* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download III Semester Botany MODULE 7 ENDOCRINOLOGY
Survey
Document related concepts
Cardiac physiology wikipedia , lookup
History of catecholamine research wikipedia , lookup
Glycemic index wikipedia , lookup
Neuroendocrine tumor wikipedia , lookup
Breast development wikipedia , lookup
Mammary gland wikipedia , lookup
Endocrine disruptor wikipedia , lookup
Hormone replacement therapy (male-to-female) wikipedia , lookup
Bioidentical hormone replacement therapy wikipedia , lookup
Hyperandrogenism wikipedia , lookup
Hyperthyroidism wikipedia , lookup
Transcript
III Semester Botany MODULE 7 ENDOCRINOLOGY Syllabi: Endocrine glands and their hormones, mode of action (in brief), Hypothalamus, Pituitary, Thyroid, Parathyroid, Thymus, Islets of Langerhands, Adrenal, Testis and ovary, Hormonal disorders. Endocrine Glands Endocrinology is the science concerned with the endocrine glands and their secretions (hormones), as well as diagnosis and treatment of disorders of the endocrine system .The body contains two kinds of glands: exocrine and endocrine. Exocrine glands secrete their products through ducts into body cavities and onto body surface, and include salivary, sweat, and digestive glands. By contrast, endocrine glands always secrete hormones into the extracellular spaces around the secretory cells, not into ducts. The secretion then typically diffuses into capillaries and is delivered to target cells by blood. The endocrine glands include the pituitary, thyroid, parathyroids, adrenal, pineal, and thymus glands. In addition, several organs in your body have areas made up of endocrine tissues that produce specific hormones, for example the endocrine pancreas, gonads (ovaries and testes), and hypothalamus. Also, the walls of the small intestine, stomach, kidneys, and heart contain small pockets of endocrine cells. Chemistry of Hormones Most hormones belong to one of the following 4 chemical classes: 1. Peptide hormones: largest, most complex, and most common hormones. Examples include insulin and prolactin 2. Steroid hormones: lipid soluble molecules synthesized from cholesterol. Examples include gonadal steroids (e.g testosterone and estrogen) and adrenocortical steroids (e.g. cortisol and aldosterone). 3. Amines: small molecules derived from individual amino acids. Include catecholamines (e.g. epinephrine produced by the adrenal medulla), and thyroid hormones. 4.Eicosanoids: small molecules synthesized from fatty acid substrates arachidonic acid) located within cell membranes. Include prostaglandins. 1 (e.g. Pituitary Gland The pea-size pituitary gland is enclosed by sella turcica (Turk's saddle) of the sphenoid bone and is connected to the hypothalamus by a funnel-shaped infundibulum. In humans, the pituitary gland has two major lobes: the anterior lobe or adenohypophysis, composed of glandular tissue and the site of production and release of 6 major hormones, as well as the posterior lobe or neurohypophysis, which is actually part of the brain and is composed of neurons and pituicytes (glia-like supporting cells) and is the site of release of two neurohormones produced by the hypothalamus. The posterior pituitary only stores hormones, it is not involved in the production of hormones. The hypothalamus and the Posterior Pituitary The hypothalamus is an important part of the brain with many functions. Neurons of the hypothalamus synthesize and secrete two hormones, antidiuretic hormone (ADH) and oxytocin. The cell bodies of these neurons are found in two nuclei of the hypothalamus and have axons that descend along a thin stalk of tissue called the infundibulum and end in the posterior pituitary gland. The hormones are released at the axon terminals, diffuse into the capillaries and are distributed throughout the body in the bloodstream. ADH and oxytocin are peptides and are considered neurohormones because of their secretion by neurons. 