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Anterior Pituitary (Adenohypophysis)
The anterior pituitary, or adenohypophysis, is the glandular portion of the pituitary gland derived embryologically from oral ectoderm. It contains specialized endocrine cells that secrete growth hormone, prolactin, ACTH, TSH, FSH, and LH under predominantly hypothalamic control through the hypothalamo-hypophyseal portal system.
Region-
SystemEndocrine System
The anterior pituitary, also called the adenohypophysis, is the glandular endocrine portion of the pituitary gland. It constitutes the larger part of the pituitary and contains specialized endocrine cells responsible for secretion of growth hormone (GH), prolactin (PRL), adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH), follicle-stimulating hormone (FSH), and luteinizing hormone (LH).
Although anatomically attached to the hypothalamus, the anterior pituitary differs fundamentally from the posterior pituitary in its structure, embryological origin, and mechanism of hypothalamic control. The adenohypophysis develops primarily from oral ectoderm associated with Rathke's pouch, whereas the neurohypophysis develops from neural ectoderm of the developing diencephalon.
The hypothalamus regulates anterior pituitary secretion mainly through releasing and inhibiting hormones delivered by the hypothalamo-hypophyseal portal system. This vascular arrangement permits hypothalamic hormones to reach anterior pituitary endocrine cells directly at relatively high concentrations.
Location
The anterior pituitary forms the glandular portion of the pituitary gland within the hypophyseal fossa of the sella turcica. The sella turcica is a depression in the body of the sphenoid bone within the middle cranial fossa.
The gland lies inferior to the hypothalamus and optic chiasm and superior to the sphenoidal sinus. It is connected functionally and anatomically with the hypothalamus through the pituitary stalk and associated vascular pathways.
Pituitary Gland Divisions
The pituitary gland consists of two developmentally and histologically distinct major components: the adenohypophysis and neurohypophysis.
| Division | Alternative Name | Embryological Origin | General Function |
|---|---|---|---|
| Anterior pituitary | Adenohypophysis | Oral ectoderm | Synthesizes and secretes several endocrine hormones |
| Posterior pituitary | Neurohypophysis | Neural ectoderm | Stores and releases hypothalamically synthesized ADH and oxytocin |
Parts of the Adenohypophysis
The adenohypophysis is classically divided into three anatomical components:
- Pars distalis
- Pars intermedia
- Pars tuberalis
The pars distalis is by far the largest and most functionally important component in humans.
Pars Distalis
The pars distalis forms the major portion of the anterior pituitary. It contains cords and clusters of hormone-producing endocrine cells separated by a rich network of sinusoidal capillaries.
Most classical anterior pituitary hormones are synthesized and secreted by endocrine cells located within the pars distalis.
Pars Intermedia
The pars intermedia is a relatively poorly developed region in adult humans located between the pars distalis and posterior pituitary.
It represents tissue associated developmentally with Rathke's pouch and may contain small colloid-filled cysts representing remnants of the pouch lumen.
Pars Tuberalis
The pars tuberalis is a thin extension of adenohypophyseal tissue that surrounds part of the infundibular stalk.
Its cells include populations expressing gonadotropins and other endocrine markers, although its functional importance in humans is less prominent than that of the pars distalis.
Structural Organization
| Component | Location | Key Feature |
|---|---|---|
| Pars distalis | Main anterior portion | Contains most hormone-secreting endocrine cells |
| Pars intermedia | Between pars distalis and neurohypophysis | Rudimentary in adult humans |
| Pars tuberalis | Extends around infundibular stalk | Thin superior extension of adenohypophysis |
Embryological Development
The adenohypophysis develops primarily from an upward-growing diverticulum of oral ectoderm called Rathke's pouch.
This developmental pathway contrasts with the neurohypophysis, which develops as a downward extension of neuroectoderm from the floor of the diencephalon.
Rathke's Pouch
Rathke's pouch arises from the roof of the primitive oral cavity. It grows dorsally toward the developing brain and comes into close association with the descending neurohypophyseal diverticulum.
The connection between Rathke's pouch and the oral cavity subsequently disappears, while its epithelial cells proliferate and differentiate into the glandular components of the adenohypophysis.
Development of the Pars Distalis
The anterior wall of Rathke's pouch proliferates extensively and forms the major part of the pars distalis.
Endocrine cell lineages subsequently differentiate within this developing tissue and acquire their characteristic hormone-producing phenotypes.
Development of the Pars Intermedia
The posterior wall of Rathke's pouch contributes to the pars intermedia.
