Glands

Anatomy or system affected: Breasts, endocrine system, gastrointestinal system, genitals, nervous system, pancreas, reproductive system, skin

Definition: Cells, organs, or tissues that produce, store, and secrete fluids.

Structure and Functions

A gland is formed from a group of specialized cells that, together, produce and secrete substances necessary for optimal bodily function. Some release fluid into a duct or tube that ultimately empties into a body cavity or an area outside the body. These glands are known as exocrine glands, meaning “externally secreting.” Other glands, known as endocrine, or “internally secreting,” glands pass their secretions directly into closely associated blood vessels. Endocrine glands secrete a certain type of molecule called a hormone. Hormones act as chemical messages that regulate almost every important function in the body.

Major endocrine glands include the pituitary, pineal gland, and hypothalamus in the brain; the thyroid and parathyroid glands in the neck; the adrenal glands and pancreas in the abdomen; the female ovaries in the pelvic cavity; and the male testes in the scrotum.

There are many exocrine glands spread throughout the human body. How they are classified depends on the method by which they produce their secreted product, and by the composition of the secretion. The cells of merocrine glands remain intact; their product is secreted by a process called exocytosis. In exocytosis, the substance to be secreted is surrounded by a membrane inside the cell, forming a structure called a secretory vesicle. At the time of release, the vesicle membrane fuses with the cell membrane, releasing the vesicle contents to the extracellular space. Examples of merocrine glands include salivary glands, which produce saliva; lacrimal glands, which produce tears; and sweat glands. In apocrine glands, such as the milk-producing mammary gland, part of the cell is pinched off and shed to release cytoplasm, which contains the secretory product, into the duct; the remaining portion of the cell then regenerates. The last class of exocrine gland is the holocrine gland. The cells within these glands collect their secretory product internally and then rupture, releasing the product and cell debris into the duct; the cell is then replaced. Sebaceous (oil-producing) glands in the skin are an example of this type of gland.

Unlike endocrine glands, which secrete only hormones, exocrine glands produce a more diverse range of substances. Mucous glands produce mucins, sugar-bound proteins that, when mixed with water, form mucus, a sticky gelatinous substance. Mucus has a lubricative role. For example, it protects reproductive and digestive tracts from friction forces. It also forms a protective barrier that traps foreign particles and bacteria that may cause irritation or infection and protects the cells of the digestive tract from digestive enzymes and acid.

Serous glands produce serous fluid which has a more watery consistency than mucus. Examples of serous glands include mammary glands, tear glands, and the pancreas.

Salivary glands are mixed glands. The largest salivary glands, the parotids, secrete purely serous fluid, and the submaxillary and sublingual glands secrete saliva containing mucin.

The function of exocrine glands is not necessarily discrete from that of the endocrine system. Exocrine secretion and hormones from the endocrine system work together in concert. For example, hormones regulate the complex process of digestion, absorption, and usage of nutrients started by exocrine secretions.

The pancreas, found behind the stomach, is both an exocrine gland, producing digestive enzymes for the intestine, and an important endocrine gland. The exocrine part makes up most of the pancreas mass. It consists of acinar cells, which secrete pancreatic juice, containing digestive enzymes, such as trypsin, chymotrypsin, lipase and amylase, and the pancreatic ductal system, which delivers the pancreatic juice to the duodenum of the small intestine. Secretion of pancreatic juice is induced by the release of the hormones secretin and cholecystokinin, produced by the duodenum in response to signals brought about by the presence of food. Cells in the lining of the pancreatic ducts release an alkaline secretion containing bicarbonate ions, which serves to neutralize the acidic juices traveling down to the duodenum from the stomach.

