thymus gland
thymus gland is mass of glandular tissue located in the neck or chest of most vertebrate animals. In humans, the thymus is a soft, flattened, pinkish-gray organ located in the upper chest under the breastbone. It is relatively large in the newborn infant (about the size of the baby's fist), and continues to grow throughout childhood up to the age of puberty when it weighs about 1.2 oz (35 grams). Then it gradually decreases in size until it blends in with the surrounding tissue. The functions of the thymus were not well understood until the early 1960s, when its role in the development of the body's system of immunity was discovered. Beginning during fetal development, the thymus processes many of the body's lymphocytes, which migrate throughout the body via the bloodstream, seeding lymph nodes and other lymphatic tissue. The main cells undergoing this processing are the T cells, a heterogeneous groups of cells essential in protecting the body against invasions by foreign organisms. If the thymus fails to develop or is removed early in fetal life, the immune system cannot develop completely. Normally, by the time the infant is a few months old, the immune system has sufficiently formed so as to function throughout life. However, further growth and development of lymphoid tissue still depends on intervention by the thymic cells. After the initial seeding process, the thymus releases a hormonal substance that stimulates further growth of lymphoidal tissue, although such a substance has not yet been isolated.
pineal gland
pineal gland is small organ (about the size of a pea) situated in the brain. Long considered vestigial in humans, the structure, which is also called the pineal body or the epiphysis, is present in most vertebrates. It is sensitive to different levels of light and is essential to the functioning of an animal's biological clock. In many animals, including humans, the pineal gland synthesizes a hormone called melatonin in periods of darkness. Melatonin synthesis is halted when light hits the retina of the eye, sending impulses to the gland via the optic nerve. Besides influencing daily, or circadian, rhythms such those of as sleep and temperature, the pineal gland and melatonin appear to direct annual rhythms and seasonal changes in animals. The pineal gland and melatonin are now being studied for their roles in sleep, reproduction, aging, and seasonal affective disorder. In humans the pineal gland begins to produce melatonin at age 3 months; production falls steadily from puberty on.
seasonal affective disorder
seasonal affective disorder (SAD), recurrent fall or winter depression characterized by excessive sleeping, social withdrawal, depression, overeating, and pronounced weight gain. SAD effects an estimated 6% of Americans; for reasons not yet understood, 80% of those affected are women. Most children who are affected have a close relative who also has SAD or another psychiatric condition. The disorder particularly affects people who live in the upper latitudes.
Although the mechanism of the disorder is not perfectly understood, it is known to be a reaction to the biological effects of light on the body (see biorhythm). Daily, or circadian, rhythms help animals keep track of the seasonal changes in the environment, such as the shortening of days in winter, so that they can make the adaptive changes necessary for their survival in each season. Two substances, the hormone melatonin and the neurotransmitter serotonin, are a part of this process and are being studied for a possible role in SAD. Melatonin is secreted by the pineal gland, which is in turn controlled by an area (the suprachiasmatic nuclei) of the hypothalamus; the hypothalamus, among other things, performs a clocklike function in the body. The eye's retinal nerves are connected to this area. Melatonin is secreted chiefly at night, and its secretion is suppressed by light. Secretion of the neurotransmitter serotonin declines in the winter and may undergo abnormal declines in those with SAD; concentrations of serotonin are increased by bright light. Serotonin is especially active in the hypothalamus. Decreased sensitivity of the retina has also been implicated as a cause of SAD.
