Other Endocrine Glands
The body has a number of other endocrine glands, including the gonads (testes in males and the ovaries in females). Other lesser-known glands, such as the thymus gland and the pineal gland, also produce hormones. Some tissues within organs produce hormones and/or growth factors. Individual body cells produce prostaglandins.
Testes and Ovaries
The testes are located in the scrotum, and the ovaries are located in the pelvic cavity. The testes produce androgens (e.g., testosterone), which are the male sex hormones, and the ovaries produce estrogens and progesterone, the female sex hormones. The hypothalamus and the pituitary gland control the hormonal secretions of these organs.
Puberty is the time of life when sexual maturation occurs. Greatly increased testosterone secretion during puberty stimulates the growth of the penis and the testes. Testosterone also brings about and maintains the male secondary sex characteristics that develop during puberty, including the growth of a beard, axillary (underarm) hair, and pubic hair. It prompts the larynx and the vocal cords to enlarge, causing the voice to change. It is partially responsible for the muscular strength of males, and this is why some athletes take supplemental amounts of anabolic steroids, which are either testosterone or related chemicals.
The contraindications of taking anabolic steroids are discussed in the end of this page. Testosterone also stimulates oil and sweat glands in the skin; therefore, it is largely responsible for acne and body odor. Another side effect of testosterone is baldness. Genes for baldness are probably inherited by both sexes, but baldness is seen more often in males because of the presence of testosterone.
Estrogen and Progesterone
The female sex hormones, estrogens and progesterone, have many effects on the body. In particular, estrogens secreted during puberty stimulate the growth of the uterus and the vagina. Estrogen is necessary for egg maturation and is largely responsible for the secondary sex characteristics in females, including female body hair and fat distribution. In general, females have a more rounded appearance than males because of a greater accumulation of fat beneath the skin. Also, the pelvic girdle is wider in females than in males, resulting in a larger pelvic cavity. Both estrogen and progesterone are required for breast development and for regulation of the uterine cycle, which includes monthly menstruation (discharge of blood and mucosal tissues from the uterus).
The lobular thymus gland, which lies just beneath the sternum (see Fig. 10.1), reaches its largest size and is most active during childhood. Lymphocytes that originate in the bone marrow and then pass through the thymus are transformed into T lymphocytes. The lobules of the thymus are lined by epithelial cells that secrete hormones called thymosins. These hormones aid in the differentiation of lymphocytes packed inside the lobules. Although the hormones secreted by the thymus ordinarily work in the thymus, researchers hope that these hormones could be injected into AIDS or cancer patients where they would enhance T-lymphocyte function.
The pineal gland, which is located in the brain (see Fig. 10.1), produces the hormone melatonin, primarily at night. Melatonin is involved in our daily sleep-wake cycle; normally we grow sleepy at night when melatonin levels increase and awaken once daylight returns and melatonin levels are low (Fig. 10.13). Daily 24-hour cycles such as this are called circadian rhythms, and circadian rhythms are controlled by an internal timing mechanism called a biological clock. Based on animal research, it appears that melatonin also regulates sexual development. It has also been noted that children whose pineal gland has been destroyed due to a brain tumor experience early puberty.
Hormones from Other Tissues
We have already mentioned that the heart produces atrial natriuretic hormone. And you will see that the stomach and small intestine produce peptide hormones that regulate digestive secretions.
Figure 10.13 Melatonin production. Melatonin production is greatest at night when we are sleeping. Light suppresses melatonin production (a), so its duration is longer in the winter (b) than in the summer (c).
Leptin is a protein hormone produced by adipose tissue. Leptin acts on the hypothalamus, where it signals satiety-that the individual has had enough to eat. Strange to say, the blood of obese individuals may be rich in leptin. It is possible that the leptin they produce is ineffective because of a genetic mutation, or else their hypothalamic cells lack a suitable number of receptors for leptin.
A number of different types of organs and cells produce peptide growth factors, which stimulate cell division and mitosis. Some, such as lymphokines, are released into the blood; others diffuse to nearby cells. Growth factors of particular interest are the following:
Granulocyte and macrophage colony-stimulating factor (GMCSF) is secreted by many different tissues. GM-CSF causes bone marrow stem cells to form either granulocyte or macrophage cells, depending on whether the concentration is low or high.
Platelet - derived growth factor is released from platelets and from many other cell types. It helps in wound healing and causes an increase in the number of fibroblasts, smooth muscle cells, and certain cells of the nervous system.
Epidermal growth factor and nerve growth factor stimulate the cells indicated by their names, as well as many others. These growth factors are also important in wound healing.
Tumor angiogenesis factor stimulates the formation of capillary networks and is released by tumor cells. One treatment for cancer is to prevent the activity of this growth factor.
Prostaglandins are potent chemical signals produced within cells from arachidonate, a fatty acid. Prostaglandins are not distributed in the blood; instead, they act locally, quite close to where they were produced. In the uterus, prostaglandins cause muscles to contract and may be involved in the pain and discomfort of menstruation. Also, prostaglandins mediate the effects of pyrogens, chemicals that are believed to reset the temperature regulatory center in the brain. For example, aspirin reduces body temperature and controls pain because of its effect on prostaglandins. Certain prostaglandins reduce gastric secretion and have been used to treat ulcers; others lower blood pressure and have been used to treat hypertension; and still others inhibit platelet aggregation and have been used to prevent thrombosis. However, different prostaglandins have contrary effects, and it has been very difficult to successfully standardize their use.
Side Effects of Anabolic Steroids
Anabolic steroids are synthetic forms of the male sex hormone testosterone. Taking doses 10 to 100 times the amount prescribed by doctors for various illnesses promotes larger muscles when the person also exercises. Trainers may have been the first to acquire anabolic steroids for weight lifters, bodybuilders, and other athletes, such as professional football players. However, being a steroid user can have serious detrimental effects. Men often experience decreased sperm counts and decreased sexual desire due to atrophy of the testicles. Some develop an enlarged prostate gland or grow breasts. On the other hand, women can develop male sexual characteristics. They grow hair on their chests and faces, and lose hair from their heads; many experience abnormal enlargement of the clitoris. Some cease ovulating or menstruating, sometimes permanently.
Some researchers predict that two or three months of highdosage use of anabolic steroids as a teen can cause death by age 30 or 40. Steroids have even been linked to heart disease in both sexes and implicated in the deaths of young athletes from liver cancer and one type of kidney tumor. Steroids can cause the body to retain fluid, which results in increased blood pressure. Users then try to get rid of “steroid bloat” by taking large doses of diuretics. A young California weight lifter had a fatal heart attack after using steroids, and the postmortem showed a lack of electrolytes, salts that help regulate the heart. Finally, steroid abuse has psychological effects, including depression, hostility, aggression, and eating disorders. Unfortunately, these drugs make a person feel invincible. One abuser even had his friend videotape him as he drove his car at 40 miles an hour into a tree!
The many harmful effects of anabolic steroids are given in Figure 10B. The Federal Food and Drug Administration now bans most steroids, and steroid use has also been banned by the National Collegiate Athletic Association (NCAA), the National Football League (NFL), and the International Olympic Committee (IOC). Of great concern is the increased use of steroids by teenagers wishing to build bulk quickly, possibly due to society’s emphasis on physical appearance and adolescents’ need to feel better about how they look.
Figure 10B The effects of anabolic steroid use.