2 Antidiuretic Hormone (ADH) "Diuresis" means urine production. Antidiuretic is a substance that inhibits formation of urine. The main target tissues for ADH are kidney tubules, which in the presence of ADH reabsorb more water from urine and return it to the bloodstream, thus maintaining blood volume. Inability to produce ADH causes diabetes insipidus (watery urine), in which a person produces copious amounts of urine with a resultant loss of ions from the blood. The condition can be corrected by the administration of ADH. Alcohol suppresses ADH production and release. When ADH is absent, the kidneys don’t reabsorb as much water. The person drinking alcohol urinates more often and may become dehydrated as a result. The symptoms of the drinker’s “hangover”—headache, nausea, dizziness—are largely due to dehydration. Oxytocin Oxytocin, the other hormone made in the hypothalamus, causes uterine contraction during childbirth and milk letdown when a baby is nursing. The more the uterus contracts during labor, the more nerve impulses reach the hypothalamus, causing oxytocin to be released. Similarly, the more a baby suckles, the more oxytocin is released. In both instances, the release of oxytocin from the posterior pituitary is controlled by positive feedback—that is, the stimulus continues to bring about an effect that ever increases in intensity. Recently, it has been suggested that oxytocin is important in sexual arousal and orgasm in both males and females. In nonsexual relationships, oxytocin may be responsible for promoting nurturing and affectionate behaviour (a "cuddle hormone"). Hypothalamus and Anterior Pituitary The hypothalamus exerts control of the anterior pituitary by secreting tropic hormones which are hormones that regulate the secretion of other hormones by either stimulating or inhibiting their secretion. The anterior pituitary is non-neural tissue which unlike the posterior pituitary has no direct nervous connection with the hypothalamus. The tropic hormones secreted by the neurosecretory cells of the hypothalamus reach the target cells of the anterior pituitary by the hypothalamicpituitary portal system. 3 Tropic hormones from the hypothalamus act upon cells that secrete tropic hormones in the anterior pituitary. Except for one, all hormones secreted by the hypothalamus and anterior pituitary are peptides. Hypothalamus Tropic Effect Anterior Pituitary Effect/ Target Organ Prolactin Mammary gland development/milk secretion Prolactin releasing hormone (PRH) + Prolactin inhibiting hormone (dopamine) - Thyrotropin releasing hormone (TRH) + Thyroid stimulating hormone (TSH) Secretion of thyroid hormone by thyroid gland Corticotropin Releasing Hormone (CRH) + Adrenocorticotropic hormone (ACTH) Adrenal Cortex Growth and energy metabolism/secretion of insulin-like growth factors by liver Growth hormone releasing hormone (GHRH) + Growth hormone Growth hormone inhibiting hormone (GHIH) - Growth hormone + Follicle stimulating hormone (FSH)/Luteinizing hormone (LH) Gonadotropin releasing hormone (GNRH) FSH: development of sex cells/LH: secretion of sex hormones and stimulation of ovulation Hormones of the anterior pituitary include: 1. Growth hormone (GH) or Somatotropin (STH) 2. Thyroid-stimulating hormone (TSH) or Thyrotropin 3. Adrenocorticotropic hormone (ACTH) or Corticotropin 4. Follicle-stimulating hormone (FSH) 5. Luteinizing hormone (LH) 6. Prolactin (PRL) (These last two are collectively known as Gonadotropins) Growth hormone GH is responsible for general somatic growth by promoting increase in cell size and the rate of mitosis. Although its major targets are the bones and skeletal muscles, the effect on bones is only indirect: instead GH stimulates the liver to produce several small proteins called somatomedins which directly stimulate growth of both cartilage and bone, including direct effect on the epiphyseal plates of long bones. In addition GH: 4 1. Increase uptake of amino acids from the blood and increase protein synthesis 2. Increase uptake of sulfur needed for the synthesis of chondroitin sulfate into cartilage matrix 3. Increase breakdown of fat leading to ↑ blood levels of fatty acids, and 4. Decrease the rate of glucose uptake and metabolism Hypersecretion of GH in children results in gigantism, a condition in which growth is extremely rapid. The person becomes abnormally tall (even 2.4 m or more) but has relatively normal body proportions. On the other hand, hypersecretion of GH in adults results in acromegaly (enlarged extremities). This condition is characterized by overgrowth of bony areas still responsive to GH, namely bones of the feet, face, and hands. Thickening of soft tissues leads to malformed facial features and an enlarged tongue. Hyposecretion of GH in children results in slowed long bone growth, a condition called pituitary dwarfism. Such individuals attain a maximum height of only 1.2 m and may have normal body proportions. However, lack of GH is typically accompanied by deficiencies of other anterior pituitary hormones, including