The pituitary gland is located at the base of the brain and is part of the endocrine system. It is sometimes called the hypophysis, from two Greek words that mean "to grow beneath." The pituitary is responsible for the hormonal regulation of several body processes, including water retention, breast milk synthesis and release, human growth, and thyroid gland secretions.
Description
The pituitary is one of the most extensively researched glands in the endocrine system. In humans, it is located at the base of the brain just beneath the hypothalamus. There are three separate lobes (or sections) of the pituitary: the anterior lobe, the posterior lobe and the intermediate lobe. Therefore, it is sometimes considered as three different glands. In addition, there is a small stem called the pituitary stalk that connects the pituitary to the hypothalamus.
The pituitary gland is formed during early fetal development. An understanding of its formation explains its position in the endocrine system as well as its neurological importance. Early in the development of the fetus, a small sac of cells forms at the top of the oral cavity and moves upward. These cells are known as Rathke's pouch. At the same time, a small fold of neural tissue extends downward from the hypothalamus. During fetal development, the two structures continue to move toward each other; they meet and fuse to form the anterior (originally Rathke's pouch) and posterior (from the hypothalamus) lobes of the pituitary.
The hypothalamus, which is located just above the pituitary gland, is a region in the forebrain that is responsible for regulating all lobes of the pituitary. The pituitary releases, but does not necessarily synthesize, nine different hormones. Neurohormones are synthesized by the hypothalamus and transported to the posterior pituitary. The release of hormones from each lobe of the pituitary is regulated differently.
Anterior pituitary
The anterior pituitary is sometimes called the adenohypophysis. It constitutes about 80% of the pituitary by weight. The cells of the anterior pituitary act like true endocrine cells. Instead of containing neurons, the anterior pituitary receives chemical signals through the blood and releases hormones in response. It has a direct connection with the hypothalamus through blood vessels. Various cells in the anterior pituitary release the following hormones:
The posterior pituitary is sometimes referred to as the neurohypophysis because it acts like an extension of the nervous system. As opposed to the anterior pituitary, which is connected to the hypothalamus via the circulatory system, the posterior pituitary receives nerve impulses from the same nerve cells that innervate the hypothalamus. The posterior pituitary releases oxytocin and antidiuretic hormone (ADH, or vasopressin).
Intermediate pituitary
The intermediate lobe is not a complete "lobe" in humans. Instead it is a simple structure comprised of just a few cells. The intermediate pituitary is an important structure in many lower vertebrates, but it has very little significance in humans. In lower vertebrates, the intermediate
KEY TERMS
Acromegaly—Abnormal enlargement of such parts of the body as the hands, face, head, and feet.
Adenohypophysis—Another name for the anterior lobe of the pituitary.
Antidiuretic hormone (ADH)—A hormone released by the posterior lobe of the pituitary gland that increases the absorption of water by the kidneys. It is also known as vasopressin.
Diabetes insipidus—A disorder characterized by increased urine production resulting from inadequate levels of ADH.
Gonadotropins—Hormones that affect the development or activity of the ovaries or testes.
Hypophysis—Another name for the pituitary gland.
Hypopituitarism—A condition produced by deficient activity of the anterior lobe of the pituitary gland. It is characterized by obesity, incomplete sexual maturation, and in extreme cases, dwarfism.
Hypothalamus—A region in the forebrain that regulates the functions of the autonomic nervous system by vascular communication. The hypothalamus governs the functions of both lobes of the pituitary gland.
Neurohypophysis—Another name for the posterior lobe of the pituitary.
Oxytocin—A hormone produced by the posterior pituitary that stimulates the movement of breast milk from the sacs in which the milk is produced to the larger ducts from which the milk is ejected.
Peptide—A compound containing two or more amino acids, in which the carboxyl group of one acid is linked to the amino group of the other.
Vasopressin—Another name for ADH.
pituitary releases melanocyto-stimulating hormone. This hormone stimulates the growth of melanocytes, which are cells that produce a dark pigment called melanin.
Function
The pituitary gland is an organ that is part of the endocrine system, along with many other glands and organs. It is regulated by the hypothalamus, and it in turn regulates the secretion of many different hormones that are essential to human health.
Role in human health
Hormones released from the anterior and posterior pituitary have far-reaching effects on many different organ systems and physiological processes.
Hormones of the anterior pituitary
Luteinizing hormone and follicle stimulating hormone are called gonadotropins. As the name suggests, the target tissues of these hormones are the gonads (ovaries and testes). They have two main functions. The first is to promote the development and maturation of sperm and eggs. Second, they stimulate the production and release of such sex steroid hormones as estradiol and testosterone in women and men respectively.
Prolactin is responsible for stimulating cells in the female breast to produce milk. Therefore, lactotrophs located in the anterior pituitary of women that are breast-feeding are large and numerous, indicating an increased amount of prolactin production. These lactotrophs comprise about 30% of the cells in the anterior pituitary. The pituitary in women doubles in size during pregnancy because of the increase in size and number of lactotrophs.
The target tissue of adrenocorticotropic hormone is the adrenal cortex (part of the adrenal gland that is located above the kidney). ACTH stimulates the production of cortisol and also causes the cells of the adrenal gland to grow. Cortisol has many effects on metabolism in various tissues.
