The anterior pituitary (also called the adenohypophysis or pars anterior) is a major organ of the endocrine system. The anterior pituitary is the glandular, anterior lobe that together with the posterior pituitary (or neurohypophysis) makes up the pituitary gland (hypophysis) which, in , is located at the base of the Human brain, protruding off the bottom of the hypothalamus.
The anterior pituitary regulates several physiological processes, including stress, growth, reproduction, and lactation. Proper functioning of the anterior pituitary and of the organs it regulates can often be ascertained via that measure hormone levels.
Structure
The
pituitary gland sits in a protective bony enclosure called the
sella 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 the
neural composition of the
pars nervosa of the posterior pituitary.
The anterior pituitary is composed of three regions, the pars distalis, pars tuberalis, and pars intermedia.
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
and
chromophil.
The chromophils can be further divided into
acidophils (alpha cells) and
basophil cell (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 a highly vascularised sheath extending up from the pars distalis, which joins with the
pituitary stalk (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.
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 for ACTH, FSH, and LH).[Ross, Michael. Histology: A Text and Atlas. 5th ed., 2006. pp 695]
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 in the embryo. Rathke's pouch is also
ectodermal in origin.
The pouch eventually loses its connection with the pharynx, 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.[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: somatotropes (GH);
prolactin cell (PRL); gonadotropes (LH and FSH); corticotropes (ACTH) and
thyrotropic cell (TSH).
[Le Tissier, P.R; Hodson, D.J; Lafont C; Fontanaud P; Schaeffer, M; Mollard, P. (2012) Anterior pituitary cell networks. Front Neuroendocrinol. Aug; 33(3):252-66] 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 (Greek: trophe, "nourishment"). Trophic hormones directly affect growth either as hyperplasia or hypertrophy on the tissue it is stimulating. are named for their ability to act directly on target tissues or other to release hormones, causing numerous cascading physiological responses.
[Malendowicz, L.K; Rucinski, M; Belloni, A.S; Ziolkowska, A; and Nussdorfer, G.C. (2007) Leptin and the regulation of the hypothalamic-pituitary-adrenal axis. Int Rev Cytol. 263: 63-102.][Sone, M. and Osamura, R.Y. (2001) Leptin and the pituitary. Pituitary. Jan-Apr; 4(1-2): 15-23.]
Role in the endocrine system
Hypothalamic control
Hormone secretion from the anterior pituitary gland is regulated by hormones secreted by the
hypothalamus. Neuroendocrine cells in the hypothalamus project
axons to the
median 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 (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 (LH) and
growth hormone (GH) and can stimulate the secretion of thyroid-stimulating hormone (TSH).
are now known to inhibit adrenocorticotropic hormone (ACTH) and also to stimulate TSH, GH and LH release.
[Hedge, G.A. (1977) Roles for the prostaglandins in the regulation of anterior pituitary secretion. Life Sci. Jan 1;20(1):17-33.] Clinical evidence supports the experimental findings of the excitatory and inhibitory effects GABA has on
Growth hormone secretion, dependent on GABA's site of action within the hypothalamic-pituitary axis.
[Racagni, G; Apud, J.A; Cocchi, D; Locatelli, V; Muller, E.E. (1982) GABAergic control of anterior pituitary hormone secretion. Life Sci. Aug 30;31(9):823-38.]
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 through a mechanism involving increased
metabolism and
cutaneous vasodilation. Increased levels of LH also result in
hypothermia but through a decreased metabolism action.
ACTH increase metabolism and induce cutaneous
vasoconstriction, increased plasma levels also result in
hyperthermia and
prolactin decreases with decreasing temperature values. Follicle-stimulating hormone (FSH) also may cause
hypothermia if increased beyond homeostatic levels through an increased metabolic mechanism only.
[Lin, M.T; Ho, L.T; and Uang, W.N. (1983) Effects of anterior pituitary hormones and their releasing hormones physiological and behavioral functions in rats. J. steroid Biochem. Vol. 19(1) 433-38.]
- ;Gonadal function
, primarily luteinising hormone (LH) secreted from the anterior pituitary stimulates the
ovulation cycle in female
, whilst in the males, LH stimulates the synthesis of
androgen 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 ACTH 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 prolactin is essential for lactation.
- ;Stress: Operating through the hypothalamic-pituitary-adrenal axis (HPA), the anterior pituitary gland has a large role in the neuroendocrine system's stress response. Stress induces a release of corticotropin-releasing hormone (CRH) and vasopressin from the hypothalamus, which activates the release of adrenocorticotropic hormone (ACTH) from the anterior pituitary gland. Then, this acts on the adrenal cortex to produce glucocorticoids such as cortisol. These glucocorticoids act back on the anterior pituitary gland and the hypothalamus with negative feedback to slow the production of CRH and ACTH.
- ;Aging: Operating through the hypothalamic-pituitary-gonadal axis, the anterior pituitary gland also affects the reproductive system. The hypothalamus releases gonadotropin-releasing hormone (GnRH), which stimulates the release of luteinizing hormone (LH) and follicle-stimulating hormone. Then the gonads produce estrogen and testosterone. The decrease in release of (LH and FSH) caused by normal aging may be responsible for impotence
- ;Tactile responding: It has been shown that infant mice who were stroked with a paintbrush (simulating motherly care) had more release and binding of growth hormone (GH) from the anterior pituitary gland.
- ;Circadian rhythms: Light information received by the eyes is transmitted to the pineal gland via the circadian clock (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.
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, where it occurs before the epiphyseal plates in bones close in puberty.
The most common type of pituitary tumour is a
prolactinoma which hypersecretes
prolactin.
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.
Decreased activity
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 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.
Hypopituitarism could be caused by the destruction or removal of the anterior pituitary tissue through traumatic brain injury, tumor,
tuberculosis, 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
Ancient Greek adeno- ("gland"),
hypo ("under"), and
physis ("growth").
==Additional images==
See also
-
Triple bolus test
-
Hypothalamic–pituitary–somatic axis
Further reading
-
Marieb, E. 2004. Human Anatomy and Physiology. Benjamin Cummings: New York.
-
Wheater, P., Burkitt, H., Daniels, V. 1987. Functional Histology. Churchill Livingstone: New York.
External links
