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17 CHAPTER The Endocrine System The Endocrine System: An Overview A system of ductless glands Secrete messenger molecules called hormones Hormones travel to distant body y cells and signal g characteristic physiological responses Interacts closely with the nervous system Controls and integrates the functions of other organ systems in the body, slow process Maintains homeostasis, controls growth, metabolism, stress defenses, blood chemistry, etc. Endocrine Organs Scattered throughout the body and small Pure endocrine organs Endocrine organs that belong to other organ systems but contain endocrine cells Pituitary, y, pineal, p , thyroid, y , parathyroid, p y , and adrenal glands Pancreas, thymus, gonads, and the hypothalamus Richly vascularized with blood and lymph vessels Mostly epithelial origin 1 Location of the Major Endocrine Glands Pineal gland Hypothalamus Pituitary gland Thyroid gland Parathyroid glands (on dorsal aspect of thyroid gland) Thymus Adrenal glands Pancreas Ovary (female) Testis (male) Figure 17.1 Hormones Classes of hormones Amino acid-based hormones – protein based Steroids- lipid based derived from cholesterol Basic hormone action Circulate throughout the body in blood vessels Influences only specific tissues – target cells that have receptor for hormone A hormone can have different effects on different target cells Effects depend on the preprogrammed response of the target cells - hormones are merely molecular “triggers” Control of Hormones Secretion Secretion triggered by three major types of stimuli. Humoral – simplest of endocrine control mechanisms Neural – secretion of endocrine glands controlled by neural stimuli Secretion in direct response to changing ion or nutrient levels in the blood Example: sympathetic nerve fibers stimulate cells in the adrenal medulla. Induces release of epinephrine and norepinephrine Hormonal – stimuli received from other glands Certain hormones signal secretion of other hormones 2 Types of Endocrine Gland Stimuli (a) Humoral stimulus 1 Capillary blood contains low concentration of Ca2+, which stimulates… (b) Neural stimulus 1 Preganglionic sympathetic fibers stimulate adrenal medulla cells… (c) Hormonal stimulus 1 The hypothalamus secretes hormones that… Hypothalamus CNS (spinal cord) Capillary (low Ca2+ in blood) 2 …stimulate the anterior pituitary gland to secrete hormones that… Thyroid gland (posterior view) Parathyroid glands Pituitary gland Preganglionic sympathetic fibers Thyroid gland Adrenal cortex Gonad (Testis) Medulla of adrenal gland Parathyroid glands PTH Capillary 2 …secretion of parathyroid hormone 2 …to secrete catecholamines (PTH) by parathyroid glands. PTH acts to (epinephrine and norepinephrine) increase blood Ca2+. 3 …stimulate other endocrine glands to secrete hormones Figure 17.2 Control of Hormones Secretion Always controlled by feedback loops Blood concentration declines below a minimum Blood concentration exceeds maximum More hormone is secreted Hormone production is halted Major Endocrine Organs Pituitary Thyroid Parathyroid Adrenal (suprarenal) Pineal- not discussed Pancreas Thymus-T-cell maturation (not discussed) gonads 3 The Pituitary Gland (Hypophysis) Secretes nine major hormones Found in hypophyseal fossa, depression found in sella turcica of sphenoid bone Attached to the hypothalamus by the infundibulum Two basic divisions of the pituitary gland Adenohypophysis (anterior) Neurohypophysis (posterior) The Pituitary Gland Corpus callosum Thalamus Pineal Hypothalamus Mammillary body Brain stem (b) Acidophil Basophil (a) Optic chiasma Median eminence of hypothalamus Anterior lobe Pars tuberalis Pars intermedia Pars distalis Pituitary (hypophysis) Chromophobe cell Mammillary body Tuber cinereum Posterior lobe Infundibulum Pars nervosa