Scattered within the exocrine pancreas are structures known as islets of Langerhans, which comprise the endocrine portion of the pancreas. The major hormones secreted by the islets of Langerhans, insulin and glucagon, are the major regulators of the blood sugar level. Soon after a meal is digested, insulin is released, enabling cells to take excess sugar out of the blood at a rapid rate. Sugar, mainly stored in the liver as glycogen, is then released steadily into the blood between meals because of glucagon production in the pancreas. Carefully balancing these hormones ensures the sugar content in the blood remains constant. Defects in insulin production or usage result in elevated blood glucose levels and diabetes mellitus. The long-term effects of the high blood sugar level of diabetics include heart disease and high blood pressure, unhealed wounds, which become gangrenous, kidney failure, endless infections, nerve damage, and possible coma.

The thyroid is wrapped around the windpipe in the throat. By means of the two iodine-containing hormones that it produces, called thyroxine (T₄) and triiodothyronine (T₃), this gland controls the rate of the body’s metabolism.

The four parathyroid glands on the back of the thyroid supply parathyroid hormone, which maintains the proper balance of calcium in the body. If there is not enough calcium, parathyroid hormone instructs the intestine to absorb more calcium and the kidneys to retain more. If the blood still has an insufficient level of calcium, parathyroid hormone causes it to be released from storage in the bones.

The hormones that bring about the most striking changes in both anatomy and behavior are known as the sex hormones. Because sex hormone levels are high in the fetus, they directly influence the development of its sex organs. In females, puberty is caused by an increase in hormones, produced by the two ovaries (located in the pelvic cavity), causes breast development, the growth of pubic and underarm hair, and the broadening of hips and thighs. Each month, the levels of estrogen and progesterone rise and fall, controlling the release of an egg from the ovary.

In addition, estrogen controls exocrine function in the female reproductive tract. Increasing levels of estrogen as ovulation approaches cause cervical crypt glands in the cervix to secrete mucus that is more conducive to sperm survival and movement. After ovulation has taken place, progesterone takes over as the main female hormone, and cervical mucus reverts to an inhospitable consistency or even dries up altogether.

In women, the mammary gland, or breast, undergoes changes throughout life. This is another example in which endocrine hormones influence exocrine function. Before birth, the nipple and some of the milk ducts are formed. At puberty, the ovaries secrete an increased level of estrogen, which leads to the formation of a more elaborate milk duct system along with connective tissue and fat deposition. This results in enlargement and rounding of the breast. At the onset of ovulation and menstruation, the milk ducts are fully mature, with the presence of alveolar cells at the terminal ends of the milk ducts. It is these cells that produce and secrete the milk.

If pregnancy occurs, then increased secretion of estrogen and progesterone from the placenta causes further development of the ductal system; by the time the baby is born most of the fat is replaced by milk ducts. Pregnancy also induces secretion of prolactin that stimulates milk production by the alveolar cells. During pregnancy, high levels of progesterone inhibit the release of milk from the milk duct. At birth, the placenta is expelled, and the level of progesterone decreases dramatically so the milk can be released. The event of the infant suckling causes yet another hormone, oxytocin, to be released, which causes contraction of the milk duct near to the alveolar cells, squeezing the milk toward the nipple. Finally, at or just prior to menopause, the ductal system undergoes a process called involution as a result of decreased estrogen levels.

In males, two testes are located in the scrotal sac. They bring about puberty by their production of testosterone and other androgen hormones. These hormones are responsible for male secondary sex characteristics, such as facial hair, deepening of the voice, and heavier muscles and bones.

The adrenal glands, perched on top of the kidneys, are responsible for regulating the body’s response to stress. One of the major hormones of the adrenal cortex is cortisol. Cortisol helps maintain blood pressure, regulates fluid levels, directs protein and sugar metabolism, increases or decreases body fat reserves, and affects the immune system. A second area of the cortex secretes aldosterone, which directs the kidney to retain sodium and excrete potassium, controlling their levels in the blood. The third cortex area produces some testosterone and estrogen in both males and females.

The adrenal medulla responds to sudden stress by pouring epinephrine, formerly called adrenaline, into the blood. Dramatic changes in the working of the heart, lungs, liver, muscles, and many other organs then enable the body to cope with sudden emergencies.

