| Anterior pituitary gland | |
|---|---|
 Diagram of anterior lobe of the pituitary, and its sections shown with the posterior lobe | |
| Details | |
| Precursor | Oral mucosa (Rathke's pouch) |
| Artery | Superior hypophyseal |
| Vein | Hypophyseal |
| Identifiers | |
| Latin | lobus anterior hypophysis |
| MeSH | D010903 |
| NeuroNames | 407 |
| NeuroLex ID | birnlex_1581 |
| TA98 | A11.1.00.002 |
| TA2 | 3855 |
| FMA | 74627 |
| Anatomical terminology | |
Theanterior pituitary (also called theadenohypophysis orpars anterior) is a majororgan of theendocrine system. The anterior pituitary is theglandular,anterior lobe that together with theposterior pituitary (or neurohypophysis) makes up thepituitary gland (hypophysis) which, inhumans, is located at the base of thebrain, protruding off the bottom of thehypothalamus.
The anterior pituitary regulates severalphysiological processes, includingstress,growth,reproduction, andlactation. Proper functioning of the anterior pituitary and of the organs it regulates can often be ascertained viablood tests that measurehormone levels.
Thepituitary gland sits in a protective bony enclosure called thesella turcica (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 theneural composition of thepars nervosa of theposterior pituitary.[citation needed]
The anterior pituitary is composed of three regions, the pars distalis, pars tuberalis, and pars intermedia.
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, includingchromophobe cells andchromophil cells.[1] The chromophils can be further divided intoacidophils (alpha cells) andbasophils (beta cells).[1] These cells all together produce hormones of the anterior pituitary and release them into the blood stream.[citation needed]
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.
The pars tuberalis (tubular part) forms a part of a highly vascularised sheath extending up from the pars distalis, which joins with thepituitary stalk (also known as the infundibular stalk orinfundibulum), 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 ofTSHB (a β subunit of TSH), informing the pars tuberalis of thephotoperiod (length of day). The expression of this subunit is regulated by the secretion ofmelatonin in response to light information transmitted to thepineal gland.[2][3] Earlier studies have shown localization of melatonin receptors in this region.[4]
Principal cells of the pars tuberalis are low columnar in form, with the cytoplasm containing numerous lipid droplets, glycogen granules, and occasional colloid droplets. A sparse population of functional gonadotrophs are present (indicated by immunoreactivity forACTH,FSH, andLH).[5]
Thepars 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.
The anterior pituitary is derived from theectoderm, more specifically from that ofRathke's pouch, part of the developinghard palate in the embryo. Rathke's pouch is alsoectodermal in origin.
The pouch eventually loses its connection with thepharynx, 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 fromneuroectoderm.[6]
The anterior pituitary contains five types of endocrine cell, and they are defined by the hormones they secrete:somatotropes (GH);lactotropes (PRL);gonadotropes (LH and FSH);corticotropes (ACTH) andthyrotropes (TSH).[7] It also contains non-endocrinefolliculostellate cells which are thought to stimulate and support the endocrine cell populations.
Hormones secreted by the anterior pituitary aretrophic hormones (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 otherendocrine glands to release hormones, causing numerous cascading physiological responses.[6]
| Hormone | Other names | Symbol(s) | Structure | Secretory cells | Staining | Target | Effect | |
|---|---|---|---|---|---|---|---|---|
| Adrenocorticotropic hormone | Corticotropin | ACTH | Polypeptide | Corticotrophs | Basophil | Adrenal gland | Secretion ofglucocorticoid,mineralocorticoid andandrogens | |
| Thyroid-stimulating hormone | Thyrotropin | TSH | Glycoprotein | Thyrotrophs | Basophil | Thyroid gland | Secretion ofthyroid hormones | |
| Follicle-stimulating hormone | - | FSH | Glycoprotein | Gonadotrophs | Basophil | Gonads | Growth ofreproductive system | |
| Luteinizing hormone | Lutropin | LH, ICSH | Glycoprotein | Gonadotrophs | Basophil | Gonads | Sex hormone production | |
| Growth hormone | Somatotropin | GH, STH | Polypeptide | Somatotrophs | Acidophil | Liver,adipose tissue | Promotes growth;lipid andcarbohydrate metabolism | |
| Prolactin | Lactotropin | PRL | Polypeptide | Lactotrophs | Acidophil | Ovaries,mammary glands,testes,prostate | Secretion ofestrogens/progesterone;lactation;spermatogenesis;prostatic hyperplasia | TSH andACTH secretion |
Hormone secretion from the anterior pituitary gland is regulated by hormones secreted by thehypothalamus.Neuroendocrine cells in the hypothalamus projectaxons to themedian eminence, 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 (thehypothalamo-hypophyseal portal vessels).
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 ofluteinizing hormone (LH) andgrowth hormone (GH) and can stimulate the secretion ofthyroid-stimulating hormone (TSH).Prostaglandins are now known to inhibitadrenocorticotropic hormone (ACTH) and also to stimulate TSH, GH and LH release.[10] Clinical evidence supports the experimental findings of the excitatory and inhibitory effects GABA has onGH secretion, dependent on GABA's site of action within the hypothalamic-pituitary axis.[11]
The homeostatic maintenance of the anterior pituitary is crucial to our physiological well being. Increased plasma levels ofTSH inducehyperthermia through a mechanism involving increasedmetabolism andcutaneousvasodilation. Increased levels ofLH also result inhypothermia but through a decreased metabolism action.ACTH increase metabolism and induce cutaneousvasoconstriction, increased plasma levels also result inhyperthermia andprolactin decreases with decreasing temperature values.Follicle-stimulating hormone (FSH) also may causehypothermia if increased beyond homeostatic levels through an increased metabolic mechanism only.[12]
Gonadotropes, primarilyluteinising hormone (LH) secreted from the anterior pituitary stimulates theovulation cycle in femalemammals, whilst in the males, LH stimulates the synthesis ofandrogen which drives the ongoing will to mate together with a constant production ofsperm.[6]
Main articleHypothalamic-pituitary-adrenal axis
The anterior pituitary plays a role in stress response.Corticotropin releasing hormone (CRH) from the hypothalamus stimulatesACTH release in a cascading effect that ends with the production of glucocorticoids from theadrenal cortex.[6]
Hyperpituitarism is the condition where the pituitary secretes excessive amounts of hormones. This hypersecretion often results in the formation of apituitary 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[20] and can lead togigantism, a hormone disorder shown in "giants" such asAndré the Giant, where it occurs before theepiphyseal plates in bones close in puberty.[13] The most common type of pituitary tumour is aprolactinoma which hypersecretesprolactin.[21] A third type of pituitary adenoma secretes excess ACTH, which in turn, causes an excess ofcortisol to be secreted and is the cause ofCushing's disease.[13]
Hypopituitarism 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 ofdwarfism. 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 ofcortisol. This is a life-threatening condition.Hypopituitarism could be caused by the destruction or removal of the anterior pituitary tissue through traumatic brain injury, tumor,tuberculosis, orsyphilis, among other causes. This disorder used to be referred to asSimmonds' disease but now according to theDiseases Database it is calledSheehan syndrome.[22] If the hypopituitarism is caused by the blood loss associated with childbirth, the disorder is referred to as Sheehan syndrome.
The anterior pituitary is also known as theadenohypophysis, meaning "glandular undergrowth", from theGreekadeno- ("gland"),hypo ("under"), andphysis ("growth").
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