17.11 Other Endocrine Glands

Pineal gland (anatomy)

A small, cone-shaped structure forming the posterior region of the epithalamus within the diencephalon

What hormone does the pineal gland secrete?

Melatonin; melatonin production tends to be cyclic; it increases at night, decreases during the day, and has the lowest levels around lunchtime. Melatonin helps regulate the circadian rhythm (24 hour body clock)

Melatonin affects the synthesis of?

Gonadotropin-releasing hormone (GnRH) from the hypothalamus.

Excessive melatonin secretion is known to

delay puberty in humans

Parathyroid glands

Small brownish red, located on the posterior surface of the thyroid gland. These glands are usually 4 small nodules, but some individuals may have as few as 2 or as many as 6.

2 different types of cells in the parathyroid gland

Chief cells and oxyphil cells

Chief cells

More common, are the source of parathyroid hormone (PTH), which is released from the parathyroid gland in response to decrease in blood calcium levels. They increase blood calcium levels by stimulating release of calcium in urine, and causing the kidney to release an enzyme to convert the inactive calcidiol hormone to the active calcitriol hormone.

Oxyphil cells

Role is not known, although these cells are associated with a rare form of cancer called oxyphil cell adenoma.

Thymus(endocrine function)

The thymus (thi'mus) is a bilobed organ that is located anterior to the heart on its superior aspect (see L figure 17.2). The thymus is relatively large in infants, continues to grow until puberty, and then begins to regress (decrease in size) after puberty. A connective tissue framework houses both epithelial cells and maturing T-lymphocytes (a specific type o white blood cell). Immature T-lymphocytes (thymocytes) migrate to the thymus following their formation in the bone marrow, and epithelial cells there secrete thymic hormones (i.e., thymosin, thymulin, thymopoietin), which participate in the maturation of T-lymphocytes (see L section 21.3b)

Heart(endocrine function)

Endocrine cells within the atria of the heart synthesize and release the hormone atrial natriuretic (nă'trē-yu-ret'ik; natrium = to carry,ouron = urine) peptide (ANP) in response toincreased stretch of the atrial wall (which indicates an increase in blood volume and blood pressure).

This peptide hormone stimulates both the kidneys to increase urine output (which decreases blood volume) and the blood vessels to dilate. Both of these actions facilitate blood pressure to decrease.

Thus, the primary function of ANP is to lower blood pressure.

Kidneys(endocrine function)

Endocrine cells within the kidneys release erythropoietin (EPO) (ě-rith'rõ-poy'ě-tin) when

specialized receptors (chemoreceptors) within the kidney detect low blood oxygen levels. EPO stimulates red bone marrow to increase the production rate of red blood cells (erythrocytes), which are the oxygen-carrying cells.

Stomach and Small Intestine(endocrine function)

The stomach and small intestine are regions of the gastrointestinal (GI) tract. The stomach both synthesizes and releases gastrin (gas'trin; gaster = stomach), a hormone that acts primarily to increase stomach activity (both its motility and its release of secretions) to facilitate digestion within the stomach (see L' section 26.2d). The small intestine is a long tube that is inferior to the stomach and located medially within the abdominal cavity. The small intestine releases both secretin and cholecystokinin, both of which function to facilitate digestion within the small intestine. A primary function of secretin (se-kre'tin) is to stimulate release of secretions from both the liver (bile) and pancreas (pancreatic juice) into the small intestine.

A primary function of cholecystokinin (CCK) (ko'lē-

sis-to-ki'nin; chole = bile, cyst = sac, kinin = to

move), and what gives the hormone its name, is to stimulate release of bile from the gallbladder (a muscular sac on the inferior surface of the liver).

Skin(endocrine function)

Ultraviolet (UV) light penetrates into surface skin cells (keratinocytes) to convert modified cholester molecules to vitamin Dz (also called cholecalciferol which is then released into the blood. Vitamin D3 converted to calcidiol by an enzyme within the live and then by an enzyme within the kidney to calcitriol, the active hormone (which is a lipid-soluble sterol hormone). Calcitriol is similar to parathyroid hormone because it increases blood calcium by stimulating release of calcium from bor tissue and decreases calcium loss in the urine.

Additionally, calcitriol enhances the absorption of calcium from the contents of our digested food within the lumen of the small intestine. Calcitriol

stimulates epithelial cells lining the small intestine to increase the number of plasma membrane Ca2+ transport proteins (e.g., calbinden). Without calcitriol, much of the calcium we ingest is not absorbed; it continues through the digestive tract and is lost in the feces.

Adipose Connective Tissue(endocrine function)

Adipose connective tissue is located throughout the body, and it releases the hormone leptin. This hormone helps to regulate food intake by binding to the neurons within the hypothalamus that control appetite (see ( section 13.4c). Leptin stimulates down-regulation (see ' section 17.6a) of other receptors for hormones that are involved in increasing appetite. Lower percentage of body fat is associated with lower blood levels of leptin, which stimulates the appetite. Thus, one of the functions of leptin is to regulate energy balance within the body.

Clinicians and researchers have become more aware of other endocrine functions of adipose connective tissue by observing the outcomes of either excess or deficiency of this tissue. Excess adipose connective tissue has been linked with various types of cancers (e.g., colon, breast) and the delay of puberty, whereas extremely low body fat can interfere with the menstrual cycle.