A major organ of the

endocrine system The endocrine system is a messenger system comprising feedback loops of the hormones released by internal glands of an organism directly into the circulatory system, regulating distant target organs. In vertebrates, the hypothalamus is the neu ...

, the anterior pituitary (also called the adenohypophysis or pars anterior) is the

glandular In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormones) for release into the bloodstream (endocrine gland) or into cavities inside the body or its outer surface (exocrine gland). Structure De ...

, anterior lobe that together with the posterior lobe (

posterior pituitary 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 ...

, or the neurohypophysis) makes up the

pituitary gland In vertebrate anatomy, the pituitary gland, or hypophysis, is an endocrine gland, about the size of a chickpea and weighing, on average, in humans. It is a protrusion off the bottom of the hypothalamus at the base of the brain. The ...

(hypophysis). The anterior pituitary regulates several physiological processes, including

stress Stress may refer to: Science and medicine * Stress (biology), an organism's response to a stressor such as an environmental condition * Stress (linguistics), relative emphasis or prominence given to a syllable in a word, or to a word in a phrase ...

, growth, reproduction, and

lactation Lactation describes the secretion of milk from the mammary glands and the period of time that a mother lactates to feed her young. The process naturally occurs with all sexually mature female mammals, although it may predate mammals. The proces ...

. Proper functioning of the anterior pituitary and of the organs it regulates can often be ascertained via blood tests that measure

hormone A hormone (from the Greek participle , "setting in motion") is a class of signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and behavior. Hormones are require ...

levels.

Structure

The

sits in a protective bony enclosure called the

sella turcica The sella turcica (Latin for 'Turkish saddle') is a saddle-shaped depression in the body of the sphenoid bone of the human skull and of the skulls of other hominids including chimpanzees, gorillas and orangutans. It serves as a cephalome ...

(''Turkish chair/saddle''). It is composed of three lobes: the anterior, intermediate, and posterior lobes. In many animals, these lobes are distinct. However, in humans, the intermediate lobe is but a few cell layers thick and indistinct; as a result, it is often considered part of the anterior pituitary. In all animals, the fleshy, glandular anterior pituitary is distinct from the neural composition of the

. The anterior pituitary is composed of three regions: :; Pars distalis The pars distalis (distal part) comprises the majority of the anterior pituitary and is where the bulk of pituitary hormone production occurs. The pars distalis contains two types of cells, including

chromophobe cell A chromophobe is a histological structure that does not stain readily, and thus appears relatively pale under the microscope. Chromophobe cells are one of three cell stain types present in the anterior and intermediate lobes of the pituitary gland ...

s and chromophil cells. The chromophils can be further divided into

acidophils In the anterior pituitary, the term "acidophil" is used to describe two different types of cells which stain well with acidic dyes. * somatotrophs, which secrete growth hormone (a peptide hormone) * lactotrophs, which secrete prolactin (a pepti ...

(alpha cells) and

basophils Basophils are a type of white blood cell. Basophils are the least common type of granulocyte, representing about 0.5% to 1% of circulating white blood cells. However, they are the largest type of granulocyte. They are responsible for inflammator ...

(beta cells). These cells all together produce hormones of the anterior pituitary and release them into the blood stream. Nota bene: The terms "basophil" and "acidophil" are used by some books, whereas others prefer not to use these terms. This is due to the possible confusion with white blood cells, where one may also find basophils and acidophils. File:Histology of pars distalis of the anterior pituitary with chromophobes, basophils, and acidophils, annotated.jpg, Microanatomy of the pars distalis showing chromophobes, basophils, and acidophils File:Histology of anterior pituitary eosinophilic follicles.jpg, Eosinophilic follicles are a common normal finding in the anterior pituitary. :; Pars tuberalis: The pars tuberalis (tubular part) forms a part of the sheath extending up from the pars distalis, which joins with the

pituitary stalk The pituitary stalk (also known as the infundibular stalk, Fenderson's funnel, or simply the infundibulum) is the connection between the hypothalamus and the posterior pituitary. The floor of the third ventricle is prolonged downward as a funnel- ...

