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Endocrine System Role of the Endocrine System Hormones: Types

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1 Endocrine System Role of the Endocrine System Hormones: Types
Transport of Hormones Interaction of Hormones with Target Cells Effects of Hormones on Target Control Mechanisms of Endocrine Glands Endocrine Organs Pituitary (Anterior and Posterior) Thyroid Parathyroids Adrenals (Cortex, Medulla) Pancreas Gonads Ovaries Testes Overview The second great controlling system after the nervous system is the endocrine system. Along with the nervous system, it coordinates and directs the activities of the body's cells. However, the nervous system is built for specific and rapid messaging to initiate movement that has to occur immediately in some cases. The endocrine system acts more broadly and slowly by means of chemical messaging that travels through the blood to all tissues of the body. Only certain organs are capable of responding to this broadcast message and the changes that occur can last for minutes to many days. Hormones of the endocrine system control reproduction, growth and development, mobilization of body defenses, maintenance of blood electrolyte, water, and nutrient balances, and regulation of cellular metabolism and available energy. The organs of the endocrine system do not amount to much mass, about 0.2 percent of the entire body mass.

2 The Endocrine System A more broad-based and long-lasting communication system than the nervous system Uses chemical messages (hormones) that are released into the blood Hormones control several major processes Reproduction Growth and development Mobilization of body defenses Maintenance of much of homeostasis Regulation of metabolism

3 Types of Hormones Protein-based hormones Steroid hormones
Prostaglandins and catecholamines Insulin Growth hormone Hormones are chemical substances secreted by cells into the extracellular fluids that regulate the metabolic activity of other cells in the body. The word hormone comes from the Greek meaning "to arouse". Nearly all hormones are either peptide-based ones, in which some number of amino acids are involved, or steroid hormones composed of steroid or sterol derivatives. A less common hormone composition is found in the prostaglandins and catecholamines which are produced from a certain class of lipids.

4 Hormones as Chemical Messengers
Endocrine gland (source) Target organs or glands Hormones released into the bloodstream circulate through the body and eventually in a matter of minutes, arrive at their target cells or target organs. Protein receptors or docking sites, set into the wall of the plasma membrane of the target, specifically bind the hormone and initiates change within the cell itself. Hormones move through the bloodstream to target organs

5 Hormone Interaction with Target Cells
Hormones bind to receptors sticking out from the plasma membrane of target cells or within target cells growth factor insulin epinephrine Hormones Receptors Examples of receptors found in the plasma membrane of cells

6 Effects Caused by Hormones
Changes in electrical state of the cell, stimulating change Causes enzymes to be made, changing metabolic abilities of target cell Turn on or off enzymes that alter metabolism inside cell Stimulate cell division and multiplication Turn on certain genes Typically one or more of the following changes occur upon binding: The plasma membrane changes permeability or electrical state Protein synthesis or synthesis of regulatory molecules are initiated inside the cell Enzymes are activated or inactivated Cellular division is initiated (e.g. mitosis) Hormones act by two major mechanisms, either at the plasma membrane or on some target within the cell.

7 Control Mechanisms of Endocrine Glands
Hormonal: Chemical stimulus (i.e. endocrine glands are activated by other hormones) Humoral/Blood-Based: Changing blood levels of certain ions stimulate hormone release Neural: Nerve impulses stimulate hormone release; most are under control of the sympathetic nervous system 1. Hormonal stimulus (the most common) occurs when an endocrine gland is itself stimulated to secrete something in response to a chemical message from another organ. The endocrine gland known as the hypothalamus, found at the base of the brain, stimulates the anterior portion of the pituitary gland (just inferior to it) using hormone stimulation. The hypothalamus then monitors the levels of the hormone released by the anterior pituitary and stops releasing initiation hormones when the pituitary hormone is at a specific level. 2. In the humoral stimulus mechanism, changing blood levels of certain ions and nutrients may also stimulate hormone release. The word humoral refers to the "humor" or bodily fluids (generally the blood). For example the parathyroid hormone (PTH) released by the parathyroid glands is secreted into the blood when blood calcium levels drop too low. Parathyroid hormone then acts to reverse declining calcium levels and is no longer produced when calcium levels rise back to a certain level (again, negative feedback). 3.Finally, neural stimuli are used to initiate hormone release in some cases. In the sympathetic nervous system, stimulation of the adrenal medulla (part of the adrenal glands) is facilitated by nervous stimulation. In response, the adrenal medulla releases norepinephrine and epinephrine (noradrenalin and adrenalin) during periods of stress.

