effective circulating volume and arterial pressure also influence vasopressin secretion.
Significant reductions in volume or pressure can strongly stimulate ADH release through cardiovascular afferent pathways.
Oxytocin
Oxytocin is synthesized by hypothalamic magnocellular neurons and released from terminals within the posterior pituitary.
Its best-established peripheral endocrine actions involve uterine smooth muscle contraction and contraction of mammary myoepithelial cells.
Oxytocin and Labor
Near term, uterine expression of oxytocin receptors increases. Oxytocin can therefore produce increasingly effective contractions of the myometrium.
Cervical stretch during labor can activate a neuroendocrine positive feedback mechanism that increases oxytocin release and strengthens uterine contractions.
Oxytocin and Lactation
Suckling activates sensory pathways that stimulate hypothalamic oxytocin neurons.
Oxytocin released from the posterior pituitary causes contraction of myoepithelial cells around mammary alveoli and ducts, producing the milk ejection reflex.
Hormone Storage
ADH and oxytocin can be stored within secretory vesicles along posterior pituitary axons and at their terminals before release.
Accumulations of these vesicles within axonal swellings contribute to the formation of Herring bodies.
Hormone Release
Posterior pituitary hormone release is controlled by electrical activity in hypothalamic neurosecretory neurons.
Action potentials reaching nerve terminals open voltage-gated calcium channels. Calcium entry triggers exocytosis of hormone-containing vesicles into the extracellular space adjacent to capillaries.
Fenestrated Capillaries
The posterior pituitary contains a rich network of fenestrated capillaries suited for rapid transfer of peptide hormones into the bloodstream.
Neurosecretory axon terminals are closely associated with this vascular network.
Blood Supply
The posterior pituitary receives its principal arterial supply from the inferior hypophyseal arteries, which usually arise from the cavernous portion of the internal carotid arteries.
Additional vascular contributions and anastomoses can occur within the pituitary region.
Inferior Hypophyseal Arteries
The inferior hypophyseal arteries form a capillary plexus within the posterior pituitary.
ADH and oxytocin released from neurosecretory terminals enter these vessels and are subsequently carried into the systemic venous circulation.
Venous Drainage
Blood from the pituitary drains through hypophyseal veins into venous channels surrounding the sella, including the cavernous sinus region.
This drainage carries posterior pituitary hormones into the systemic circulation.
Posterior Pituitary Versus Anterior Pituitary
| Feature | Posterior Pituitary | Anterior Pituitary |
|---|---|---|
| Alternative name | Neurohypophysis | Adenohypophysis |
| Primary tissue | Neural | Glandular epithelial |
| Embryological origin | Neuroectoderm of diencephalon | Oral ectoderm of Rathke's pouch |
| Connection with hypothalamus | Direct axonal connection | Primarily portal vascular connection |
| Major hormones associated | ADH and oxytocin | GH, prolactin, ACTH, TSH, LH and FSH |
| Hormone synthesis | Occurs in hypothalamus | Occurs in pituitary endocrine cells |
| Supporting cells | Pituicytes | Multiple endocrine cell populations and folliculostellate cells |
Neurohypophysis Versus Adenohypophysis
The terms neurohypophysis and adenohypophysis emphasize the fundamentally different tissue organization of the posterior and anterior portions of the pituitary gland.
The neurohypophysis develops as an extension of the brain and contains neural tissue, whereas the adenohypophysis develops from oral ectoderm and forms glandular endocrine tissue.
Embryological Development
The posterior pituitary develops from a downward extension of neuroectoderm from the floor of the developing diencephalon.
This neural evagination forms the infundibulum and neural components of the neurohypophysis.
Development of the Pituitary Gland
The pituitary gland has a dual embryological origin. The posterior pituitary develops from neuroectoderm, while the anterior pituitary develops from an upward-growing diverticulum of oral ectoderm known as Rathke's pouch.
The close association of these independently derived tissues produces the mature pituitary gland.
Embryological Comparison
| Structure | Embryological Origin |
|---|---|
| Posterior pituitary | Neuroectoderm of the developing diencephalon |
| Anterior pituitary | Oral ectoderm of Rathke's pouch |
Relationship to the Sella Turcica
The posterior pituitary lies within the sella turcica together with the anterior pituitary. Superiorly, the pituitary stalk passes through an opening in the diaphragma sellae toward the hypothalamus.
The gland's confined sellar location creates important relationships with the optic chiasm superiorly and cavernous sinuses laterally.
Relationship to the Optic Chiasm
The optic chiasm lies superior to the pituitary region. Expanding sellar or suprasellar lesions can therefore affect visual pathways.
Large pituitary masses extending superiorly may compress the optic chiasm and produce visual field abnormalities.
Relationship to the Cavernous Sinus
The cavernous sinuses lie lateral to the pituitary gland. They contain important neurovascular structures, including the internal carotid artery and several cranial nerves.
Large lesions involving the pituitary region can extend laterally and affect these structures.
Pituitary Stalk Lesions
Lesions affecting the pituitary stalk can disrupt axonal transport of posterior pituitary hormones and interfere with hypothalamic control of the anterior pituitary.
The clinical consequences depend on the anatomical level, severity, and cause of the lesion.
Central Diabetes Insipidus
Central diabetes insipidus results from insufficient secretion of vasopressin caused by dysfunction of the hypothalamic-neurohypophyseal system.
Reduced ADH action decreases renal water reabsorption, resulting in excretion of large volumes of dilute urine and increased thirst.
Causes of Central Diabetes Insipidus
Central diabetes insipidus can result from hypothalamic or pituitary surgery, trauma, tumors, inflammatory disorders, infiltrative disease, genetic abnormalities, or other lesions affecting vasopressin-producing neurons or their axons.