TSH and gonadotropins (FSH and LH) resulting in malproportioned individuals who fail to mature sexually. When diagnosed before puberty, pituitary dwarfism can be treated with GH replacement therapy. Hypersecretion of GH in adults generally causes no problems. Thyroid-Stimulating Hormone (TSH) TSH controls development and secretory activity of the thyroid gland. TSH release is induced by the TRH from the hypothalamus. Adrenocorticotropic Hormone (ACTH) ACTH stimulates the adrenal cortex to release corticosteroid hormones. ACTH release is induced by hypothalamic corticotropin-releasing hormone (CRH). FSH and LH (Gonadotropins) In both sexes, gonadotropins release is stimulated by gonadotropin-releasing hormone (GnRH) from the hypothalamus. Gonadotropins regulate the function of the gonads (testes and ovaries): FSH stimulates production of gametes (sperm or egg) LH promotes production of sex steroids (testosterone or estrogen). 5 Prolactin (PRL) In humans, it is responsible for mammary growth and development and lactogenesis (milk production) at the end of pregnancy in the female, as well as regulation of testicular function in the male. PRL release is stimulated by prolactin-releasing hormone (PRH - possibly serotonin) and inhibited by prolactin-inhibiting hormone (PIH or dopamine). In females, a brief rise in PRL levels just before the menstrual period partially accounts for breast swelling and tenderness experienced by some women at that time, but since this PRL stimulation is very brief, the breasts do not produce milk. In contrast, PRL levels increase dramatically toward the end of pregnancy and milk production by the breasts becomes possible. After birth, the infant's suckling stimulates PRH release in the mother allowing for continued milk production. Pineal gland The pineal gland which is located in the brain secretes the hormone melatonin. Melatonin appears to regulate circadian rhythms (day-night cycles). Thyroid Gland The thyroid gland is a large gland located in the neck, where it is attached to the trachea just below the larynx. The parathyroid glands are embedded in the posterior surface of the thyroid gland. The thyroid gland is composed of a large number of follicles, each a small spherical structure made of thyroid cells filled with triiodothyronine (T3), which contains three iodine atoms, and thyroxine (T4), which contains four iodine atoms. These are the two forms of thyroid hormone; T3 is thought to have the greatest effect on the body. Effects of Thyroid Hormones To produce triiodothyronine and thyroxine, the thyroid gland actively acquires iodine. The concentration of iodine in the thyroid gland can increase to as much as 25 times that of the blood. If iodine is lacking in the diet, the thyroid gland is unable to produce the thyroid hormones. In response to constant stimulation by the anterior pituitary, the thyroid enlarges, resulting in a simple, or endemic, goiter. Some years ago, it was discovered that the use of iodized salt allows the thyroid to produce the thyroid hormones, and therefore helps prevent simple goiter. 6 Thyroid hormones increase the metabolic rate. They do not have a single target organ; instead, they stimulate all cells of the body to metabolize at a faster rate. More glucose is broken down, and more energy is utilized. If the thyroid fails to develop properly, a condition called congenital hypothyroidism results. Severe hypothyroidism during the fetal development and in infants is called cretinism. The child has a short disproportionate body, a thick tongue and neck, and is mentally retarded. Cretinism may reflect a genetic deficiency of the fetal thyroid gland or maternal factors such as lack of iodine in the diet. The occurrence of hypothyroidism in adults produces the condition known as myxedema, which is characterized by lethargy, weight gain, loss of hair, slower pulse rate, lowered body temperature, and thickness and puffiness of the skin. The administration of adequate doses of thyroid hormones restores normal function and appearance. . In the case of hyperthyroidism (over secretion of thyroid hormone), as seen in Graves’ disease, the thyroid gland is overactive, and a goiter forms. This type of goiter is called exophthalmic goiter. The eyes protrude because of edema in eye socket tissues and swelling of the muscles that move the eyes. The patient usually becomes hyperactive, nervous, and irritable, and suffers from insomnia. Parathyroid Glands The parathyroid glands are small in size and are found on the posterior aspect of the thyroid gland. Typically, there are four of them but the actual number may vary. PTH, a small protein, is the single most important hormone controlling calcium homeostasis. Its release is triggered by falling blood calcium levels and inhibited by hypercalcemia (high blood calcium). PTH release has the following consequences on these target organs: 1. Osteoclasts (bone absorbing cells) are stimulated to digest bone and release ionic calcium and phosphates to the blood. 