8 Location of Major Endrocrine Organs
MAJOR ENDOCRINE ORGANS We will concern ourselves with ten different major endocrine organs of the body. One of these organs, the hypothalamus, is part of the nervous system but is also considered a major endocrine organ because it also produces several hormones. By the same token, the pancreas is considered an exocrine organ within the digestive system because it also produces digestive enzymes in addition to the hormone insulin. Pure endocrine-only glands are ductless and release their products into the blood or lymph. Ducts are used to deliver exocrine products to the body cavities or the body surface. Figure 9.3

9 Nervous system that hormones can drip down on
Pituitary Gland Flip and enlarge Nervous system that hormones can drip down on blood portal system Pituitary The pituitary gland is about the size of a grape and hangs by a stalk from the inferior surface of the brain's hypothalamus. It is surrounded by the "Turks' saddle" of the sphenoid bone. It has two functional lobes which we will study separately, the anterior and the posterior pituitary. glandular tissue nervous tissue

10 Hormones of the Anterior Pituitary
Six anterior pituitary hormones Two affect non-endocrine targets Four stimulate other endocrine glands (tropic hormones) Characteristics of all anterior pituitary hormones They are proteins (or peptides) They act through second-messenger systems They are regulated by hormonal stimuli, mostly negative feedback Anterior Pituitary The anterior pituitary is that portion that faces the front of the body. It is distinguished from the posterior lobe of the pituitary by the hormones that it produces and the method by which it is stimulated. The anterior pituitary is sometimes called the master endocrine gland because its removal or destruction has a dramatic effect on the body. The anterior pituitary is controlled by releasing and inhibiting hormones produced by the brain's hypothalamus. Hormone released by the hypothalamus reach the anterior pituitary by a short and directed blood pathway called a portal system (portal circulation: two capillary beds connected by a vein. Four of the six hormones produced in the anterior pituitary are targeted towards other endocrine glands; they are therefore called tropic hormone. The other remaining two hormones (growth hormone and prolactin) act on nonendocrine targets.

11 Hormones of the Anterior Pituitary
Causes aldosterone, glucocorticoid, or androgen release 1. Growth hormone (GH) is a general protein metabolic hormone produced in the anterior pituitary. It is directed largely towards the growth of skeletal muscles and long bones, and thus plays a key role in determining final body size. In muscles GH causes amino acids to be built into proteins and stimulates most target cells to grow and divide. It also causes fats to be broken down and used for energy (while saving glucose). Deficits in GH production (hyposecretion) cause pituitary dwarfism. Excess of GH production (hypersecretion) products gigantism, often as result of tumors of the pituitary. Acromegaly is hypersecretion after long bone growth has ended, producing bony eyebrow ridges and larger feet and hands. 2. Prolactin (PRL) is a protein hormone similar to growth hormone in structure. It targets the breast and is active after childbirth in females to stimulate and maintain milk production. It has no known function in males. 3. Adrenocorticotropic hormone (ACTH) regulates endocrine activity of the cortex (outer) portion of the adrenal gland. 4. Thyroid-stimulating hormone (TSH), also called thyrotropic hormone (TH) regulates growth and activity of the thyroid gland. 5. Follicle-stimulating hormone (FSH) is a gonadotropic hormones that stimulates follicle development in the ovaries (causing them to produce estrogen) in females and stimulates sperm development in the testes of males. Hyposecretion leads to sterility while hypersecretion causes major problems (save multiple births). 6. Luteinizing hormone (LH) triggers ovulation of an egg from the ovary, causing it to produce progesterone and estrogen. In males, LH is called interstitial cell-stimulating hormone (ICSH) because it stimulates testosterone production by the testes interstitial cells. Hyposecretion leads to sterility while hypersecretion going cause major problems (save multiple births). Increases bone and skeletal mass Causes milk secretion in breasts Stimulates thyroid hormone secretion Causes follicle and sperm maturation, ovulation and testosterone production

