pituitary gland

pituitary gland

pituitary gland, small oval endocrine gland that lies at the base of the brain. It is sometimes called the master gland of the body because all the other endocrine glands depend on its secretions for stimulation (see endocrine system).

Anatomy and Function

Physiologically, the pituitary is divided into two distinct lobes that arise from different embryological sources. The anterior lobe, or adenohypophysis, grows upward from the pharyngeal tissue at the roof of the mouth. An intermediate lobe also originates in the pharynx, but in humans it is greatly reduced in structure and function. The posterior lobe, or neurohypophysis, grows downward from neural tissue. It is structurally continuous with the hypothalamus of the brain, to which it remains attached by the hypophyseal, or pituitary, stalk. The hypothalamus controls almost all secretions of the pituitary. The posterior lobe is controlled by nerve fibers that originate in hypothalamic neurons and the anterior lobe by substances that are transported from the hypothalamus by tiny blood vessels.

Pituitary Hormones

The tissues in the anterior lobe consist of extensive vascular areas interspersed among glandular cells that secrete at least six different hormones. It was formerly believed that a master molecule was stimulated by various enzymes to produce these hormones, but present evidence indicates that each is individually synthesized, probably by a specific type of glandular cell. Three such types of cells exist in the anterior pituitary gland: acidophils, basophils, and chromophobes. The growth hormone, thought to be synthesized by certain acidophils, stimulates all the tissues in the body to grow by effecting protein formation.

The remaining five important hormones influence body functions by stimulating target organs. Adrenocorticotropic hormone (ACTH) controls the secretion of steroid hormones by the adrenal cortex, which affects glucose, protein, and fat metabolism; thyrotropin controls the rate of thyroxine synthesis by the thyroid gland, which is the principal regulator of body metabolic rate; prolactin, which regulates the formation of milk after the birth of an infant; and three separate gonadotropic hormones (follicle-stimulating hormone, luteinizing hormone, and luteotropic hormone) control the growth and reproductive activity of the gonads.

The release of each of the hormones from the anterior lobe is controlled by a specific substance secreted by nerve cells in the hypothalamus. These substances, called releasing factors, are transmitted by nerve fibers to tiny capillaries in the hypophyseal stalk. They move through blood vessels to the anterior lobe, where each releasing factor is responsible for the release of a specific pituitary hormone.

The two hormones that are produced by the posterior lobe are synthesized by nerve cells in the hypothalamus. They are transported by nerve fibers to nerve endings in the posterior lobe, where they are released. The hormones are antidiuretic hormone (ADH or vasopressin), which alters the permeability of the kidney tubules, permitting more water to be retained by the body; and oxytocin, which aids in the release of milk from mammary glands and causes uterine contractions. The only hormone that is synthesized by the intermediate lobe is the melanocyte-stimulating hormone, which appears to control skin pigmentation.

Disorders of Pituitary Hormone Secretion

Oversecretion of the pituitary hormone human growth hormone can cause gigantism if it occurs before growth of the long bones is complete, or acromegaly if it begins during adulthood. Undersecretion of human growth hormone can lead to dwarfism if experienced during childhood, and decreased endocrine function accompanied by lethargy and loss of sexual capacity in the adult.

or hypophysis

Endocrine gland lying on the underside of the brain that plays a major role in regulating the endocrine system. The anterior pituitary lobe secretes six hormones that play specific roles in stimulating production of cortisol and androgens by the adrenal cortex (corticotropin), growth of eggs and sperm (follicle-stimulating hormone), production of progesterone and testosterone (luteinizing hormone), linear growth in children and bone maintenance in adults (growth hormone), milk production (prolactin), and production of thyroid hormone (thyrotropin). The posterior lobe stores and releases two hormones, oxytocin and vasopressin, from nerve cells in specialized regions of the hypothalamus that control pituitary function. These hormones stimulate uterine contraction and milk secretion (oxytocin) and blood pressure and fluid balance (vasopressin).

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The pituitary gland, or hypophysis, is an endocrine gland about the size of a pea. It is a protrusion off the bottom of the hypothalamus at the base of the brain, and rests in a small, bony cavity (sella turcica) covered by a dural fold (diaphragma sellae). The pituitary fossa, in which the pituitary gland sits, is situated in the sphenoid bone in the middle cranial fossa at the base of the brain.

The pituitary gland secretes hormones regulating homeostasis, including tropic hormones that stimulate other endocrine glands. It is functionally connected to the hypothalamus by the median eminence.

Sections

Located at the base of the brain, the pituitary is functionally linked to the hypothalamus. It is composed of two lobes: the adenohypophysis and neurohypophysis. The adenohypophysis, also referred to as the anterior pituitary is divided into anatomical regions known as the pars tuberalis, pars intermedia, and pars distalis. The neurohypophysis, also referred to as the posterior pituitary. The pituitary is functionally linked to the hypothalamus by the pituitary stalk, whereby hypothalamic releasing factors are released and in turn stimulate the release of pituitary hormones.

Anterior pituitary (Adenohypophysis)

The anterior pituitary synthesizes and secretes important endocrine hormones, such as ACTH, TSH, PRL, GH, endorphins, FSH, and LH. These hormones are released from the anterior pituitary under the influence of hypothalamus. Hypothalamic hormones are secreted to the anterior lobe by way of a special capillary system, called the hypothalamic-hypophyseal portal system.it is developed from dorsalwall of pharynx(stomodial part) i.e called as 'ruthke's pouch'. they all transport by special nerve cells present in the hypothalamus.such nerve cells are located in various parts of hypothalamus & send their nerve fibre into median eminence & tubar cinerium(b/w ant. &post. lobe).

Posterior pituitary (Neurohypophysis)

The hormones secreted by the posterior pituitary are

Oxytocin is one of the few hormones to create a positive feedback loop. For example, uterine contractions stimulate the release of oxytocin from the posterior pituitary, which in turn increases uterine contractions. This positive feedback loop continues until the baby is born.

Intermediate lobe

There is also an intermediate lobe in many animals. For instance in fish it is believed to control physiological colour change. In adult humans it is just a thin layer of cells between the anterior and posterior pituitary. The intermediate lobe produces melanocyte-stimulating hormone (MSH), although this function is often (imprecisely) attributed to the anterior pituitary.

Functions

The pituitary hormones help control some of the following body processes:

Pathology

Disorders involving the pituitary gland include:

Condition Direction Hormone
Acromegaly overproduction growth hormone
Growth hormone deficiency underproduction growth hormone
Syndrome of inappropriate antidiuretic hormone overproduction vasopressin
Diabetes insipidus underproduction vasopressin
Sheehan syndrome underproduction any pituitary hormone
Pituitary adenoma overproduction any pituitary hormone
Hypopituitarism underproduction any pituitary hormone

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See also

References

External links

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