The original lumen of Rathke's pouch largely disappears, although small epithelial or colloid-containing remnants may persist between the anterior and posterior pituitary.
Development of the Pars Tuberalis
Adenohypophyseal cells extend superiorly around the developing infundibulum to form the pars tuberalis.
This creates the characteristic sleeve of glandular tissue surrounding part of the pituitary stalk.
Histological Organization
The pars distalis is composed of endocrine cells arranged in cords, nests, and clusters around a dense network of fenestrated sinusoidal capillaries.
This arrangement allows pituitary cells to receive hypothalamic regulatory hormones efficiently and release their own hormones rapidly into the circulation.
Anterior Pituitary Cell Types
Modern classification of anterior pituitary cells is based primarily on the hormones they produce and their transcriptional differentiation rather than solely on staining characteristics.
| Cell Type | Principal Hormone | Major Target |
|---|---|---|
| Somatotroph | Growth hormone | Liver and multiple peripheral tissues |
| Lactotroph | Prolactin | Mammary gland |
| Corticotroph | ACTH | Adrenal cortex |
| Thyrotroph | TSH | Thyroid gland |
| Gonadotroph | FSH and LH | Ovaries and testes |
Somatotrophs
Somatotrophs synthesize and secrete growth hormone. They constitute a substantial proportion of endocrine cells in the anterior pituitary.
Growth hormone secretion is stimulated principally by growth hormone-releasing hormone and inhibited by somatostatin. Ghrelin also stimulates GH secretion.
Growth Hormone
Growth hormone (GH), or somatotropin, promotes growth and influences carbohydrate, lipid, and protein metabolism.
Many of its growth-promoting effects are mediated by insulin-like growth factor 1, particularly IGF-1 produced by the liver and other tissues.
Lactotrophs
Lactotrophs, also called mammotrophs, synthesize and secrete prolactin.
Unlike most anterior pituitary endocrine cells, lactotrophs are normally under strong tonic inhibitory control from hypothalamic dopamine.
Prolactin
Prolactin (PRL) acts primarily on the mammary glands and is important for the initiation and maintenance of milk production after childbirth.
Dopamine inhibits prolactin secretion, while several factors, including suckling and TRH, can increase prolactin release.
Corticotrophs
Corticotrophs synthesize proopiomelanocortin (POMC), which is processed to produce ACTH and related peptides.
Hypothalamic CRH provides the principal stimulatory signal for ACTH secretion, with vasopressin capable of augmenting the response.
ACTH
Adrenocorticotropic hormone (ACTH) acts primarily on the adrenal cortex, particularly the zona fasciculata and zona reticularis.
ACTH stimulates cortisol synthesis and secretion and provides trophic support to ACTH-responsive regions of the adrenal cortex.
Thyrotrophs
Thyrotrophs synthesize and secrete thyroid-stimulating hormone.
TRH from the hypothalamus stimulates thyrotroph activity, while circulating thyroid hormones provide negative feedback at the pituitary and hypothalamic levels.
Thyroid-Stimulating Hormone
Thyroid-stimulating hormone (TSH), or thyrotropin, acts on thyroid follicular cells and stimulates thyroid hormone synthesis, secretion, and glandular growth.
TSH is a glycoprotein hormone composed of alpha and beta subunits.
Gonadotrophs
Gonadotrophs produce the gonadotropins FSH and LH.
Secretion of both hormones is controlled primarily by pulsatile hypothalamic gonadotropin-releasing hormone.
Follicle-Stimulating Hormone
Follicle-stimulating hormone (FSH) acts on the gonads. In the ovaries, it contributes to follicular development and granulosa cell function. In the testes, it acts primarily on Sertoli cells and supports spermatogenesis.
Luteinizing Hormone
Luteinizing hormone (LH) also acts on the gonads. In females, it participates in ovulation, corpus luteum formation, and ovarian steroidogenesis. In males, LH stimulates Leydig cells to produce testosterone.
Anterior Pituitary Hormones
| Hormone | Cell Type | Major Hypothalamic Regulation | Major Target |
|---|---|---|---|
| GH | Somatotroph | GHRH stimulates, somatostatin inhibits | Liver and multiple tissues |
| Prolactin | Lactotroph | Dopamine predominantly inhibits | Mammary gland |
| ACTH | Corticotroph | CRH stimulates | Adrenal cortex |
| TSH | Thyrotroph | TRH stimulates, somatostatin inhibits | Thyroid gland |
| FSH | Gonadotroph | GnRH stimulates | Gonads |
| LH | Gonadotroph | GnRH stimulates | Gonads |
Hypothalamic Control
The anterior pituitary is functionally linked to the hypothalamus through specialized releasing and inhibiting hormones.