(also known as the infundibular stalk or infundibulum), arising from the posterior lobe. (The pituitary stalk connects the hypothalamus to the posterior pituitary.) The function of the pars tuberalis is poorly understood. However, it has been seen to be important in receiving the endocrine signal in the form of TSHB (a β subunit of TSH), informing the pars tuberalis of the photoperiod (length of day). The expression of this subunit is regulated by the secretion of melatonin in response to light information transmitted to the pineal gland. Earlier studies have shown localization of melatonin receptors in this region. :; Pars intermedia: The pars intermedia (intermediate part) sits between the pars distalis and the posterior pituitary, forming the boundary between the anterior and posterior pituitaries. It is very small and indistinct in humans.

Development

The anterior pituitary is derived from the ectoderm, more specifically from that of Rathke’s pouch, part of the developing

hard palate The hard palate is a thin horizontal bony plate made up of two bones of the facial skeleton, located in the roof of the mouth. The bones are the palatine process of the maxilla and the horizontal plate of palatine bone. The hard palate spans t ...

in the embryo. Rathke’s pouch is also

ectodermal The ectoderm is one of the three primary germ layers formed in early embryonic development. It is the outermost layer, and is superficial to the mesoderm (the middle layer) and endoderm (the innermost layer). It emerges and originates from the o ...

in origin. The pouch eventually loses its connection with the

pharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its st ...

, giving rise to the anterior pituitary. The anterior wall of Rathke's pouch proliferates, filling most of the pouch to form the pars distalis and the pars tuberalis. The posterior wall of the anterior pituitary forms the pars intermedia. Its formation from the soft tissues of the upper palate contrasts with the posterior pituitary, which originates from

neuroectoderm Neuroectoderm (or neural ectoderm or neural tube epithelium) consists of cells derived from ectoderm. Formation of the neuroectoderm is first step in the development of the nervous system. The neuroectoderm receives bone morphogenetic protein-inhi ...

.Nelson, R. J. (2011) An Introduction to Behavioral Endocrinology, 4th Edition. Sunderland, MA: Sinauer Associates, Inc.

Function

The anterior pituitary contains five types of endocrine cell, and they are defined by the hormones they secrete:

somatotrope Somatotropes (from the Greek ''sōmat'' meaning "body" and ''tropikós'' meaning "of or pertaining to a turn or change") are cells in the anterior pituitary that produce growth hormone. Structure Somatotropic cells constitute about 30−40% of ant ...

s (GH); lactotropes (PRL); gonadotropes (LH and FSH);

corticotrope Corticotropes (or corticotrophs) are basophilic cells in the anterior pituitary that produce pro-opiomelanocortin (POMC) which undergoes cleavage to adrenocorticotropin (ACTH), β-lipotropin (β-LPH), and melanocyte-stimulating hormone (MSH). Thes ...

s (ACTH) and thyrotropes (TSH). It also contains non-endocrine folliculostellate cells which are thought to stimulate and support the endocrine cell populations. Hormones secreted by the anterior pituitary are

trophic hormone Trophic hormones are hormones of the anterior lobe of the pituitary. These hormones affect growth, function, or nutrition of other endocrine cells. Trophic hormones can be found in body systems including the endocrine, gastrointestinal, urinary, a ...

s (Greek: trophe, “nourishment”). Trophic hormones directly affect growth either as hyperplasia or hypertrophy on the tissue it is stimulating. Tropic hormones are named for their ability to act directly on target tissues or other

endocrine gland Endocrine glands are ductless glands of the endocrine system that secrete their products, hormones, directly into the blood. The major glands of the endocrine system include the pineal gland, pituitary gland, pancreas, ovaries, testes, ...

s to release hormones, causing numerous cascading physiological responses.

Role in the endocrine system

Hypothalamic control

Hormone secretion from the anterior pituitary gland is regulated by hormones secreted by 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 hypothalamu ...