12 Hormones of the Posterior Pituitary
Posterior Pituitary: Release of Hypothalamus-produced Hormones The posterior pituitary is technically not an endocrine gland but a storage depot. The posterior pituitary stores hormones produced by the hypothalamus gland and releases these hormones when they are needed. The hormones made in the hypothalamus travel along axons of neurons connecting the hypothalamus to the posterior pituitary. 1. Oxytocin made in the hypothalamus is released by the posterior pitutiary in significant amounts only during sex, childbirth, and nursing. It stimulates uterine muscle contraction in labor and sexual intercourse. It causes milk ejection in a nursing woman. Synthetic drugs like pitocin are oxytocin substitutes that induce labor. Less frequently, oxytocics are used to stop postpartum bleeding and to stimulate milk ejection. 2. Antidiuretic hormone (ADH) made in the hypothalamus is released by the posterior pitutiary to inhibit urine production (called diuresis). ADH causes the kidneys to reabsorb more water from the forming urine, decreasing urine volume and increasing blood volume. In large amounts, ADH (renamed as vasopressin) also increases blood pressure by causing constriction of the arterioles. Drinking alcohol inhibits ADH secretion and results in large amounts of urine production (causing dehydration later). Diuretic drugs block the effect s of ADH causing water flushing (used to manage edema - excess water retention). Hyposecretion of ADH causes diabetes insipidus where excessive urine output and constant thirst are symptoms. Stimulates uterine muscle contraction in labor and intercourse; causes milk ejection Inhibits diuresis (urine production) and increases blood pressure

13 Thyroid Gland Thyroid hormone Calcitonin
Consists of two lobes and a connecting isthmus Thyroid hormone Controls rate of cellular respiration. All body cells are targets. Calcitonin Decreases blood calcium levels by causing bone calcium deposition. Made by parafollicular cells. Thyroid The thyroid gland is an endocrine gland found at the base of the throat, just inferior to the Adam's apple. It is a fairly large gland consisting of two lobes connected by an isthmus. Internally the thyroid gland is composed of hollow structures called follicles that store a sticky colloidal material from which thyroid hormone is made and stored. C or parafollicular cells found in the connective tissue between the follicles produces the second hormone called calcitonin. Two hormones are made by the thyroid. 1. Thyroid hormone is actually two active iodine-containing hormones called thyroxine (T4) and triiodothyronine (T3). Thyroxine is the major hormone secreted by the thyroid follicles. Triiodothyronine is formed at the target tissues by conversion of thyroxine to triiodothyronine. Both hormones are constructed from two tyrosine amino acids linked together (thyroxine binds four I atoms while triiodothyronine as three (hence the names T4 and T3). Figure 9.6

14 Thyroid Hormone Lack of Dietary Iodine: Goiter
Hyposecretion of thyroxine ( hypothyroidism in adults) Hyperthyroidism (Grave's disease) Thyroid hormone controls the rate of cellular respiration (glucose oxidation). All cells in the body are targets for this hormone. It is important for normal tissue growth and development, especially the nervous and reproductive systems. i. A lack of dietary iodine causes enlargement of the thyroid (a goiter) in an attempt to produce more thyroxine. Hyposecretion of thyroxine can be cause by lack of TSH as in cretinism of children, resulting in dwarfism (adult body proportions remain childlike), dry skin, scanty hair and mental retardation. Hormone therapy can address this. In adults hypothyroidism causes physical and mental sluggishness, puffy face, fatigue, low body temp, and dry skin. ii. Hyperthyroidism results from a tumor or the thyroid and causes high BMR, intolerance of heat, rapid heartbeat, weight loss, and agitation (Graves' disease). The thyroid may enlarge and the eye bulge in hyperthyroidism. Radiation of the thyroid can treat this.

15 Calcitonin (Calcium deposition)
Decreases blood calcium levels by causing its deposition on bone Antagonistic to parathyroid hormone Produced by C (parafollicular) cells 1. Calcitonin or thyrocalcitonin decreases blood calcium levels by causing calcium deposition in the bones. It acts antagonistically to parathyroid hormone made by the parathyroids. Calcitonin is made by C (parafollicular) cells in the connective tissue between follicles. It is released directly into the blood in response to increasing levels of blood calcium. Calcitonin production probably declines in elderly adults, exacerbating decalcification of the bones. Figure 9.9

16 Parathyroid Glands (Calcium removal)
Found on posterior surface of thyroid Parathyroid hormone Stimulates osteoclasts to remove calcium from bone Stimulates the kidneys and intestine to absorb more calcium Raises calcium levels in the blood Parathyroids The parathyroid glands are tiny masses of glandular tissue found on the posterior surface of the thyroid gland. Usually there are two glands on each lobe (four total) but there can be more and in other regions of the neck. The parathyroids secrete parathyroid hormone (PTH) or parathormone which, when released into the blood, stimulate bone destruction by osteoclasts. This increases blood calcium levels. PTH is a hypercalcemic hormone whereas calcitonin (from the thyroid) is a hypocalcemic hormone. The negative feedback interaction between these two hormones control blood calcium level. PTH also stimulates kidneys and intestines to absorb more calcium. Low blood calcium causes overactive nervous action. Delivery of impulses to muscles occurs at such a rapid rate that muscles go into uncontrollable spasm (tetany) that can be fatal. Removal of the parathyroids can cause this situation.