These regulatory hormones are secreted by hypothalamic neurosecretory neurons into capillaries at the median eminence and delivered to the adenohypophysis through portal vessels.
Hypothalamic Regulatory Hormones
| Hormone | Principal Anterior Pituitary Effect |
|---|---|
| CRH | Stimulates ACTH secretion |
| TRH | Stimulates TSH and can stimulate prolactin |
| GnRH | Stimulates LH and FSH secretion |
| GHRH | Stimulates GH secretion |
| Somatostatin | Inhibits GH and TSH secretion |
| Dopamine | Inhibits prolactin secretion |
Median Eminence
The median eminence is a specialized region at the base of the hypothalamus where hypophysiotropic neurons release regulatory hormones into the primary capillary plexus.
Its fenestrated capillaries allow hypothalamic hormones to enter the portal circulation efficiently.
Hypothalamo-Hypophyseal Portal System
The hypothalamo-hypophyseal portal system is a vascular network that directly connects capillary beds in the hypothalamic region with those of the anterior pituitary.
This arrangement is essential because it allows small quantities of hypothalamic regulatory hormones to reach adenohypophyseal cells without first becoming extensively diluted in the systemic circulation.
Portal Circulation Pathway
| Step | Structure or Event |
|---|---|
| 1 | Superior hypophyseal arteries supply the median eminence and upper infundibulum |
| 2 | A primary capillary plexus is formed |
| 3 | Hypothalamic releasing and inhibiting hormones enter the primary plexus |
| 4 | Hypophyseal portal veins descend toward the anterior pituitary |
| 5 | A secondary capillary plexus forms within the adenohypophysis |
| 6 | Hypothalamic hormones leave the blood and regulate endocrine cells |
| 7 | Anterior pituitary hormones enter venous blood and reach the systemic circulation |
Superior Hypophyseal Arteries
The superior hypophyseal arteries contribute importantly to the vascular supply of the median eminence and pituitary stalk and generate the primary capillary plexus of the portal system.
They generally arise from branches associated with the internal carotid circulation.
Primary Capillary Plexus
The primary capillary plexus lies in the median eminence and upper portion of the infundibular region.
Hypothalamic releasing and inhibiting hormones enter these fenestrated vessels before being transported through portal veins.
Hypophyseal Portal Veins
Hypophyseal portal veins carry blood containing hypothalamic regulatory hormones from the primary capillary plexus toward the adenohypophysis.
They provide the crucial vascular connection between the hypothalamus and anterior pituitary.
Secondary Capillary Plexus
The portal vessels form a secondary capillary plexus among the endocrine cells of the anterior pituitary.
Hypothalamic hormones diffuse from these capillaries to their target pituitary cells, while pituitary hormones subsequently enter the vascular system for systemic distribution.
Blood Supply
The vascular organization of the pituitary is specialized for endocrine communication. The superior hypophyseal arterial system is particularly important for portal delivery of hypothalamic hormones to the adenohypophysis.
Additional arterial contributions and anastomoses support pituitary tissue, while venous drainage carries secreted hormones toward the cavernous sinus and systemic circulation.
Relationship to the Cavernous Sinus
The pituitary gland lies between the paired cavernous sinuses. These venous spaces contain the internal carotid arteries and several cranial nerves along or within their walls.
This close anatomical relationship is important because expanding pituitary lesions can extend laterally into the cavernous sinus and affect nearby neurovascular structures.
Relationship to the Optic Chiasm
The optic chiasm lies superior to the pituitary gland. Enlargement of the anterior pituitary, particularly from a pituitary macroadenoma, can extend superiorly and compress the chiasm.
Compression classically affects crossing nasal retinal fibers and can produce bitemporal visual field loss.
Relationship to the Sphenoidal Sinus
The sphenoidal sinus lies inferior to the pituitary gland and sella turcica.
This anatomical relationship provides the basis for the transsphenoidal surgical approach commonly used to access pituitary lesions.
Acidophils
In traditional histological staining, some anterior pituitary cells are classified as acidophils because their cytoplasmic granules stain strongly with acidic dyes.
Somatotrophs and lactotrophs are traditionally included in this category.
Basophils
Basophils contain secretory granules that demonstrate affinity for basic staining techniques in conventional histology.
Corticotrophs, thyrotrophs, and gonadotrophs are traditionally classified among the basophilic endocrine cells.