2. Kidneys are stimulated to reabsorb calcium and excrete phosphate 3. Absorption of calcium by the intestine increases. Vitamin D is required for absorption of calcium from ingested food. For vitamin D to exert this effect, it must first be converted by the kidneys to its active form and it is this conversion that is directly stimulated by PTH. 7 Calcium is essential for so many functions, including transmission of action potentials, muscle contraction, pacemaker activity in the heart, and blood clotting, precise control of ionic calcium levels in body fluids is absolutely critical. As a result both hyper- and hypoparathyroidism can have severe consequences. Hyperparathyroidism Is rare, usually the result of a parathyroid gland tumor, and results in severe loss of calcium from the bones. The bones soften and deform as their mineral salts are replaced by fibrous connective tissue. The resulting hypercalcemia leads to, among others, depression of the nervous system leading to abnormal reflexes and weakness of the skeletal muscles, as well as formation of kidney stones as excess calcium salts are deposited in kidney tubules. Hypoparathyroidism It is a PTH deficiency, which is a common consequence of parathyroid trauma or removal during thyroid surgery. The resulting hypocalcemia increases excitability of neurons and may lead to tetany resulting in uncontrollable muscle twitches and convulsions, which if untreated may progress to spasms of the larynx, respiratory paralysis and death. Thymus The thymus lies close to the heart and is critical for normal immune function because T-lymphocytes mature here (which is the reason they are called T (T for thymus) - lymphocytes). The thymus also secretes the hormone thymosin which regulates T cell function. Adrenal Glands The adrenal glands sit atop the kidneys. Each adrenal gland consists of an inner portion called the adrenal medulla and an outer portion called the adrenal cortex. The adrenal medulla is under nervous control, and the adrenal cortex is under the control of ACTH (also called corticotropin), an anterior pituitary hormone. Stress of all types, including emotional and physical trauma, prompts the hypothalamus to stimulate the adrenal glands. 8 Adrenal Medulla Epinephrine (adrenaline) and norepinephrine (noradrenaline) produced by the adrenal medulla rapidly bring about all the body changes that occur when an individual reacts to an emergency situation. The release of epinephrine and norepinephrine achieves the same results as sympathetic stimulation—the “fight-orflight” responses: increased heart rate, rapid respiration, dilation of the pupils, etc. Thus, these hormones assist sympathetic nerves in providing a shortterm response to stress. Adrenal Cortex There are three layers in the adrenal cortex, and each produces a different set of steroid hormones. The hormones produced by the adrenal cortex provide a longterm response to stress. The two major types of hormones produced by the adrenal cortex are the mineralocorticoids and the glucocorticoids. The mineralocorticoids regulate salt and water balance, leading to increases in blood volume and blood pressure. Glucocorticoids influence metabolism of most body cells, help us resist stress, and are considered to be absolutely essential to life. The most important glucocorticoid in humans is cortisol, but small amounts of cortisone and corticosterone are also produced. The glucocorticoids regulate carbohydrate, protein, and fat metabolism, leading to an increase in blood glucose level. Cortisone, the medication often administered for inflammation of joints, is a glucocorticoid. 