The pineal gland is embedded close to the very center of the brain. Although the exact function structure is a mystery, it appears to be the only gland producing the hormone melatonin.

The careful and precise control of most of these glands is the work of the pituitary, causing it to be called “the master gland.” This small gland has three distinct parts: the front or anterior pituitary, the back or posterior pituitary, and a tiny middle section.

The anterior pituitary sends thyroid-stimulating hormone (TSH) to the thyroid, causing it to release T₃ and T₄. It also secretes adrenocorticotropic hormone (ACTH), which causes the adrenal cortex to give off its many secretions. In the female, the follicle-stimulating hormone (FSH) that the anterior pituitary sends to the ovary causes an egg to mature, while in the male, it fosters sperm development. Luteinizing hormone (LH), from the anterior pituitary, triggers the monthly release of an egg. The male’s LH causes clusters of cells in the testes, called interstitial cells, to produce testosterone. The anterior pituitary also secretes growth hormone (GH), which stimulates body growth until maturity, and secretes prolactin hormone to cause the female breast to produce milk for a nursing baby.

The posterior pituitary secretes only two hormones: oxytocin, which brings about labor and birth, and antidiuretic hormone (ADH), which is also known as vasopressin. ADH causes the kidney to reabsorb the proper amount of water needed by the body.

The hypothalamus is a small area of tissue below the brain that relays messages from the brain and the rest of the body to the pituitary. It produces a number of releasing and inhibiting hormones which, in turn, carefully control the anterior pituitary’s secretion of FSH, LH, TSH, ACTH, prolactin, and growth hormone.

Many endocrine glands' pathways are regulated by negative feedback. For example, an increase in thyrotropin-releasing hormone (TRH) from the hypothalamus causes the pituitary to release TSH. This, in turn, triggers the production of T₃ and T₄. Increased levels of T₃ and T₄ stop the hypothalamus from releasing TRH, dampening the pathway.

Disorders and Diseases

Complex interactions in the endocrine system involving the concept of feedback have made many treatments possible for defective glands. Appropriate medical treatment of thyroid disease, for example, requires careful understanding of its feedback mechanism.

Hypothyroidism arises when insufficient thyroid hormones are produced. It is quite common, with one in every one thousand men and two in every one hundred women afflicted at some time during life. Symptoms include sensitivity to cold temperatures, weight gain, high blood pressure, yellowed skin, and loss of hair. It is often caused by an inability of the thyroid gland to produce enough hormones, the result of a disease of the pituitary that prevents it from producing TSH, or a disease of the hypothalamus that prevents it from producing TRH. Surprisingly, an underactive thyroid is often enlarged but still unable to produce enough T₃ or T₄. This enlargement is called a goiter. In the type called Hashimoto’s thyroiditis, hypothyroidism develops because the body mistakenly produces antibodies that destroy thyroid tissue.

In addition, the thyroid can oversecrete as well as undersecrete. Hypersecretion is also quite common, with three or four out of one thousand people having this disease, the majority being women. The resulting excessive rate of metabolism causes many possible symptoms including intolerance to heat, irritability, excessive perspiration, heart palpitations, rapid weight loss, weakness, shortness of breath, and sore, bulging eyes.

There are two main types of hyperthyroidism. By far, the most common is called Graves’ disease. It is brought about when the patient’s own immune system creates antibodies that cause continuous excess hormone production by the thyroid even though the pituitary is sending the normal amount of TSH. The less common cases of hyperthyroidism involve lumps or nodules that form in the thyroid and oversecrete T₃ and T₄ for no apparent reason. One of three treatments is usually used to cure hyperthyroidism: Radioactive iodine is given to destroy part of the overactive gland, surgery is used to remove part of the gland, or certain drugs are prescribed that prevent the thyroid from producing its hormones.

Although no one objects to attempts to aid sufferers of thyroid disorders or diabetes mellitus, ethical questions have arisen concerning defects in some endocrine glands. A striking example involves a lack of growth hormone from the pituitary. Doctors, parents, and youngsters often disagree over whether GH therapy should be given to a child who is noticeably shorter than peers at a given age. Because GH, like any hormone, may produce many unwanted side effects, there is much controversy over this particular medical application of endocrine gland research.

