Pituitary gland - the "master gland" of the endocrine system
The pituitary gland, also called as hypohysis, is located at the base of the brain and connected to the hypothalamus of the brain.
A short but complex network of blood vessels, called a portal system, extends from the hypothalamus to the pituitary gland. This is the critical link by which the nervous system exerts its control over hormone production in the pituitary gland and other endocrine glands.
The portal system carries small peptide molecules called “releasing hormones” secreted by neurosecretory cells in the hypothalamus directly to tissues in the pituitary gland.
The pituitary gland produces hormones that regulate the hormone production of many other endocrine glands in the body. Such substances are referred to as tropic hormones.
The pituitary gland is typically referred to as the “master gland” of the endocrine system.
The pituitary gland is actually composed of two glands:
As the human embryo develops, the anterior pituitary is formed from cells from the roof of the mouth that migrate toward the brain.
The posterior pituitary is composed of neural tissue.
The anterior and posterior pituitary hormones and their target organs
Anterior and posterior pituitary hormones
The two sections of the pituitary gland produce a number of different hormones which act on different target glands or cells.
- Adrenocorticotropic hormone (ACTH)
- Thyroid-stimulating hormone (TSH)
- Luteinising hormone (LH)
- Follicle-stimulating hormone (FSH)
- Prolactin (PRL)
- Growth hormone (GH)
- Melanocyte-stimulating hormone (MSH)
The anterior pituitary hormones
The anterior pituitary gland produces the four tropic hormones - the adrenocorticotropic hormone (ACTH), the thyroid-stimulating hormone, the follicle stimulating hormone (FSH), and the luteinizing hormone (LH).
Adrenocorticotropic hormone stimulates the adrenal gland to produce a hormone called cortisol. ACTH is also known as corticotropin.
Thyroid-stimulating hormone stimulates the thyroid gland to secrete its own hormone, which is called thyroxine. TSH is also known as thyrotropin.
Luteinising and follicle-stimulating hormones control reproductive functioning and sexual characteristics. Stimulates the ovaries to produce oestrogen and progesterone and the testes to produce testosterone and sperm.
LH and FSH are known collectively as gonadotropins.
Luteinising hormone is also referred to as interstitial cell stimulating hormone (ICSH) in males.
Exact role of melanocyte-stimulating hormone in humans is unknown.
Functions and effects of the human growth hormone (somatotropin, HGH)
Human growth hormone spurs body growth by increasing:
- intestinal absorption of calcium
- cell division and development (especially in bone and cartilage)
- protein synthesis and lipid metabolism
- the release of fatty acids from fat cells, and prompts the conversion of fatty acids into fragments that can then form acetyl CoA for use as an energy source for the body.
Human growth hormone also suppresses glycolysis and increases glycogen production in the liver.
In summary, HGH spares proteins and carbohydrates by enhancing the use of lipids as an energy source for cell functions.
Somatotropin has a half-life of about 20 hours after secretion, after which it is no longer chemically active.
HGH, acting as a tropic hormone, triggers the production of growth factors in the liver and other tissues. These growth factors (composed of protein molecules) prolong the effects of somatotropin on bone and cartilage tissues.
Levels of human growth hormone tend to decrease with age. The resulting decline in protein synthesis may be responsible for some of the characteristic signs of aging, such as diminished muscle mass and wrinkles.
Insufficient HGH production during childhood results in a condition called pituitary dwarfism.
An excess of HGH production prior to puberty causes a disorder known as gigantism.
Excess somatotropin production during adult years produces acromegaly, symptoms of which include excessive thickening of bone tissue.
Function and secretion of prolactin hormone
Prolactin - a non-steroid hormone produced by the anterior pituitary and, in smaller quantities, by the immune system, the brain, and the pregnant uterus.
Prolactin stimulates the development of mammary gland tissue and milk production (lactogenesis).
The hypothalamic regulation of prolactin production is unusual.
The hypothalamus secretes the neurotransmitter dopamine, which inhibits rather than stimulates the production and secretion of prolactin by the pituitary. Severing the connection between the hypothalamus and the pituitary gland results in an increase in prolactin production.
After birth, however, the stimulation of nerve endings in the nipples during infant feeding will trigger the release of prolactin-secreting hormones by the hypothalamus. This spinal reflex (known as a neuroendocrine reflex) stimulates the production of prolactin.
Increasing estrogen levels also stimulate prolactin production in late pregnancy to prepare the mammary glands for lactation after the birth of a baby. Increased prolactin levels in pregnancy also inhibit ovulation by suppressing the production of Luteinising hormone.
The posterior pituitary hormones
The posterior pituitary secretes two types of hormones.
This part of the pituitary gland is composed of secretory nerve cells that originate in the hypothalamus.
Secretory nerve cells of the hypothalamus produce oxytocin and antidiuretic hormone (ADH). These hormones migrate down their axons to the tissues of the posterior pituitary gland, where they are stored and then later released.
Antidiuretic hormone (ADH) function
Specialized cells in the hypothalamus, called osmoreceptor cells, monitor the concentration of sodium ions in blood.
An increase in sodium levels triggers the secretion of antidiuretic hormone.
In the kidneys, vasopressin makes the walls of the distal tubules more permeable to water. This increases the rate of re-absorption of water back into the blood and produces more concentrated urine.
Since alcohol suppresses antidiuretic hormone secretion, the consumption of alcohol results in the production of more dilute urine by the kidney.
The pituitary also secretes ADH in response to decreased blood pressure resulting from loss of blood from torn or damaged blood vessels. Antidiuretic hormone stimulates severed arteries to constrict (vasoconstriction), reducing blood loss and increasing blood pressure. These mechanisms help maintain adequate blood supply to the organs and tissues, reducing potential cell damage.
Inappropriate antidiuretic hormon secretion
Insufficient production of vasopressin can cause diabetes insipidus. Symptoms of this endocrine disorder include increased thirst and dehydration, production of abnormally high volumes of very dilute urine, and an enlarged urinary bladder.
Abnormally high antidiuretic hormone levels prompt the kidneys to retain water and produce more concentrated urine. This increases blood volume and decreases sodium concentration in the blood. The loss of sodium can cause nerve fibres and muscle tissue to become “twitchy.”
Oxytocin effects and childbirth
In women, the hormone oxytocin plays an important role during and after childbirth.
The hormone oxytocin triggers muscle contractions during childbirth and the release of milk from the breasts. Oxytocin stimulates the uterine muscles to contract more forcefully. Each contraction increases the stimulus on the pressure receptors and the release of more oxytocin. This oxytocin positive feedback loop ends with the birth of the baby.
The action of an infant feeding from the mother’s breast initiates the “suckling” reflex. This reflex triggers oxytocin secretion. Oxytocin stimulates contractions in the smooth muscles of the mammary ducts, which causes the expulsion of milk from the mammary glands.