How Hormones Are Produced
ENDOCRINE SYSTEM
The endocrine system controls many of the functions of the human body, including but not limited to metabolism, reproduction, growth, and activity level.
Hormones
The chemical mediators of the endocrine system are collectively termed hormones and are distributed throughout the body via the circulatory system. The intensity of a physiological response depends on the concentration of hormones produced and the density of receptors expressed on the cell surface of the appropriate tissues. If, for example, no receptors are available to bind to even a high level of hormone, no cellular response is initiated.
What exactly is a hormone?
Any substance that is transported in the blood and that elicits a cellular response may be termed a hormone.
Amino Acid Derivatives
Individual amino acids, while usually used to create proteins, may be transformed into other biologically significant molecules such as hormones. This is the case for the amino acid tyrosine, the starting material for a group of hormones called the catecholamines. The amino acid phenylalanine is converted into tyrosine, which may be further processed into dopamine, a type of catecholamine. Dopamine functions as either a hormone or neurotransmitter depending on where it is produced and how it is transported.
What is the function of dopamine?
In the brain, dopamine is essential for motor control, as well as for mood associated with reward and gratification. Outside of the nervous system, dopamine is a potent vasodilator and regulator of another catecholamine, norepinephrine.
In the next phase in the processing of tyrosine, dopamine is first converted into norepinephrine before it is processed into an essential body hormone, epinephrine (adrenaline). Additionally, tyrosine may be processed by thyroid cells into a differently iodinated form, either thyroxine or triiodothyronine.
Tryptophan is another amino acid that may be processed into hormones. Serotonin and melatonin are derived from tryptophan in the pineal gland, and play a role in diurnal (sleep/wake) cycles.
Proteins
The largest molecular group of hormones is the protein hormones. Many of these are produced in the pituitary gland (the master endocrine gland) and control functions from water retention in the kidneys (antidiuretic hormone, ADH), thyroid hormone secretion (thyroid-stimulating hormone, TSH), and reproduction (luteinizing hormone, LH; follicle-stimulating hormone, FSH).
Pancreatic islet cells also produce and secrete protein hormones such as insulin and glucagon, which regulate the carbohydrate level in the blood stream. Other protein hormones (calcitonin and parathormone) regulate the level of calcium in the blood stream.
Steroids
While cholesterols are often viewed in a negative light, without them the human body would not have steroid hormones. Cholesterol is the starting material upon which the steroid hormones are based. These include testosterone, estrogen, and progesterone, which function in human reproduction. Cortisol (cortisone) is another steroid hormone produced in the adrenal gland that takes part in many functions of metabolism, especially carbohydrate release into the blood stream.
Pituitary Gland
The pituitary gland (hypophysis) is the master regulatory gland of the human body. This endocrine gland produces and secretes hormones into the blood stream that lead to the release of other endocrine hormones from other glands.
The pituitary gland is found at the base of the brain and suspended by a stalk (hypophyseal stalk) just inferior to the hypothalamus. Only about the size of a pea, the pituitary is composed of two lobes that are derived from distinctly different embryological tissues.
Anterior Pituitary
The anterior pituitary (also called pars distalis and adenohypophysis) resembles glandular epithelium because of its embryonic source. The cells of the pituitary are called chromophils and include:
1. Acidophils, which are the most abundant cell type in the anterior pituitary. These cells are further subdivided based upon the hormones they secrete:
· The somatotrophs produce the growth hormone somatotropin.
· The mammotrophs secrete prolactin. This hormone, stimulated by the pituitary hormone oxytocin, promotes mammary gland development as well as lactation.
2. Basophils are subdivided into:
· The corticotrophs that produce adrenocorticotropic hormone (ACTH). This hormone stimulates the cortical cells in the adrenal gland to release other endocrine hormones.
· Thyrotrophs, another basophil cell type, produce and secrete thyroid-stimulating hormone (TSH, thyrotropin). As the name of this hormone implies, it stimulates the production and release of the thyroid hormones thyroxine and triiodothyronine.
· The final type of basophil is the gonadotrophs, which produce follicle-stimulating hormone (FSH) and luteinizing hormone (LH). In females, these hormones lead to the maturation of ovarian follicles, ovulation of a mature follicle, and lactation of the breasts at the end of pregnancy. In males, FSH stimulates the development of sperm stem cells (spermatocytes) and LH stimulates testicular cells to produce testosterone.
Posterior Pituitary
This portion of the pituitary isn’t glandular at all. In fact, the pituicytes (cells of the posterior pituitary) closely resemble neuroglia cells. This tissue is derived from embryonic forebrain tissue and these cells support neurons whose axons extend from the hypothalamus to the posterior lobe of the pituitary gland.
Hormones produced in the hypothalamus are transported down the hypophyseal stalk and terminate in the posterior pituitary gland. Here they are stored in granules called Herring bodies in the axon terminals.
The hypothalamic hormones released by the posterior pituitary include antidiuretic hormone (ADH) and oxytocin. ADH (vasopressin) is released when low blood pressure is detected.