It was estimated in the early-2020s that 2,500 children with cystic fibrosis, a disorder which affects many exocrine glands, were born in the US each year. This disease is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. The protein product of this gene is critical for the transport of salts into secretory cells of the lungs, intestine, pancreas, liver, reproductive tract, and skin. In individuals with cystic fibrosis, salt imbalance in these cells results in thick, sticky mucus that cannot be cleared by cilia. This means that blockages occur and secretory products cannot function, leading to problems, such as breathing difficulty, inadequate digestion, and infertility. In addition, the thick mucus is an ideal site for bacteria to thrive, resulting in chronic infections in cystic fibrosis patients. Due to the complications of cystic fibrosis, the average life expectancy of Americans with this disease was under fifty years in the 2020s. Cystic fibrosis is diagnosed by testing the salt content of sweat and by genetic analysis. Treatment consists of a strict diet that is supplemented with enzymes, vitamins, and bile salts to replace those deficient in the patient; physical therapy and medication to reduce the mucus buildup in the lungs; and antibiotics to treat bacterial infection.

Sjögren’s syndrome is an autoimmune disease in which certain types of exocrine cells are destroyed by white blood cells. The Sjögren’s Foundation reported in 2023 that the disease affected four million Americans, mostly women. Often, the lacrimal and salivary glands are affected, and without sufficient tears and saliva, the patient experiences dry eyes and mouth accompanied by recurring eye and mouth infections, difficulty swallowing certain foods, and discomfort of the eyes, such as a gritty feeling. Additionally, secretory cells in the skin, respiratory passages, and vaginal tissues can be affected, causing dryness in these tissues. The symptoms of dry eyes and mouth are treated using medications that stimulate saliva production and artificial tears applied as drops or as an ointment.

Perspective and Prospects

Glands, together with the nervous system, are the body’s means of control and coordination. Given this fact, the discovery of each gland’s functions had ramifications for medical science as a whole.

Although the ancient Greeks, Romans, and Chinese suspected the importance of some glands, it was only in the seventeenth century that scientists began to acquire useful knowledge of them. At that time, the Englishman Thomas Wharton first recognized the difference between duct and ductless glands. In the 1660s, Théophile Bordeu, a Frenchman regarded by many as the founder of endocrinology, declared that some parts of the body gave off “emanations” that had dramatic effects on other parts of the body. Following Bordeu’s lead, the Dutchman Fredrik Ruysch claimed in the 1690s that the thyroid poured important substances into the bloodstream.

Two major breakthroughs occurred in the late 1800s when Paul Langerhans found the actual pancreas cell clusters, called islets, that produce insulin and when Charles-Edouard Brown-Séquard developed a technique to use extracts from glands to determine their function.

The year 1900 brought three major discoveries: William Bayliss and Ernest Starling found that a chemical messenger from the intestine causes the pancreas to excrete digestive juice; Jokichi Takamine discovered that adrenaline increases heart rate and blood pressure; and Alfred Frölich described dwarfed individuals who had suffered previous pituitary damage.

In 1914, in Minnesota, Edward Kendall obtained the chemical he named thyroxine from animal thyroids. Similarly, in 1921, Frederick Banting and Charles Best isolated insulin from the pancreases of animals. By the mid-1970s, Rosalind Yalow and her colleagues had perfected a technique called radioimmunoassay, which uses radioactive materials to measure minute quantities of hormones. This enables physicians to measure the circulating level of nearly every hormone and diagnose anyone with an excess or deficiency. Many hormones, such as insulin, growth hormone, and estrogen, can then be given to supplement what the body is underproducing; they have been very expensive and hard to obtain in quantity from animals or deceased humans. By the 1980s, however, recombinant DNA technology and the polymerase chain reaction (PCR) offered unlimited, pure, and readily accessible hormones.

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