A major organ of the endocrine system, the anterior pituitary, is the glandular, anterior
lobe that together with the posterior lobe, the makes up the pituitary gland.
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 blood tests that measure hormone levels.
Structure
The pituitary gland is a pea-sized gland that sits in a protective bony enclosure called
the sella turcica.
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 as part of the anterior pituitary.
In all animals, the fleshy, glandular anterior pituitary is distinct from the neural composition
of the posterior pituitary.
The anterior pituitary is composed of three regions:
Pars distalis The pars distalis,, comprises the majority
of the anterior pituitary and is where the bulk of pituitary hormone production occurs.
Pars tuberalis The pars tuberalis,, forms a part of the sheath
extending up from the pars distalis which joins with the pituitary stalk, arising from
the posterior lobe..
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 informing the pars tuberalis of the photoperiod.
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 a localisation
of melatonin receptors in this region.
Pars intermedia The pars intermedia,, 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.
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.
Function The anterior pituitary contains five types
of endocrine cell, and they are defined by the hormones they secrete: somatotropes; prolactins;
gonadotropes; corticotropes and thyrotropes.
Hormones secreted by the anterior pituitary are trophic hormones and tropic hormones.
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 glands 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 releasing 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.
Other Control 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 and growth hormone
and can stimulate the secretion of thyroid-stimulating hormone.
Prostaglandins are now known to inhibit adrenocorticotropic hormone and also to stimulate TSH, GH and
LH release.
GABA, through action with the hypothalamus, has been shown experimentally to influence
the level of GH secretion.
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
unit.
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 hypothermia through a mechanism involving decreased metabolism
and cutaneous vasodilation.
Increased levels of LH also result in hypothermia but through a decreased metabolism action.
ACTH and prolactin increase metabolism and induce cutaneous vasoconstriction, increased
plasma levels also result in hypothermia; follicle-stimulating hormone also may cause
hypothermia if increased beyond homeostatic levels through an increased metabolic mechanism
only.
Gonadal Function
Gonadotropes, primarily luteinising hormone secreted from the anterior pituitary stimulates
the ovulation cycle in female mammals, 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 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, the anterior pituitary gland has a large role in the neuroendocrine system’s stress
response.
Stress induces a release of corticotropin-releasing hormone and vasopressin from the hypothalamus,
which activates the release of adrenocorticotropic hormone 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.
Increased cortisol under stress conditions can cause the following: metabolic effects,
bone re-absorption, activation of the sympathetic nervous system response, anti-inflammatory
effects, and inhibition of reproduction/growth.
When the anterior pituitary gland is removed in rats, their avoidance learning mechanisms
were slowed down, but injections of ACTH restored their performance.
In addition, stress may delay the release of reproductive hormones such as luteinizing
hormone and follicle-stimulating hormone.
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 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, which stimulates the release of luteinizing
hormone and follicle-stimulating hormone.
Then the gonads produce estrogen and testosterone.
The decrease in release of gonadotropins caused by normal aging may be responsible for impotence
and frailty in elderly men because of the eventual decrease in production of testosterone.
This lower level of testosterone can have other effects, such as reduced libido, 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 had more release and binding of growth hormone from the anterior
pituitary gland.
Circadian Rhythms Light information received by the eyes is
transmitted to the pineal gland via the circadian pacemaker.
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 estrogen and
testosterone.
In addition, melatonin may affect production of prolactin.
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, 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 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, Addison's Disease can be caused by hypo-secretion of ACTH and a lack of cortisol.
Hypopituitarism could be caused by the destruction or removal of the anterior pituitary tissue
through traumatic brain injury, tumor, tuberculosis, or syphilis, among others 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 adeno, hypo, and physis.
Additional images
See also This article uses anatomical terminology;
for an overview, see anatomical terminology.
Tropic hormones Pituitary adenoma
Triple bolus test Posterior pituitary
Hypopituitarism References
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 Histology image: 14002loa - Histology Learning
System at Boston University Embryology at unc.edu
The Pituitary Network Association -- www.pituitary.org