In some cases, no specific cause is identified.
Central Versus Nephrogenic Diabetes Insipidus
| Feature | Central Diabetes Insipidus | Nephrogenic Diabetes Insipidus |
|---|---|---|
| Primary defect | Insufficient ADH secretion | Reduced renal response to ADH |
| Major anatomical level | Hypothalamic-neurohypophyseal system | Kidney |
| ADH availability | Reduced or inadequate | Usually present |
| Renal water conservation | Impaired | Impaired |
SIADH
Syndrome of inappropriate antidiuretic hormone secretion (SIADH) involves excessive vasopressin effect relative to physiological needs.
Excessive renal water retention can produce dilutional hyponatremia and reduced plasma osmolality.
Posterior Pituitary on Imaging
The posterior pituitary often demonstrates a characteristic area of high signal intensity on unenhanced T1-weighted magnetic resonance imaging, commonly called the posterior pituitary bright spot.
The appearance is associated with neurosecretory material within the posterior pituitary, although its presence or absence must be interpreted within the broader clinical and imaging context.
Posterior Pituitary Bright Spot
The posterior pituitary bright spot can provide useful information when evaluating disorders of the hypothalamic-neurohypophyseal system.
Its absence is not by itself diagnostic of a specific disorder, and imaging findings must be correlated with endocrine testing and clinical features.
Clinical Importance of the Pituitary Stalk
The pituitary stalk is a narrow anatomical pathway carrying posterior pituitary axons as well as vessels involved in anterior pituitary regulation.
Consequently, a single stalk lesion can potentially affect both posterior and anterior pituitary function.
Posterior Pituitary Hormone Deficiency
Clinically significant dysfunction of the posterior pituitary most commonly involves vasopressin deficiency.
Isolated oxytocin deficiency is less readily recognized clinically because labor and lactation involve multiple interacting neural, endocrine, and local mechanisms.
Neuroendocrine Integration
The posterior pituitary is a classic example of neuroendocrine integration. Hormones are synthesized by neurons, transported through axons, and released directly into the circulation in response to electrical activity.
This allows the nervous system to generate rapid endocrine responses to changes in osmolality, blood volume, cervical stretch, and suckling.
Posterior Pituitary Functional Organization
| Anatomical Level | Function |
|---|---|
| Hypothalamic nuclei | Synthesize ADH and oxytocin |
| Neuronal axons | Transport neurosecretory vesicles |
| Infundibulum | Provides connection between hypothalamus and pars nervosa |
| Pars nervosa | Stores and releases neurohormones |
| Fenestrated capillaries | Receive hormones into systemic blood |
Key Histological Features
| Feature | Description |
|---|---|
| Axons | Numerous unmyelinated neurosecretory axons |
| Pituicytes | Specialized supporting glial cells |
| Herring bodies | Axonal dilations containing neurosecretory granules |
| Capillaries | Dense fenestrated vascular network |
| Endocrine epithelial cells | Not the principal hormone-producing component |
Key Features of the Posterior Pituitary
| Feature | Key Point |
|---|---|
| Alternative name | Neurohypophysis |
| Tissue type | Neural tissue |
| Embryological origin | Neuroectoderm of the diencephalon |
| Major component | Pars nervosa |
| Connection to hypothalamus | Infundibulum and hypothalamo-hypophyseal tract |
| Major hypothalamic nuclei | Supraoptic and paraventricular nuclei |
| Supporting cells | Pituicytes |
| Characteristic axonal structures | Herring bodies |
| Hormones released | ADH and oxytocin |
| Principal arterial supply | Inferior hypophyseal arteries |
| Major clinical association | Disorders of vasopressin secretion, including central diabetes insipidus |
Anatomical and Physiological Importance
The posterior pituitary represents the terminal endocrine component of the hypothalamic-neurohypophyseal system. Its structure differs fundamentally from that of the anterior pituitary because the neurohypophysis is composed predominantly of neural tissue rather than hormone-producing glandular epithelium.
Magnocellular neurons located principally in the supraoptic and paraventricular nuclei synthesize ADH and oxytocin. Their unmyelinated axons descend through the hypothalamo-hypophyseal tract within the infundibulum and terminate in the pars nervosa. Hormone-containing vesicles are transported along these axons and may accumulate within characteristic swellings known as Herring bodies.
When hypothalamic neurons are activated, action potentials propagate to their posterior pituitary terminals. Calcium-dependent exocytosis releases ADH or oxytocin into the dense network of fenestrated capillaries. In this way, neuronal electrical signals are converted directly into endocrine signals distributed through the systemic circulation.
ADH connects the neurohypophysis to the regulation of water balance. Changes in plasma osmolality and effective circulating volume alter vasopressin secretion, which in turn regulates water permeability of the renal collecting ducts. Oxytocin connects the same neuroendocrine system to reproductive physiology through uterine contraction and milk ejection.
The neurohypophysis is also anatomically important because its axons traverse the pituitary stalk. Damage involving the hypothalamus, stalk, or posterior pituitary can therefore interrupt hormone synthesis, transport, or release. Vasopressin deficiency can produce central diabetes insipidus, while abnormalities of excessive vasopressin action can contribute to disorders of water and sodium balance.
Through its direct neural continuity with the hypothalamus, specialized neurosecretory axons, pituicytes, Herring bodies, fenestrated capillary network, and release of ADH and oxytocin, the posterior pituitary provides one of the clearest anatomical examples of integration between the nervous and endocrine systems.
Published on October 1, 2026
Last updated on October 1, 2026
Last updated on October 1, 2026