Endocrine System and Glands | Research & Encyclopedia Articles

Endocrine System and Glands

The endocrine system controls and regulates body metabolism and other activities by the production of hormones. Hormones are a class of molecules, varying in actual biochemical composition, that act as messenger molecules. Hormones may be steroids, protein peptides, or lipids. Regardless of their unique chemical structure, hormones, are secreted by endocrine glands and in some circumstances, specialized tissues.

Just as there are agonist muscle groups that work in opposition to one another to coordinate movements, a number of hormones exist as pairs of hormones that stimulate opposite responses in target tissues. Antagonistic hormones include such chemicals as glucagons and insulin that act to respectively increase or decrease blood levels of glucose, and thus affect the availability of glucose to cells to use in cellular metabolism.

Endocrine glands release their homes into the blood stream for distribution to target tissues. Target cells have receptor sites (hormone receptors) on their membranes that specifically bind specific hormone sequences or structures.

Major human endocrine glands include the pituitary gland, hypothalamus, thyroid, and pancreas.

The pituitary gland is the major and controlling gland of the endocrine system. Often referred to as the master gland of the endocrine system, the structure of pituitary gland, along with the hormones it secretes combine to form the primary site in the body for the coordination of nervous system and endocrine system function. The pituitary gland, also known as the hypophysis or hypophysis cerbri, produces a number of hormones that control hormone production in other endocrine glands.

The pituitary gland is situated in the sella turcica, a small indentation or recess located near the base of the brain. About the size of a pea, the pituitary gland is connected to the hypothalamus by a slender pituitary stalk (hypophyseal stalk). This connection is not trivial. The posterior pituitary is really an extension of the hypothalamus, and its tissue type differs from the anterior pituitary gland. In addition to an anatomical connection via the hypophyseal-hypothalamic tract, the hypothalamus regulates pituitary function--therefore provides a bridge between the nervous system and the endocrine system--through the hypophyseal portal system. The hypophyseal portal system is a localized collection blood vessels that allow regulating agents secreted by the hypothalamus to travel directly to the pituitary gland.

Neural input (e.g., information gathered by the nervous system) can be interpreted by the hypothalamus and translated into pituitary control, and therefore affect endocrine response. The connection of the pituitary gland to the hypothalamus establishes a clear chain of control: nervous signals in the hypothalamus cause changes in pituitary hormone secretion that, in turn, control the production of hormone in other endocrine glands.

The pituitary gland has two distinct anatomical and physiological regions that produce different hormones. The front (anterior) lobe, termed the adenohypophysis, produces different hormones than the posterior lobe (neurohypophysis). The adenohypophysis secretes seven different hormones. Growth hormone (GH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), the gonadotropins (LH and FSH), prolactin (PRL), and melanocyte-stimulating hormone (MSH). The neurohypophysis produces oxytocin and antidiuretic hormone (ADH).

Growth hormone (GH), or somatotropin, is a hormone that acts directly on tissues to stimulate overall body growth. GH acts to stimulate growth by increasing protein synthesis and acts metabolically to shunt glucose from ATP synthesis pathways, while at the same time promoting fat usage. GH stimulates the secretion of somatomedin hormones. Abnormalities in the pituitary gland that result in the overproduction or underproduction of GH can result in gigantism (generalized enlargement), acromegaly (enlargements of the extremities), or--in cases of underproduction--pituitary dwarfism.

Thyroid-stimulating hormone (TSH) stimulates thyroid growth and the secretion of thyroid hormones, including thyroxine and triiodothyronine.

Pituitary secretion of adrenocorticotropic hormone (ACTH) stimulates adenocortical growth (growth of cortex of the adrenal glands locate on the top (apex) of the kidneys) and the subsequent secretion of corticosteroids. There is also a feedback mechanism involved in ACTH production. Cortisol (a hormone also important in controlling the inflammatory response) is the principal corticosteroid produced by the adrenal glands. As levels of cortisol increase in the bloodstream, they act to inhibit further production of CTH by the anterior pituitary.

The pituitary gland is also an important regulator of the reproductive system through the secretion of the gonadotropins LH (lutenizing hormone) and FSH (follicle stimulating hormone). In the male, LH is usually referred to as interstitial cell stimulating hormone (ICSH). In the female, LH acts to stimulate ovulation and the formation of the corpus luteum on an ovarian follicle. In the male, the action of LH or ICSH acts to stimulate the production of testosterone. In the female, FSH stimulates estrogen secretion and supports the growth and maturation of the ovarian follicle. In the male, the primary role of FSH is to stimulate spermatogenesis (the formation of sperm cells).

Pituitary stimulation of melanocyte stimulating hormone (MSH) contributes to the regulation of pigmentation of the skin by stimulating melanocytes to produce melanin. Some anatomists actually divide the pituitary into three lobes and argue that the intermediate lobe (pars intermedia) is the area that secretes MSH. Other anatomists argue that the region described as the intermediate lobe of the pituitary gland is too poorly vascularized to be anatomically of functionally distinguishable from the anterior pituitary.

Prolactin (a lactotrophic hormone secreted by anterior pituitary gland) is a multifunctional hormone that stimulates a number of different cell types and cell sites in addition to its primary functions of stimulating milk production by the mammary glands and stimulating progesterone production by the corpeus luteum located the ovarian follicle. Receptor sites for PRL are found on many cell types scattered throughout the body.

Oxytocin is actually synthesized in the hypothalamus and is stored in the neurohypophysis. Oxytocin stimulated by the neurohypophysis stimulates the contraction of smooth muscle in the uterus during birth. Oxytocin also stimulates the ejection of milk by the mammary glands and the release of oxytocin is stimulated when infants suckle.

Antidiuretic hormone (ADH), also termed vasopressin, is also stored in the posterior pituitary gland. When released ADH acts on the renal systems to promote water retention. ADH also stimulates smooth muscle contractions in the smooth muscles lining blood vessels and the digestive tract. Because they act on the smooth muscle of blood vessels, vasopressors are important in the hormonal regulation of blood pressure.

The thyroid gland produces iodine-requiring hormones that regulate metabolism including temperature regulation and weight control. In addition to pancreatic role in producing digestive enzymes, the special cells with the islet of Langerhans of the pancreas secrete glucagons and insulin to regulate blood sugar levels. Differing regions of the adrenal glands secrete epinephrine and corticosteroids. The pineal gland plays a role in metabolic regulation.

In addition to producing sex cells (sperm and ova), the male testes and the female ovaries secrete the sex hormones testosterone, estrogen, and progesterone in sex-specific amounts for males and females.