17 Adrenal Glands Two parts that act as if separate glands
Cortex – outer glandular region in three layers Medulla – inner neural tissue region Sits on top of the kidneys Ad-renal both mean “upon or Epi-nephros next to the kidney” &

18 Adrenal Hormones and Their Locations
Adrenal Cortex Outer portion produces mineralocorticoids like aldosterone Middle layer produces glucocorticoids like cortisone and cortisol Innermost layer produces androgens like estrogens and testosterone Medulla produces catecholoamines like epinephrine and norepineprhine Adrenal Glands The adrenals are called such because they sit upon the renals (kidneys) as bean-shaped organs. A single adrenal gland is really structurally and functional two organs; the outer glandular cortex and the central medulla region. The adrenal cortex produces three kinds or corticosteroid hormones known as mineralocorticoids, glucocorticoids and sex hormones.

19 Adrenal Cortex (Outer Portion)
Hormones of the Adrenal Cortex 1. Of the mineralocorticoids, aldosterone is the main one produced by the outermost adrenal cortex cell layer. Mineralocorticoids regulate the mineral (salt) content of the blood, specifically sodium and potassium ion concentrations. Their target is the kidney tubules that selectively reabsorb minerals or allow them to be flushed out in the urine. An increase in blood aldosterone levels causes increased retention of sodium ions and more secretion of potassium ions into urine. The reabsorption of sodium increases the osmolarity of the surrounding tissue, causing water to follow and be retained as well. Aldosterone release is stimulated by humoral factors such as low blood sodium or high blood potassium. Other organs can control aldosterone by the adrenal cortex too. The kidneys under the adrenals produce the enzyme renin when blood pressure drops; low blood pressure triggers a series of reactions that form angiotensin II that in turn simulates aldosterone release. Atrial natriuretic peptide (ANP) produced by the heart prevents aldosterone release and thus reduces blood volume and pressure. Hyperaldosteronism results in high blood pressure and edema. Aldosterone release causes salt and water retention, increasing blood pressure

20 Hormones of the Innermost Cortex Layer
Sex hormones (steroids) Produced in the inner layer of the adrenal cortex Androgens (male) and some estrogen (female) -- both produced regardless of gender Hypersecretion causes masculinization (regardless of gender) - most obvious effects in females Hyposecretion causes Addison's disease 1. The innermost adrenal cortex in known to produce androgens (male sex homrones) and some estrogens (female sex hormones). Both are produced regardless of the gender. Hypersecretion of sex hormones leads to masculinization, regardless of sex. In males these effects may not be clear but in females a beard develops and male body hair patterns are seen. Hyposecretion of all adrenal cortex hormones causes Addison's disease symptomized by bronze colored skin, lost of electrolytes, weakened muscles hypoglycemia, poor stress coping, and a suppressed immune system. Hypersecretion (usually from tumors) cause different effects depending on the cortical area involved.

21 Hormones of the Adrenal Medulla
Produces two similar hormones (catecholamines) Epinephrine Norepinephrine These hormones prepare the body to deal with short-term stress Sympathetic system stimulates catecholamine release in fight or flight: increased heart rate, blood pressure, blood gluocose, respiratory rate Hormones of the Adrenal Medulla The adrenal medulla develops from a knot of nervous tissue like the posterior pituitary. When the sympathetic nervous system signals the medulla, two similar hormones are released: epinephrine (adrenaline) and norepinephrine (noradrenalin). Together these hormones care catecholamines. It is thought that the adrenal medulla might be a modified sympathetic nervous system ganglion since the sympathetic nerves use norepinephrine as a neurotransmitter. In the fight-or-flight response, the sympathetic nervous system stimulates the adrenal medulla that pumps hormones into the blood stream to prolong the effects of the neurotransmitter of the sympathetic nervous system. Catecholamines increase heart rate, blood pressure, and blood glucose levels while dilating bronchioles in the lungs. More oxygen and glucose circulate faster to the brain, muscles and heart, allowing readiness to combat short term stress. Such a reaction can be called the alarm stage of the stress response. Glucocorticoids conversely help the body cope with prolonged or continuing stressors during what is called the resistance stage of the stress response. When stress is continuous the adrenal cortex may fail, which is usually fatal. Damage or destruction of the adrenal medulla isn't a problem if the sympathetic nervous system functions normally. Hypersecretion of catecholamines causes excessive sympathetic nervous system activity (rapid heartbeat, high blood pressure, and perspiration).