. Neuroendocrine cells in the hypothalamus project axons to the

median eminence The median eminence, part of the inferior boundary of the hypothalamus in the brain, is attached to the infundibulum. The median eminence is a small swelling on the tuber cinereum, posterior to and atop the pituitary stalk; it lies in the area rou ...

, at the base of the brain. At this site, these cells can release substances into small blood vessels that travel directly to the anterior pituitary gland (the hypothalamo-hypophyseal portal vessels).

Other mechanisms

Aside from hypothalamic control of the anterior pituitary, other systems in the body have been shown to regulate the anterior pituitary’s function. GABA can either stimulate or inhibit the secretion of

luteinizing hormone Luteinizing hormone (LH, also known as luteinising hormone, lutropin and sometimes lutrophin) is a hormone produced by gonadotropic cells in the anterior pituitary gland. The production of LH is regulated by gonadotropin-releasing hormone (GnRH) ...

(LH) and

growth hormone Growth hormone (GH) or somatotropin, also known as human growth hormone (hGH or HGH) in its human form, is a peptide hormone that stimulates growth, cell reproduction, and cell regeneration in humans and other animals. It is thus important in h ...

(GH) and can stimulate the secretion of

thyroid-stimulating hormone Thyroid-stimulating hormone (also known as thyrotropin, thyrotropic hormone, or abbreviated TSH) is a pituitary hormone that stimulates the thyroid gland to produce thyroxine (T4), and then triiodothyronine (T3) which stimulates the metabolism ...

(TSH). Prostaglandins are now known to inhibit

adrenocorticotropic hormone Adrenocorticotropic hormone (ACTH; also adrenocorticotropin, corticotropin) is a polypeptide tropic hormone produced by and secreted by the anterior pituitary gland. It is also used as a medication and diagnostic agent. ACTH is an important c ...

(ACTH) and also to stimulate TSH, GH and LH release. Clinical evidence supports the experimental findings of the excitatory and inhibitory effects GABA has on GH secretion, dependent on GABA’s site of action within the hypothalamic-pituitary axis.

Effects of the anterior pituitary

:;Thermal homeostasis The homeostatic maintenance of the anterior pituitary is crucial to our physiological well being. Increased plasma levels of TSH induce

hyperthermia Hyperthermia, also known simply as overheating, is a condition in which an individual's body temperature is elevated beyond normal due to failed thermoregulation. The person's body produces or absorbs more heat than it dissipates. When extreme ...

through a mechanism involving increased

metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run c ...

and

cutaneous Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other animal coverings, such as the arthropod exoskeleton, have different d ...

vasodilation Vasodilation is the widening of blood vessels. It results from relaxation of smooth muscle cells within the vessel walls, in particular in the large veins, large arteries, and smaller arterioles. The process is the opposite of vasoconstrictio ...

. Increased levels of LH also result in

hypothermia Hypothermia is defined as a body core temperature below in humans. Symptoms depend on the temperature. In mild hypothermia, there is shivering and mental confusion. In moderate hypothermia, shivering stops and confusion increases. In severe ...

but through a decreased metabolism action.

ACTH Adrenocorticotropic hormone (ACTH; also adrenocorticotropin, corticotropin) is a polypeptide tropic hormone produced by and secreted by the anterior pituitary gland. It is also used as a medication and diagnostic agent. ACTH is an important c ...

increase metabolism and induce cutaneous vasoconstriction, increased plasma levels also result in

and

prolactin Prolactin (PRL), also known as lactotropin, is a protein best known for its role in enabling mammals to produce milk. It is influential in over 300 separate processes in various vertebrates, including humans. Prolactin is secreted from the pit ...

decreases with decreasing temperature values. follicle-stimulating hormone (FSH) also may cause

if increased beyond homeostatic levels through an increased metabolic mechanism only. :;Gonadal function Gonadotropes, primarily luteinising hormone (LH) secreted from the anterior pituitary stimulates the ovulation cycle in female mammals, whilst in the males, LH stimulates the synthesis of

androgen An androgen (from Greek ''andr-'', the stem of the word meaning "man") is any natural or synthetic steroid hormone that regulates the development and maintenance of male characteristics in vertebrates by binding to androgen receptors. This in ...

which drives the ongoing will to mate together with a constant production of sperm. :;HPA axis Main article '' Hypothalamic-pituitary-adrenal axis'' The anterior pituitary plays a role in stress response. Corticotropin releasing hormone (CRH) from the hypothalamus stimulates

release in a cascading effect that ends with the production of glucocorticoids from the adrenal cortex.