9 The adrenal cortex also secretes small amounts of both male and female sex hormones -Gonadocorticoids (Sex Hormones) regardless of one’s gender. Both male and female sex hormones promote skeletal growth in adolescents. The male hormones from the adrenal gland stimulate the growth of axillary and pubic hair at puberty. In addition, male hormones help to sustain the sex drive, or libido, in both men and women. Hyperadrenalism When the level of adrenal cortex hormones is high due to hypersecretion, a person develops Cushing’s syndrome. It can be caused by a cortisol-secreting tumor in the adrenal glands, ACTH-secreting tumor of the pituitary, or ACTH secreted by abdominal carcinoma. The symptoms include a persistent hyperglycemia, dramatic loss of muscle and bone proteins, and water and salt retention, leading to hypertension and edema. The face is moon-shaped due to edema. Masculinization may occur in women because of excess adrenal male sex hormones. Hypoadrenalism When the level of adrenal cortex hormones is low due to hyposecretion, a person develops Addison’s disease. It involves significant reduction in plasma glucose and sodium, very high levels of potassium and loss of weight. Without cortisol, glucose cannot be replenished when a stressful situation arises. Even a mild infection can lead to death. The usual treatment is corticosteroid replacement therapy. Pancreas The pancreas is a long organ that lies transversely in the ab-domen between the kidneys and near the duodenum of the small intestine. It is composed of two types of tissue. Exocrine tissue produces and secretes digestive juices that go by way of ducts to the small intestine. Endocrine tissue, called the pancreatic islets (islets of Langerhans), produces and secretes the hormones insulin and glucagon directly into the blood. 10 The two antagonistic hormones, insulin and glucagon, both produced by the pancreas, help maintain the normal level of glucose in the blood. Insulin is secreted when the blood glucose level is high, which usually occurs just after eating. Insulin stimulates the uptake of glucose by most body cells. Insulin is not necessary for the transport of glucose into brain or red blood cells, but muscle cells and adipose tissue cells require insulin for glucose transport. In liver and muscle cells, insulin stimulates enzymes that promote the storage of glucose as glycogen. In muscle cells, the glucose supplies energy for muscle contraction, and in fat cells, glucose enters the metabolic pool and thereby supplies glycerol for the formation of fat. In these ways, insulin lowers the blood glucose level. Glucagon is secreted from the pancreas, usually between meals, when the blood glucose level is low. The major target tissues of glucagon are the liver and adipose tissue. Glucagon stimulates the liver to break down glycogen to glucose and to use fat and protein in preference to glucose as energy sources. Adipose tissue cells break down fat to glycerol and fatty acids. The liver takes these up and uses them as substrates for glucose formation. In these ways, glucagon raises the blood glucose level. Diabetes Mellitus Diabetes mellitus is a fairly common hormonal disease in which insulinsensitive body cells are unable to take up and/or metabolize glucose. Therefore, the blood glucose level is elevated—a condition called hyperglycemia. Because body cells cannot access glucose, starvation occurs at the cell level. The person becomes 11 extremely hungry—a condition called polyphagia. As the blood glucose level rises, glucose will be lost in the urine (glycosuria). Glucose in the urine causes excessive water loss through urination (polyuria). The loss of water in this way causes the diabetic to be extremely thirsty (polydipsia). Glucose is not being metabolized, so the body turns to the breakdown of protein and fat for energy. Fat metabolism leads to the buildup of ketones in the blood, and excretion of ketones in the urine (ketonuria). Because ketones are acidic, their buildup in the blood causes acidosis (acid blood), which can lead to coma and death. Secondary Endocrine Organs A number of organs, in addition to their primary functions, secrete hormones or participate in their activation. These secondary endocrine organs include: Heart - Secretes atrial natriuretic peptide which regulates sodium reabsorption by the kidneys. Kidneys - Secretes erythropoietin (EPO) which stimulates production of red blood cells in the bone marrow. Digestive organs - Secrete several hormones that regulate digestion and absorption of food. Liver - Secretes insulin-like growth factors (IGF's) which promote tissue growth. Skin and Kidneys - Activate vitamin D3 which regulates blood calcium levels. Gonads The ovaries and testes in addition to producing the sex cells are major sites for the production of sex hormones. 12