22 Pancreatic Islets The pancreas is a mixed gland
The islets of the pancreas produce hormones These hormones are antagonists that maintain blood sugar homeostasis Pancreatic Islets The pancreas is a mixed gland producing digestive enzymes as an exocrine gland and hormones as an endocrine gland. The endocrine function arises from the pancreatic islets (formerly called the islets of Langerhans). The islets are separated by exocrine cells but each clump acts as an organ within an organ. The islets produce insulin, glucagon, and small amounts of other hormones. High blood glucose levels stimulate the release of insulin from the beta cells of the islets. Insulin targets nearly all body cells and increases their ability to import glucose where it can be oxidized or converted to glycogen or fat. Insulin also increases metabolism of glucose inside the cell. It is therefore a hypoglycemic hormone and is absolutely necessary for proper body function.

23 Pancreatic Islets Insulin Glucagon
Allows glucose to cross plasma membranes into cells from beta cells (hypoglycemic hormone) Glucagon Allows glucose to enter the blood from alpha cells (hyperglycemic hormone) Figure 9.13

24 Pancreatic Hormones and Blood Sugar
Blood levels of glucose normally range from mg/100ml of blood and can soar as high as 600 mg/100 ml. Such a hyperglycemic condition is seen in diabetes mellitus when glucose spills into the urine, flushes from the body, and is accompanied by water and consequently dehydration. Fats and proteins are broken down instead of glucose and body weight drops. Loss of protein leads to a depressed immune system, the blood becomes acidic from ketones (ketosis) and coma and death result. Diabetes is diagnosed from polyuria (excessive urination), polydipsia (excessive thirst), and polyphagia (hunger). Adult-onset or Type II diabetes results from the inability of insulin receptors to respond (insulin resistance) and can be treated with special diets and sometimes extra insulin. In Type I (juvenile) diabetes, insulin is infused continuously via a pump. When blood glucose levels fall, insulin is no longer secreted due to negative feedback control. Glucagon, released by alpha cells of the islets, appears when blood glucose is low. It has hyperglycemic effects (as do glucocorticoids and epinephrine). It acts as an antagonist of insulin and targets the liver to break down stored glycogen to glucose and release it into the blood. No hypo or hypersecretory conditions are known for glucagon. Figure 9.14

25 Pineal Gland Found on the third ventricle of the brain
Secretes melatonin Helps establish the body’s wake and sleep cycles May have other as-yet-unsubstantiated functions Pineal Gland The pineal body or pineal gland is a small, cone-shaped gland in the roof of the third brain ventricle. Only the hormone melatonin appears to be secreted in substantial amounts from this gland. Melatonin levels rise and fall through the course of the day, peaking at night and making us drowsy. Melatonin is therefore an important regulator of our biorhythms (day-night cycle) and presumably coordinates the hormones of fertility and prevents sexual maturation before proper body size has been obtained. In other animals, like white-crowned sparrows, the pineal gland is responsible for the enlargement of the testes and singing behavior in males in springtime. It has been proposed that the pineal gland accepts sensory input from the eyes and tracks day length and therefore season.

26 Hormones of the Ovaries
Estrogens Produced by Graafian follicles or the placenta Stimulates the development of secondary female characteristics Matures female reproductive organs Helps prepare the uterus to receive a fertilized egg Helps maintain pregnancy Prepares the breasts to produce milk Gonads Male and female gonads produce sex hormones identical to those produced by adrenal cortex cells but different obvious with regard to source and in relative amounts. The ovaries are paired, almond-sized organs that produce female ova and two groups of steroid hormones: estrogens and progesterone. The ovaries don't begin to function until puberty when the anterior pituitary stimulates them through gonadotropic hormone release. Rhythmic ovarian cycles then begin as blood levels of ovarian hormones rise and fall. Estrogens, primarily estrone and estradiol produced by the Graafian folicles of the ovaries stimulate development of the secondary sex characteristics in females (maturation of infant ovaries and hair in pubic and axillary regions). Together with progesterone, estrogens prepare the uterus for a fertilized egg in the menstrual cycle. Estrogens also maintain pregnancy and prepare the breasts for milk production. Progesterone acts with estrogen to facilitate the menstrual cycle: the growth and sloughing of the endometrial layer. Progesterone quiets the muscles of the uterus during pregnancy and is produced by the corpus luteum, after ovulation, in large amounts. Both estrogen and progesterone are released in a cyclic way when stimulated by the cyclic release of anterior pituitary gonadotropic hormones (FSH and LH). More on this will be discussed with the reproductive system.