Behavioral effects

:;Development: The release of GH, LH, and FSH are required for correct human development, including gonadal development. :;Breast-feeding: Release of the hormone

is essential for

. :;Stress: Operating through the hypothalamic-pituitary-adrenal axis (HPA), the anterior pituitary gland has a large role in the

neuroendocrine system Neuroendocrinology is the branch of biology (specifically of physiology) which studies the interaction between the nervous system and the endocrine system; i.e. how the brain regulates the hormonal activity in the body. The nervous and endocrine ...

’s stress response. Stress induces a release of corticotropin-releasing hormone (CRH) 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 trave ...

from the

, which activates the release of

(ACTH) from the anterior pituitary gland. Then, this acts on the adrenal cortex to produce

glucocorticoids Glucocorticoids (or, less commonly, glucocorticosteroids) are a class of corticosteroids, which are a class of steroid hormones. Glucocorticoids are corticosteroids that bind to the glucocorticoid receptor that is present in almost every vertebr ...

such as cortisol. These

act back on the anterior pituitary gland and the

with negative feedback to slow the production of CRH and ACTH. Increased cortisol under stress conditions can cause the following: metabolic effects (mobilization of

glucose Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, u ...

, fatty acids, and amino acids), bone re-absorption (calcium mobilization), activation of the sympathetic nervous system response (fight or flight), anti-inflammatory effects, and inhibition of reproduction/growth. When the anterior pituitary gland is removed ( hypophysectomy) in rats, their avoidance learning mechanisms were slowed, but injections of ACTH restored their performance. In addition, stress may delay the release of reproductive hormones such as

(LH) and follicle-stimulating hormone (FSH). This shows that the anterior pituitary gland is involved in behavioral functions as well as being part of a larger pathway for stress responses. It is also known that (HPA) hormones are related to certain skin diseases and skin homeostasis. There is evidence linking hyperactivity of HPA hormones to stress-related skin diseases and skin tumors. :;Aging: Operating through the hypothalamic-pituitary-gonadal axis, the anterior pituitary gland also affects the reproductive system. The hypothalamus releases

gonadotropin-releasing hormone Gonadotropin-releasing hormone (GnRH) is a releasing hormone responsible for the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary. GnRH is a tropic peptide hormone synthesized and release ...

(GnRH), which stimulates the release of

(LH) and follicle-stimulating hormone. Then the

gonads A gonad, sex gland, or reproductive gland is a mixed gland that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm. The male gonad, the testicle, produces sper ...

produce

estrogen Estrogen or oestrogen is a category of sex hormone responsible for the development and regulation of the female reproductive system and secondary sex characteristics. There are three major endogenous estrogens that have estrogenic hormonal ac ...

and

testosterone Testosterone is the primary sex hormone and anabolic steroid in males. In humans, testosterone plays a key role in the development of male reproductive tissues such as testes and prostate, as well as promoting secondary sexual characteristi ...

. The decrease in release of

gonadotropin Gonadotropins are glycoprotein hormones secreted by gonadotropic cells of the anterior pituitary of vertebrates. This family includes the mammalian hormones follicle-stimulating hormone (FSH) and luteinizing hormone (LH), the placental/ chorioni ...

s (LH and FSH) caused by normal aging may be responsible for

impotence Erectile dysfunction (ED), also called impotence, is the type of sexual dysfunction in which the penis fails to become or stay erect during sexual activity. It is the most common sexual problem in men.Cunningham GR, Rosen RC. Overview of mal ...

and frailty in elderly men because of the eventual decrease in production of testosterone. This lower level of

can have other effects, such as reduced

libido Libido (; colloquial: sex drive) is a person's overall sexual drive or desire for sexual activity. Libido is influenced by biological, psychological, and social factors. Biologically, the sex hormones and associated neurotransmitters that act u ...

