8.6: Endocrine glands (2023)

Hypothalamic-Pituitary Axis

Thehypothalamusin vertebrates it integrates the endocrine and nervous systems. The hypothalamus is an endocrine organ located in the diencephalon of the brain. It receives information from the body and other areas of the brain and activates endocrine responses to environmental changes. The hypothalamus acts as an endocrine organ, synthesizing hormones and transporting them along axons to the posterior pituitary. It synthesizes and secretes regulatory hormones that control endocrine cells in the anterior pituitary gland. The hypothalamus contains autonomous centers that control the endocrine cells in the adrenal medulla through neuronal control.

Themucosa, sometimes called the pituitary gland or "master gland" is located at the base of the brain in the sella turcica, a groove of the sphenoid bone of the skull, shown in Figure \(\PageIndex{1}\). It is connected to the hypothalamus through a stalk calledpituitary stalk(or background). The anterior pituitary is regulated by releasing or inhibiting the release of hormones produced by the hypothalamus, and the posterior pituitary receives signals through neurosecretory cells to release hormones produced by the hypothalamus. The pituitary gland has two distinct regions—the anterior pituitary and the posterior pituitary—which between them secrete nine different peptide or protein hormones. The posterior lobe of the pituitary gland contains axons of the neurons of the hypothalamus.

Thyroid gland

Thethyroid glandit is located in the throat, just below the larynx and in front of the trachea, as shown in figure \(\PageIndex{2}\). It is a butterfly-shaped gland with two lobes connected to itisthmus. It has a dark red color due to its extensive vascular system. When the thyroid becomes enlarged due to dysfunction, it can be felt under the skin of the neck.

The thyroid gland consists of several spherical thyroid follicles, which are lined with a simple cuboidal epithelium. These follicles contain a thick fluid, calledcolloid, which stores the glycoprotein thyroglobulin, the precursor of thyroid hormones. The follicles produce hormones that can be stored in the colloid or released into the surrounding capillary network for transport to the rest of the body via the circulatory system.

Thyroid follicle cells synthesize the hormone thyroxine, also known as T₄because it contains four iodine atoms and triiodothyronine, also known as T₃because it contains three iodine atoms. Follicular cells are stimulated to release stored T₃and T₄by thyroid-stimulating hormone (TSH), which is produced by the anterior pituitary gland. These thyroid hormones increase mitochondrial ATP production rates.

A third hormone, calcitonin, is produced byfollicular cellsof the thyroid either releasing hormones or inhibiting hormones. Calcitonin release is not controlled by TSH, but is released when calcium ion concentrations in the blood increase. Calcitonin works to help regulate calcium concentrations in body fluids. It acts on the bones to inhibit osteoclast activity and on the kidneys to stimulate calcium excretion. The combination of these two events lowers calcium levels in body fluid.

Parathyroid glands

Most people have fourparathyroid glands; However, the number can vary from two to six. These glands are located on the posterior surface of the thyroid gland, as shown in Figure \(\PageIndex{3}\). Normally, there is a superior gland and an inferior gland associated with each of the two lobes of the thyroid. Each parathyroid gland is covered by connective tissue and contains many secretory cells connected by a capillary network.

The parathyroid glands produce parathyroid hormone (PTH). PTH increases blood calcium concentrations when calcium ion levels fall below normal. PTH (1) enhances Ca reabsorption²⁺by the kidney, (2) stimulates osteoclast activity and inhibits osteoblast activity, and (3) stimulates renal calcitriol synthesis and secretion, which enhances Ca²⁺absorption from the digestive system. PTH is produced by the chief cells of the parathyroid. PTH and calcitonin work in opposition to each other to maintain Ca homeostasis²⁺levels in body fluids. Another type of cells, oxyphil cells, are present in the parathyroid but their function is not known. These hormones encourage bone growth, muscle mass, and blood cell formation in children and women.

Adrenal glands

Theadrenal glandsrelated to the kidneys. a gland is located on top of each kidney, as shown in Figure \(\PageIndex{4}\). The adrenal glands consist of an outer adrenal cortex and an inner adrenal medulla. These areas secrete different hormones.

Pancreas

Thepancreas, depicted in Figure \(\PageIndex{5}\), is an elongated organ located between the stomach and the proximal part of the small intestine. It contains both exocrine cells that secrete digestive enzymes and endocrine cells that release hormones. It is sometimes referred to as a heterocrine gland because it has both endocrine and exocrine functions.

The endocrine cells of the pancreas form groups called pancreatic islets or theislets of Langerhans, as shown in the thumbnail shown in Figure \(\PageIndex{6}\). Pancreatic islets contain two main types of cells:alpha cells, which produce the hormone glucagon, andbeta cells, which produce the hormone insulin. These hormones regulate blood glucose levels. As blood glucose levels fall, alpha cells release glucagon to raise blood glucose levels by increasing the rates of glycogen breakdown and glucose release from the liver. When blood glucose levels rise, such as after a meal, beta cells release insulin to lower blood glucose levels by increasing the rate of glucose uptake in most cells of the body and increasing glycogen synthesis in skeletal muscle and the liver. Together, glucagon and insulin regulate blood glucose levels.