27 Hormones of the Ovaries
Progesterone Produced by the corpus luteum Acts with estrogen to bring about the menstrual cycle Helps in the implantation of an embryo in the uterus

28 Hormones of the Testes Interstitial cells of testes are hormone-producing Produce several androgens Testosterone is the most important androgen Responsible for adult male secondary sex characteristics Promotes growth and maturation of male reproductive system Required for sperm cell production The testes suspended in the scrotal sac produces sperm but also produces male sex hormones (androgens), mostly testosterone. Testosterone is produced by the interstitial cells and promotes development of the adult male sex characteristics. It also promotes growth and maturation of the male reproductive organs and is necessary for continuous production of sperm. Hyposecretion of testosterone renders a man sterile and can be treated by injections. Testosterone production is stimulated by luteinizing hormone.

29 Other Hormone-Producing Tissues and Organs
Parts of the small intestine Parts of the stomach Thymus Kidneys Heart Many other areas have scattered endocrine cells Other Hormone-Producing Organs and Tissues Pockets of hormone-producing cells are found in fatty tissue and in the wall of the small intestine, stomach, kidneys, and heart. These organs are not considered endocrine organs proper but nevertheless have some endocrine function. Certain tumor cells, especially lung and pancreatic cancers produce hormones identical to those made in normal endocrine glands but in an excessive and uncontrolled fashion. The thymus is located in the upper thorax, posterior to the sternum and frequently just above the heart. It is large in infants and children but decreases in size through adulthood since it is involved in "educating" white blood cells (specifically T lymphocytes), killing off cells that attack our own cells. The thymus is an endocrine gland because it produces thymosin. We will return to the thymus when we discuss the immune system. The placenta is a temporary organ formed during pregnancy and produces hormones normally associated with the ovaries. In addition to its roles as the respiratory, excretory, and nutrient-delivery system for the fetus, it produces estrogen, progesterone, hCG, hPL, and relaxin. Very early in pregnancy, human chorionic gonadotropin (hCG) is produced by the developing embryo and then by the fetal part of the placenta. Similar to LH, hCG stimulates the corpus luteum of the ovary to continue producing estrogen and progesterone to maintaining the endometrial lining (countering mensus). The home pregnancy tests sold over the counter sense this hormone in the urine. In the third month of pregnancy, the placenta takes over the job of making estrogen and progesterone with the corpus luteum degenerates. High estrogen and progesterone blood levels maintaining the endometrium and preparing the breast for lactation. Human placental lactogen (hPL) is produced the placenta to work with the estrogen and progesterone to prepare the breasts. Relaxin is also made to make the mother's pelvic ligaments and pubic symphysis more flexible for the birth.

30 Developmental Aspects of the Endocrine System
Most endocrine organs operate smoothly until old age Menopause is brought about by lack of efficiency of the ovaries Problems associated with reduced estrogen are common Growth hormone production declines with age Many endocrine glands decrease output with age Developmental Aspects The endocrine glands mature at different times in development. The pituitary is derived from the epithelium of the oral cavity and a neural tissue projection of the hypothalamus. The pineal body is entirely neural tissue. Most strictly epithelial glands develop as outpockings of the digestive tract, including the thyroid, thymus, and pancreas. The gonads, adrenals, and parathyroids arise from more complex combinations. Most endocrine organs operate smoothly until old age. In late middle age, the efficiency of the ovaries declines, causing menopause. In this period, the female reproductive organs atrophy causing problems associated with estrogen deficiency like arteriosclerosis, osteoporosis decreases skin elasticity, and promotes hot flashes of the sympathetic nervous system. Depression is common. Men do not experience these effects. In old age growth hormone output declines and the elderly cannot resist stress and infection as well. Older people are often mildly hypothyroid and have a decline in insulin production.

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