, well-being and mood, muscle and bone strength, and metabolism. :;Tactile responding: It has been shown that infant mice who were stroked with a paintbrush (simulating motherly care) had more release and binding of

(GH) from the anterior pituitary gland. :;Circadian rhythms: Light information received by the eyes is transmitted to the pineal gland via the circadian pacemaker (the suprachiasmatic nucleus). Diminishing light stimulates the release of melatonin from the pineal gland which can also affect the secretion levels in the hypothalamic-pituitary-gonadal axis. Melatonin can lower levels of LH and FSH, which will decrease levels of

and testosterone. In addition, melatonin may affect production of

Clinical significance

Increased activity

Hyperpituitarism is the condition where the pituitary secretes excessive amounts of hormones. This hypersecretion often results in the formation of a pituitary adenoma (tumour), which are benign apart from a tiny fraction. There are mainly three types of anterior pituitary tumors and their associated disorders. For example, acromegaly results from excessive secretion of growth hormone (GH) often being released by a pituitary adenoma. This disorder can cause disfigurement and possibly death and can lead to gigantism, a hormone disorder shown in “giants” such as

André the Giant André René Roussimoff (; 19 May 1946 – 28 January 1993), better known by his ring name André the Giant, was a French professional wrestler and actor. Roussimoff was known for his great size, which was a result of gigantism caused by excess ...

, where it occurs before the epiphyseal plates in bones close in puberty. The most common type of pituitary tumour is a

prolactinoma A prolactinoma is a tumor ( adenoma) of the pituitary gland that produces the hormone prolactin. It is the most common type of functioning pituitary tumor. Symptoms of prolactinoma are due to abnormally high levels of prolactin in the blood (hyp ...

which hypersecretes

. A third type of pituitary adenoma secretes excess ACTH, which in turn, causes an excess of cortisol to be secreted and is the cause of

Cushing's disease Cushing's disease is one cause of Cushing's syndrome characterised by increased secretion of adrenocorticotropic hormone (ACTH) from the anterior pituitary (secondary hypercortisolism). This is most often as a result of a pituitary adenoma (spe ...

Decreased activity

Hypopituitarism Hypopituitarism is the decreased (''hypo'') secretion of one or more of the eight hormones normally produced by the pituitary gland at the base of the brain. If there is decreased secretion of one specific pituitary hormone, the condition is know ...

is characterized by a decreased secretion of hormones released by the anterior pituitary. For example, hypo-secretion of GH prior to puberty can be a cause of dwarfism. In addition, secondary adrenal insufficiency can be caused by hypo-secretion of ACTH which, in turn, does not signal the adrenal cortex to produce a sufficient amount of cortisol. This is a life-threatening condition.

could be caused by the destruction or removal of the anterior pituitary tissue through traumatic brain injury, tumor,

tuberculosis Tuberculosis (TB) is an infectious disease usually caused by '' Mycobacterium tuberculosis'' (MTB) bacteria. Tuberculosis generally affects the lungs, but it can also affect other parts of the body. Most infections show no symptoms, i ...

, or syphilis, among other causes. This disorder used to be referred to as Simmonds' disease but now according to the Diseases Database it is called Sheehan syndrome. If the hypopituitarism is caused by the blood loss associated with childbirth, the disorder is referred to as Sheehan syndrome.

History

Etymology

The anterior pituitary is also known as the adenohypophysis, meaning "glandular undergrowth", from the

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

''adeno-'' ("gland"), ''hypo'' ("under"), and ''physis'' ("growth").

Additional images

Image:Gray1180.png, The anterior pituitary is the anterior, glandular lobe of the

. Image:Nasennebenhöhlen.gif

References

External links

*
Embryology at unc.edu

The Pituitary Network Association -- www.pituitary.org

Secretion of hormones from the Anterior Pituitary Gland
{{Authority control Endocrine system Human head and neck