Epiphysis

The pineal gland produces melatonin. The rate of melatonin production is affected by photoperiod. In addition to the visual pathways, they innervate the pineal gland. During the photoperiod of the day, some melatonin is produced. However, melatonin production increases during the dark photoperiod (night). In some mammals, melatonin has an inhibitory effect on reproductive functions by reducing the production and maturation of sperm, eggs and reproductive organs. Melatonin is an effective antioxidant, protecting the CNS from free radicals such as nitric oxide and hydrogen peroxide. Finally, melatonin is involved in biological rhythms, particularly circadian rhythms such as the sleep-wake cycle and eating habits.

Gonads

The gonads—male testicles and female ovaries—produce steroid hormones. The testes produce androgens, with testosterone being the most important, which allow the development of secondary sex characteristics and the production of sperm. The ovaries produce estradiol and progesterone, which cause secondary sex characteristics and prepare the body for childbirth.

Organs with Secondary Endocrine Functions

There are many organs whose primary functions are non-endocrine but also possess endocrine functions. These include the heart, kidneys, intestines, thymus, gonads and adipose tissue.

The heart has endocrine cells in the walls of the atria that are specialized cardiac muscle cells. These cells release the hormoneatrial natriuretic peptide (ANP)in response to increased blood volume. The high blood volume causes the cells to stretch, resulting in the release of hormones. ANP acts on the kidneys to decrease Na reabsorption⁺, causing Na⁺and water that must be excreted in the urine. ANP also reduces the amounts of renin released by the kidneys and aldosterone released by the adrenal cortex, further preventing water retention. In this way ANP causes a decrease in blood volume and blood pressure and decreases Na concentration⁺in the blood.

The gastrointestinal tract produces several hormones that aid in digestion. Endocrine cells are found in the lining of the gastrointestinal tract throughout the stomach and small intestine. Some of the hormones produced include gastrin, secretin and cholecystokinin, which are secreted in the presence of food and some of which act on other organs such as the pancreas, gall bladder and liver. They trigger the release of gastric juices, which help break down and digest food in the gastrointestinal tract.

While the adrenal glands associated with the kidneys are largeendocrine glands, the kidneys themselves also have an endocrine function. Renin is released in response to reduced blood volume or pressure and is part of the renin-angiotensin-aldosterone system that leads to the release of aldosterone. Aldosterone then causes Na retention⁺and water, increasing blood volume. The kidneys also release calcitriol, which helps absorb Ca²⁺and phosphate ions.Erythropoietin (EPO)is a protein hormone that triggers the formation of red blood cells in the bone marrow. EPO is released in response to low oxygen levels. Because red blood cells are oxygen carriers, increased production results in a greater supply of oxygen throughout the body. EPO has been used by athletes to improve performance, as greater oxygen delivery to muscle cells allows for greater endurance. Because red blood cells increase blood viscosity, artificially high levels of EPO can cause serious health risks.

Theangerlocated behind the sternum. it is more prominent in infants, and becomes smaller in size during adulthood. The thymus produces hormones called thymosins, which help develop the immune response.

Adipose tissue is a connective tissue found throughout the body. It produces the hormoneleptinin response to food intake. Leptin increases the activity of anorexigenic neurons and decreases the activity of orexigenic neurons, creating a feeling of satiety after eating, thereby affecting appetite and reducing the desire for further food. Leptin is also related to reproduction. It must be present for GnRH and gonadotropin synthesis to occur. Extremely thin females may enter puberty late. However, if adipose tissue levels increase, more leptin will be produced, improving fertility.

Summary

The pituitary gland is located at the base of the brain and is connected to the hypothalamus via the fundus. The anterior pituitary receives products from the hypothalamus from the pituitary portal system and produces six hormones. The posterior pituitary gland is an extension of the brain and releases hormones (antidiuretic hormone and oxytocin) produced by the hypothalamus.

The thyroid gland is located in the neck and consists of two lobes connected by the isthmus. The thyroid consists of follicular cells that produce the hormones thyroxine and triiodothyronine. Parafollicular cells of the thyroid produce calcitonin. The parathyroid glands are located on the back surface of the thyroid gland and produce parathyroid hormone.

The adrenal glands are located on top of the kidneys and consist of the renal cortex and the renal medulla. The adrenal cortex is the outer part of the adrenal glands and produces corticosteroids, glucocorticoids and mineralocorticoids. The adrenal medulla is the inner part of the adrenal gland and produces the catecholamines epinephrine and norepinephrine.

The pancreas is located in the abdomen between the stomach and the small intestine. Clusters of endocrine cells in the pancreas form the islets of Langerhans, which consist of alpha cells that release glucagon and beta cells that release insulin.

Some organs have endocrine activity as a secondary function but have another primary function. The heart produces the hormone atrial natriuretic peptide, which works to reduce blood volume, pressure, and Na⁺concentration. The gastrointestinal tract produces various hormones that aid in digestion. The kidneys produce renin, calcitriol and erythropoietin. Adipose tissue produces leptin, which promotes satiety signals to the brain.