Thyroid Gland
The thyroid gland is a bilobed endocrine gland positioned just inferior and ventral to the larynx. Often, the two lobes are joined together across the midline of the trachea by a narrow strip (isthmus) of thyroid tissue. Additionally, about half of the population will have a small upward projecting lobe (pyramidal lobe) from the midline isthmus upward toward the larynx.
The thyroid tissue is organized into pools of hormones and stabilizing proteins that are stored outside the cell as colloid bounded by a sphere of thyroid cells (follicular cells). They make up the basic unit of the thyroid: the thyroid follicle. The function of the follicular cells is to produce, store, and release thyroid hormones upon the stimulation of the thyroid cells by the pituitary-derived TSH.
Follicular cells begin their production of thyroid hormone with a base glycoprotein called thyroglobulin. This protein is rich in the amino acid tyrosine. Cells will add iodine onto tyrosine.
Triiodothyronine (T3) and thyroxine (T4) are recovered from the colloid in the center of the thyroid follicle and function primarily to increase carbohydrate metabolism, heart rate, appetite, and respiration. At the same time, they decrease the production of cholesterol and triglycerides and aid in the reduction of body weight.
In addition to the thyroid follicular cells, other cells reside just on the periphery of the follicles. These cells are seen as aggregates of larger, more rounded cells than the cube-shaped follicular cells. These are the parafollicular cells. Parafollicular cells produce and secrete calcitonin, a hormone that inhibits bone reabsorption, thereby leaving more calcium stored in the matrix of the bone and reducing the free circulating levels of calcium in the blood stream. Parafollicular cells are often referred to as C cells.
Parathyroid Glands
Present on the posterior portion of each lobe of the thyroid gland, the parathyroid glands exist in four clusters (superior and inferior portions on both the right and left lobes). The primary cell of this gland is the chief cell. Endocrine in nature, chief cells produce parathormone (PTH), which increases blood calcium levels. It does so by stimulating bone reabsorption, preventing calcium loss in the urine and increasing vitamin D production in the kidneys.
Adrenal Glands
Best known for its production of adrenaline under stressful conditions, the adrenal gland is positioned as a cap on the superior portion of each kidney. This gland is often referred to as the suprarenal gland. The gland has an outer portion called the cortex and an inner portion called the medulla. Making up approximately 80–90 percent of the mass of the adrenal gland, the outer cortex can be divided into three distinct and functionally diverse zones:
· Zona glomerulosa. The most external of the cortical layers is the zona glomerulosa. Its cells are arranged in spherical structures. The principal function of these cells is the production of mineralocorticoids, such as aldosterone. Once secreted, aldosterone causes the tubules of the kidneys to absorb more sodium and secrete potassium. The result is the increased conservation of water and a reduction in urine volume.
· Zona fasciculata. The middle cortical layer is made up of rows of cells whose cytoplasm is filled with vesicles that give it a rather spongy appearance. These cells are called spongiocytes, and they produce glucocorticoids, such as cortisol. These hormones control general metabolism and have both anabolic (building up) and catabolic (tearing down) effects.
· Zona reticularis. This is the smallest layer and the one adjacent to the adrenal medulla. These cells produce androgens that produce a weakly masculinizing effect.
The medulla is populated almost exclusively by large spherical chromaffin cells and is the site of adrenaline (epinephrine) production. While this is the bulk of what is secreted by the chromaffin cells, about 15 percent of their secretion is also norepinephrine, which is necessary to convert tyrosine into adrenaline.
Pancreatic Islets
Named because they look like islands of endocrine cells surrounded by the exocrine cells of the pancreas, the cells of the pancreatic islets produce hormones that relate to metabolism, digestion, and pancreatic function:
· Alpha cells in the islets produce glucagon, a hormone secreted when low blood glucose levels are detected. This hormone results in the glycolysis of glycogen and the addition of glucose to the blood stream, thus increasing blood glucose levels.
· Beta cells are insulin-producing cells. This pancreatic endocrine hormone is antagonistic to glucagon and is secreted when high blood glucose levels are detected. In response, cells remove glucose from the blood, polymerize glucose into glycogen, and store it intracellularly. The liver and muscle cells are well adept at this type of carbohydrate storage.
· Delta cells produce somatostatin (growth-inhibiting hormone). As the name implies, its job is to regulate the secretion of other endocrine hormones that stimulate growth, and by doing so, slow or halt the growth process in humans.
· Gamma cells are stimulated after a meal rich in protein and after fasting or exercise. These activities result in a lowering of the blood sugar level and cause the cells to release pancreatic polypeptide (PP). This hormone regulates the pancreas itself, affects glycogen stores in the liver, and stimulates the alimentary canal.
· Finally, epsilon cells secrete ghrelin when the stomach is empty so you feel the sensation of hunger.
Pineal Gland
This small pinecone-shaped gland (hence the name pineal) is located near the center of the hemispheres of the brain in the region of the epithalamus. It is involved in the production of melatonin, which is known to control sleep cycles and circadian rhythms.