Chromophobes
Chromophobes appear relatively pale with routine histological staining and contain fewer conspicuous cytoplasmic secretory granules.
This category can include degranulated endocrine cells and other poorly staining cellular populations and is less functionally precise than modern immunohistochemical classification.
Traditional Histological Classification
| Category | Typical Associated Cells |
|---|---|
| Acidophils | Somatotrophs and lactotrophs |
| Basophils | Corticotrophs, thyrotrophs, and gonadotrophs |
| Chromophobes | Poorly staining or degranulated cells and other populations |
Fenestrated Capillaries
The anterior pituitary contains an extensive network of fenestrated capillaries, a characteristic feature of endocrine organs.
The fenestrations facilitate rapid exchange of hypothalamic regulatory signals and pituitary hormones between endocrine cells and the circulation.
Endocrine Axes
Several anterior pituitary hormones form the intermediate level of hierarchical endocrine axes in which the hypothalamus regulates the pituitary and the pituitary regulates a peripheral endocrine organ.
| Axis | Hypothalamus | Anterior Pituitary | Peripheral Endocrine Output |
|---|---|---|---|
| HPA axis | CRH | ACTH | Cortisol |
| HPT axis | TRH | TSH | T3 and T4 |
| HPG axis | GnRH | LH and FSH | Gonadal steroids and inhibins |
| GH axis | GHRH and somatostatin | GH | IGF-1 and direct GH effects |
Negative Feedback
Peripheral hormones commonly provide negative feedback to the anterior pituitary and hypothalamus.
For example, cortisol suppresses ACTH and CRH, thyroid hormones suppress TSH and TRH, and gonadal steroids influence gonadotropin and GnRH secretion.
Anterior Pituitary Versus Posterior Pituitary
| Feature | Anterior Pituitary | Posterior Pituitary |
|---|---|---|
| Alternative name | Adenohypophysis | Neurohypophysis |
| Embryological origin | Oral ectoderm | Neural ectoderm |
| Predominant tissue | Glandular endocrine tissue | Neural tissue |
| Hypothalamic connection | Predominantly vascular portal system | Direct axonal tract |
| Hormone synthesis | Hormones synthesized by pituitary endocrine cells | ADH and oxytocin synthesized in hypothalamus |
| Major hormones | GH, PRL, ACTH, TSH, FSH, LH | Releases ADH and oxytocin |
Pituitary Adenomas
Pituitary adenomas, now commonly classified within the spectrum of pituitary neuroendocrine tumors, arise from adenohypophyseal endocrine cells. They may secrete excessive amounts of a pituitary hormone or present primarily through mass effects.
The clinical manifestations depend on the cell lineage, hormone secretion, tumor size, and relationship to surrounding structures.
Prolactin-Secreting Tumors
Lactotroph tumors can produce excessive prolactin, causing hyperprolactinemia. Elevated prolactin can suppress reproductive function by interfering with hypothalamic GnRH activity.
Hyperprolactinemia may also occur when a sellar mass disrupts delivery of hypothalamic dopamine to otherwise normal lactotrophs.
Growth Hormone-Secreting Tumors
Somatotroph tumors can cause excessive growth hormone secretion. The resulting clinical syndrome depends substantially on whether excess GH begins before or after epiphyseal closure.
In adults, chronic GH excess produces acromegaly.
ACTH-Secreting Tumors
Corticotroph tumors can secrete excessive ACTH and chronically stimulate the adrenal cortex.
Pituitary ACTH-dependent hypercortisolism is specifically termed Cushing disease.
Nonfunctioning Pituitary Tumors
Some pituitary tumors do not produce a clinically obvious hormone hypersecretion syndrome and may therefore present after becoming sufficiently large to produce local mass effects.
Possible consequences include visual field abnormalities, headache, pituitary hormone deficiencies, and compression of surrounding structures.
Pituitary Macroadenoma and Optic Chiasm
A sufficiently large sellar mass can expand superiorly through the diaphragma sellae toward the optic chiasm.
Compression of crossing optic nerve fibers at the chiasm classically produces loss of temporal visual fields in both eyes.
Hypopituitarism
Hypopituitarism refers to deficient secretion of one or more pituitary hormones. Causes can include tumors, surgery, radiation, vascular injury, inflammation, trauma, congenital abnormalities, and hypothalamic or pituitary stalk disease.
The physiological consequences depend on which hormonal axes are affected.
Panhypopituitarism
Panhypopituitarism refers to extensive loss of anterior pituitary hormone function affecting multiple endocrine axes.