Growth hormones have many different target tissues and promote the growth of each of them. For this reason human growth hormone (GH) is considered an anabolic hormone, indicating that it is responsible for building tissue proteins. For example, GH directly increases protein synthesis in muscles and the liver; and it decreases the size of adipose tissue. It also has an indirect effect by stimulating other hormones. Growth hormones indirectly affect the bones by increasing protein synthesis, collagen synthesis and cell proliferation. In many other tissues, the indirect effects of growth hormone are responsible for protein, RNA and DNA synthesis. The overall effect of growth hormone is to promote skeletal growth and a lean body mass.
As its name implies, thyroid-stimulating hormone (TSH) promotes cell growth in the thyroid gland. TSH also triggers the secretion of thyroid hormones that affect many metabolic processes in the body.
Hormones of the posterior pituitary
Both oxytocin and antidiuretic hormone (ADH) are peptide hormones that are synthesized in the cell bodies of the nerves originating in the hypothalamus and then delivered through the axons to the posterior pituitary. Thus, they are good examples of neuroendocrine hormones.
The primary target organ of ADH is the kidney. ADH is responsible for increasing water retention by the kidney, resulting in an increase in extracellular fluid and a decrease in urine volume. Receptors in the hypothalamus called osmoreceptors can sense the concentration of water in the extracellular fluid through changes in extracellular fluid osmolarity. The osmoreceptors in turn determine the release of ADH by the posterior pituitary. The consumption of alcohol decreases the amount of ADH released. As a result, more fluid is lost through urination, resulting in excessive water loss and thirst.
The primary site of action of oxytocin is female breast tissue. Oxytocin stimulates the contraction of smooth muscle cells in the breast, transferring milk from the place of synthesis to the larger ducts of the breast. Oxytocin is secreted by the stimulation of touch sensors when an infant is suckling. Other psychological factors, such as the sound of a baby crying, can stimulate the release and action of oxytocin. The role of oxytocin in the onset of labor contractions is not fully clear. There is no known stimulus for the secretion of oxytocin in the human male.
Hypopituitarism
Disorders of the pituitary gland can have severe effects on normal growth and sexual maturation. A general condition known as hypopituitarism, also known as pituitary dwarfism, is characterized by a decrease in one or more of the hormones produced by the anterior pituitary. Sexual immaturity and metabolic dysfunction leading to obesity are symptoms of this syndrome. When hypopituitarism occurs in childhood, growth is slowed. Tumors are often the cause of hypopituitarism; however, sometimes there is no identifiable cause. If there is a decrease in the levels of hormones released from the hypothalamus, then hypopituitarism results. The symptoms vary according to the number and amount of hormones that are deficient. The most effective treatment is the administration of replacement hormones.
Hyperpituitarism
The overproduction of growth hormone during childhood produces a condition known as gigantism or acromegaly. Excessive secretion of anterior pituitary hormones is known as hyperpituitarism. Growth hormone influences the overgrowth of the skeleton and all other tissues. A person may grow to 8 ft (2.4 m) or more in height. It is still unclear, but researchers think that over-production of growth hormone may be caused by an adenoma (tumor) on the anterior pituitary. Sometimes, this condition occurs in more than one member of the family, suggesting that there is a genetic component. Gigantism is treated by removing the tumor and administering medications (bromocriptine and octreotide) that inhibit the production of growth hormone.
Diabetes insipidus
A disorder related to both the hypothalamus and the posterior lobe of the pituitary is diabetes insipidus, not to be confused with diabetes mellitus. Diabetes insipidus, or DI, is caused by a deficiency of antidiuretic hormone (ADH). As a result, water is rapidly released from the body through large volumes of urine (3–30 quarts per day). DI may result from an inherited trait; from damage to the hypothalamus, which synthesizes ADH; or from damage to the posterior pituitary, which stores the ADH. Diabetes insipidus occurs more frequently in men than in women. In mild cases, no treatment is necessary other than water replacement. In extreme cases, the patient can be treated by hormone replacement therapy.
BOOKS
Cahill, Matthew, ed. Professional Guide to Diseases. 6th ed. Springhouse, PA: Springhouse Corporation, 1998.
Greenspan, Francis S., and David G. Gardner. Basic and Clinical Endocrinology. 6th ed. New York: Lange Medical Books/McGraw-Hill, 2001.
"Hypothalamic-Pituitary Relationships." Chapter 6 in The Merck Manual of Diagnosis and Therapy, edited by Mark H. Beers, MD, and Robert Berkow, MD. Whitehouse Station, NJ: Merck Research Laboratories, 1999.
Martin, John H., PhD. Neuroanatomy: Text and Atlas. 2nd ed. Norwalk, CT: Appleton & Lange, 1996.
"Pituitary Disorders." Chapter 7 in The Merck Manual of Diagnosis and Therapy, edited by Mark H. Beers, MD, and Robert Berkow, MD. Whitehouse Station, NJ: Merck Research Laboratories, 1999.
Vander, Arthur, et al, eds. Human Physiology: The Mechanisms of Body Function, 7th ed. Boston, MA: WBC/McGraw-Hill, 1998.