Treatment with bright light (about five to twenty times brighter than normal lighting) often alleviates symptoms within a period of days. Unwieldy lighting paraphernalia has given way to smaller, portable light boxes and lighted visors. Doses range from 30 minutes to a few hours per day, often undergone in the morning to simulate the dawn.
progesterone
female sex hormone that induces secretory changes in the lining of the uterus essential for successful implantation of a fertilized egg. A steroid, progesterone is secreted chiefly by the corpus luteum, a group of cells formed in the ovary after the follicle ruptures during the release of the egg cell. If fertilization does not take place, the secretion of progesterone decreases and menstruation occurs. If fertilization does occur, progesterone is secreted during pregnancy by the placenta and acts to prevent spontaneous abortion; the hormone also prepares the mammary glands for milk production. Progesterone is also synthesized from cholesterol in the cortex of the adrenal gland where it is a precursor for the synthesis of other steroids including testosterone. Synthetic compounds with progesteronelike activity have been developed that, along with estrogen, are used in oral contraceptives.
parathyroid glands
four small endocrine bodies, located behind the thyroid gland, that govern calcium and phosphorus metabolism. These four masses of tissue (each about the size of a pea) are difficult to distinguish from the thyroid and are often embedded in it. Consequently, before their significance was known they were sometimes accidently removed during thyroid surgery, causing a deficiency in parathormone, the parathyroid hormone. Parathormone increases the concentration of calcium ions in the blood, with accompanying bone absorption and increased reabsorption of calcium ions by the kidneys. The hormone's effect on phosphate ion concentration is the opposite, i.e., phosphate ion concentration in the bloodstream decreases as a result of increased phosphate excretion by the kidneys. Excessive secretion of parathormone, e.g., caused by tumor of the parathyroid glands, is a serious disorder, for excessive blood calcium can cause kidney stones and long-term weakening of the bones. Undersecretion of parathormone, which can be caused by congenital and metabolic disorders, results in too little calcium in the bloodstream, and too much phosphorus. The result is tetany, i.e., violent muscle spasms.
prostate gland
prostate gland, gland that is part of the male reproductive system. It is an organ about the size of a chestnut and consists of glandular and muscular tissue. It is situated below the neck of the bladder, encircling the urethra. The prostate produces a thin, milky, alkaline fluid that is secreted into the urethra at the time of emission of semen, providing an added medium for the life and motility of sperm. It is probable that prostatic fluid enhances fertility since the fluid flowing from the testes and seminal vesicles is acidic and sperm are not optimally mobile unless their medium is relatively alkaline.
In men over 50 enlargement of the prostate (benign prostatic hypertrophy) is common. Sometimes the result is pressure on the urethra and bladder, which interferes with urination, precipitating urinary retention and kidney disease. Balloon dilatation of the urethra and medication with alpha blockers, finasteride (Proscar), and extracts of saw palmetto have joined traditional surgical removal of the prostate (prostatectomy) as therapies.
salivary glands
in humans, three pairs of glands that secrete the alkaline digestive fluid, saliva, into the mouth. Most animals have salivary glands that resemble those in humans; however, in some animals these glands perform other functions. For example, the salivary glands of many blood-sucking species secrete a substance that prevents blood coagulation. In humans the largest pair of salivary glands is situated just below and in front of each ear (parotid glands), the second pair is below the jaw (submandibular), and the third is under the tongue (sublingual). Ducts carry the secretions of the salivary glands into the mouth cavity. Together with the mucus secreted by the membrane of the mouth and the secretions of other small glands in the mouth, saliva helps to keep the mouth moist, softens the food as it is chewed, and by means of salivary amylase—the digestive enzyme contained in saliva—converts starch to sugar, thus initiating the process of digestion
sebaceous gland
It is a gland in the skin of mammals that secretes an oily substance called sebum. In humans, sebaceous glands are primarily found in association with hair follicles but also occur in hairless areas of the skin, except for the palms of the hand and soles of the feet. Sebum is a mixture of fat and the debris of dead fat-producing cells. These cells are constantly replaced by new growth at the base of the glands. Generally the sebum is deposited on the hairs inside the follicles and is brought up to the surface of the skin along the hair shaft. In hairless areas, the sebum surfaces through ducts. Sebum lubricates and protects the hair and skin and prevents drying and irritation of membranes. Sebum may collect excessively as a result of poor hygiene, a diet rich in fats, or accelerated glandular activity, especially during adolescence. Excessive secretions of sebum may be related to acne, certain forms of baldness, and other skin disorders.