Deficiencies may involve ACTH, TSH, gonadotropins, GH, and other pituitary functions to varying degrees.
Pituitary Stalk Interruption
Damage to the pituitary stalk can disrupt delivery of hypothalamic releasing and inhibiting hormones to the adenohypophysis.
Most anterior pituitary secretions may decline because stimulatory hypothalamic input is lost. Prolactin can behave differently because dopamine normally exerts tonic inhibition, so interruption of dopamine delivery may increase prolactin concentrations.
Pituitary Apoplexy
Pituitary apoplexy refers to acute hemorrhage or infarction involving the pituitary, often occurring within a pre-existing pituitary tumor.
Rapid expansion within the confined sellar region can produce severe headache, visual abnormalities, ophthalmoplegia, and acute endocrine deficiencies, particularly ACTH deficiency.
Sheehan Syndrome
Sheehan syndrome is postpartum hypopituitarism resulting from ischemic injury to the enlarged anterior pituitary associated with severe obstetric hemorrhage or hypotension.
The condition illustrates the vulnerability of adenohypophyseal tissue to compromised perfusion.
Rathke's Cleft Cyst
A Rathke's cleft cyst is an epithelial cyst arising from remnants associated with Rathke's pouch.
Many are asymptomatic, but sufficiently large lesions can compress pituitary tissue or adjacent structures and produce endocrine or neurological manifestations.
Craniopharyngioma
Craniopharyngiomas are epithelial tumors arising in the sellar or suprasellar region and are related developmentally to remnants of the craniopharyngeal duct or Rathke's pouch pathway.
Their location can affect the pituitary, pituitary stalk, hypothalamus, and optic apparatus.
Transsphenoidal Approach
The close relationship between the pituitary gland and sphenoidal sinus permits surgical access through a transsphenoidal approach.
This route provides access to the sella while avoiding direct traversal of large portions of the cranial cavity and is widely used for pituitary surgery.
Key Features of the Anterior Pituitary
| Feature | Key Point |
|---|---|
| Alternative name | Adenohypophysis |
| Location | Hypophyseal fossa of the sella turcica |
| Embryological origin | Oral ectoderm through Rathke's pouch |
| Largest component | Pars distalis |
| Other components | Pars intermedia and pars tuberalis |
| Major cell types | Somatotrophs, lactotrophs, corticotrophs, thyrotrophs, gonadotrophs |
| Major hormones | GH, prolactin, ACTH, TSH, FSH, LH |
| Hypothalamic connection | Hypothalamo-hypophyseal portal circulation |
| Major vascular feature | Primary and secondary capillary plexuses connected by portal veins |
| Superior relationship | Optic chiasm and hypothalamic region |
| Inferior relationship | Sphenoidal sinus |
| Lateral relationship | Cavernous sinuses |
Anatomical and Functional Importance
The anterior pituitary occupies a central position in endocrine regulation because it converts hypothalamic neural signals into hormonal signals capable of controlling distant endocrine glands and peripheral tissues. Its endocrine cells are organized around a dense network of fenestrated capillaries that supports rapid communication between the hypothalamus, pituitary, and systemic circulation.
The pars distalis contains the principal hormone-producing cell populations. Somatotrophs secrete GH, lactotrophs secrete prolactin, corticotrophs produce ACTH, thyrotrophs produce TSH, and gonadotrophs produce FSH and LH. Together these hormones regulate growth, lactation, adrenal function, thyroid function, and reproduction.
The adenohypophysis is controlled primarily through the hypothalamo-hypophyseal portal system. Hypothalamic regulatory hormones enter a primary capillary plexus near the median eminence, travel through portal veins, and reach a secondary capillary plexus within the anterior pituitary. This vascular architecture permits precise control without requiring direct axonal contact between hypothalamic neurons and most anterior pituitary endocrine cells.
The gland's embryological origin from Rathke's pouch explains its glandular epithelial character and several clinically important developmental lesions. Its position within the sella turcica also creates characteristic anatomical relationships with the optic chiasm, cavernous sinuses, and sphenoidal sinus. These relationships explain visual abnormalities caused by expanding pituitary masses and provide the anatomical basis for transsphenoidal surgical access.
Through its distinctive embryology, specialized cell populations, portal vascular system, endocrine axes, and close anatomical relationship with the hypothalamus, the anterior pituitary functions as one of the major integrating centers of the human endocrine system.
Published on October 1, 2026
Last updated on October 1, 2026
Last updated on October 1, 2026