Capillary with red blood cells (d) (c) Spherical cluster of cells Figure 17.3 The Anterior Lobe Has five different types of endocrine cells to make the following hormones: 1. Growth Hormone (GH) (a.k.a. somatotropic hormone/ somatotropin) 2. Prolactin 3. Thyroid stimulating hormone (TSH) 4. Adrenocorticotropic hormone (ACTH) 5. Melanocyte-stimulating hormone (MSH) 6. Follicle-stimulating hormone (FSH) 7. Luteinizing Hormone (LH) 4 The Anterior Lobe Hypothalamic neuron cell bodies Superior hypophyseal artery Hypophyseal portal system Primary capillary plexus Hypophyseal portal veins Secondary capillary plexus Anterior lobe of pituitary Hypothalamus TSH, FSH, LH, ACTH, GH, PRL 1 When appropriately stimulated, hypothalamic neurons secrete releasing and inhibiting hormones into the primary capillary plexus. 2 Hypothalamic hormones travel through the portal veins to the anterior pituitary where they stimulate or inhibit release of hormones from the anterior lobe. 3 Anterior pituitary hormones are secreted into the secondary capillary plexus. (a) Relationship between the anterior pituitary and the hypothalamus Figure 17.4a Hypothalamic Control of Hormone Secretion from the Anterior Lobe The hypothalamus Controls secretion of anterior lobe hormones Exerts control by secreting Releasing hormones—prompt anterior lobe to release hormones Inhibiting hormones—turn off secretion of anterior lobe hormones Table 17.1 5 Table 17.1 The Posterior Lobe Structurally part of the brain Does not make hormones, merely stores and produced byy hypothalamus yp releases hormones p Secretes two hormones Antidiuretic hormone (ADH; a.k.a. vasopressin)targets kidneys and arterioles to regulate fluid volume and blood pressure Oxytocin- uterine contraction and milk secretion Relationship Between the Posterior Pituitary and Hypothalamus 1 Hypothalamic neurons synthesize oxytocin and ADH. Paraventricular nucleus Hypothalamus Supraoptic nucleus Optic chiasma Infundibulum (connecting stalk) Hypothalamichypophyseal tract Inferior hypophyseal artery Axon terminals Posterior lobe of pituitary Oxytocin ADH 2 Oxytocin O t i and d ADH are transported along the hypothalamichypophyseal tract to the posterior lobe. 3 Oxytocin and ADH are stored in axon terminals in the posterior pituitary. 4 Oxytocin and ADH are released into the blood when hypothalamic neurons fire. (b) Relationship between the posterior pituitary and the hypothalamus Figure 17.4b 6 Table 17.1 The Thyroid Gland Located in the anterior neck-inferior to larynx Two lateral lobes connected by isthmus Largest pure endocrine gland Composed of follicular and parafollicular cells. Produces two hormones Thyroid hormone (TH)- controls metabolic rate Calcitonin- depresses blood calcium levels (via osteoclasts and kidney function). Hyoid bone The Thyroid Gland Thyroid cartilage Common carotid artery Inferior thyroid artery Right subclavian artery Trachea Epiglottis External carotid artery Superior thyroid artery Isthmus of thyroid y gland g Left subclavian artery Left lateral lobe of thyroid gland Aorta (a) Gross anatomy of the thyroid gland, anterior view Figure 17.5a 7 The Thyroid Gland Figure 17.5b Disorders of Thyroid Hypothyroidism-person always cold, gains weight but doesn’t feel hungry. Hyperthyroidism- Person is “hot” hot , loses weight, very hungry, anxious. Must ablate thyroid (radioactive iodine). Goiter Figure 17.10 8 The Parathyroid Gland Lie on the posterior surface of the thyroid gland Could be embedded in thyroid and numbers and, sometimes so et es location, ocat o , vary ay Contain two types of endocrine cells Chief cells – produce parathyroid hormone (PTH) Oxyphil cells – function unknown Increases blood concentration of Ca2+ The Parathyroid Glands Pharynx (posterior aspect) Thyroid gland Esophagus Trachea Parathyroid glands Parathyroid cells (secrete parathyroid hormone) Oxyphil cells Capillary (a) Location of parathyroid glands, posterior view (b) Photomicrograph of parathyroid gland tissue (360×) Figure 17.6a, b The Pancreas Located in the posterior abdominal wall Contains both endocrine and exocrine cells Exocrine E i cells ll – acinar i cells ll – secrete t di digestive ti enzymes into small intestine Endocrine cells – pancreatic islets (islets of Langerhans) About one million islets – scattered throughout the pancreas 9 The Pancreas Main endocrine cell types Alpha cells (A cells) – secrete glucagon Signals g liver to release glucose g from glycogen gy g Raises blood sugar Beta cells (B cells) – secrete insulin Signals most body cells to take up glucose from the blood Lowers blood sugar Figure 17.9 Diabetes Mellitus Caused by: Insufficient secretion of insulin Resistance of bodyy cells to the effects of insulin Type I diabetes – develops suddenly, usually before age 15 T cell-mediated autoimmune response destroys beta cells 10 Diabetes Mellitus Type II diabetes – adult onset Usually occurs after age 40 Cells have lowered sensitivity to insulin Controlled by dietary changes and regular exercise The Adrenal Glands Located on the superior surface of the kidneys Nerve supply is almost exclusively sympathetic fibers Two endocrine glands in one Adrenal medulla – a knot of nervous tissue Adrenal cortex – bulk of the adrenal gland Cortex is composed of three layers Zona glomerulosa Zona fasciculata Zona reticularis The Adrenal Medulla Chromaffin cells – modified postganglionic sympathetic neurons Secrete catecholamines (amino acidsacids norepinephrine and epinephrine). Active in “fight, flight, and fright” response 11 The Adrenal Cortex Secretes a variety of hormones- all are steroids and are grouped into two main classes: Mineralocorticoids Aldosterone is secreted by the zona glomerulosa Glucocorticoids Cortisol – secreted by zona fasciculata and zona reticularis Helps the body deal with stressful situations (Androgenic) Dehydroepiandrosterone (DHEA)- converted to testosterone and estrogen. May help with stress or elevating immune system, mood modifier. The Adrenal Gland–Gross and Microscopic Hormones secreted Capsule Zona glomerulosa Aldosterone Medulla Cortex Zona fasciculata Adrenal gland Cortisol and a d androgens Cortex Zona reticularis Medulla Kidney Adrenal medulla (a)Drawing of the histology of the adrenal cortex and a portion of the adrenal medulla Epinephrine and norepinephrine (b) Photomicrograph (140X) Figure 17.7a, b Stress and the Adrenal Gland Short-term stress More prolonged stress Stress Nerve impulses Hypothalamus CRH (corticotropinreleasing hormone) Spinal cord Corticotroph cells of anterior pituitary Preganglionic sympathetic fibers To target in blood Adrenal medulla (secretes amino acid– based hormones) Catecholamines (epinephrine and norepinephrine) Short-term stress response 1.Increased heart rate 2.Increased blood pressure 3.Liver converts glycogen to glucose and releases glucose to blood 4.Dilation of bronchioles 5.Changes in blood flow patterns leading to decreased digestive system activity and reduced urine output 6.Increased metabolic rate Adrenal cortex (secretes steroid hormones) ACTH Mineralocorticoids Glucocorticoids Long-term stress response 1.Retention of sodium and water by kidneys 2.Increased blood volume and blood pressure 1.Proteins and fats converted to glucose or broken down for energy 2.Increased blood glucose 3.Suppression of immune system Figure 17.8 12 The Gonads Main sources of sex hormones – testes and ovaries Male l – interstitiall cells ll secrete androgens d Primarily testosterone Promotes the formation of sperm Maintains secondary sex characteristics The Gonads Female – ovaries Androgens secreted by the theca folliculi, Converted to estrogen by follicular granulosa cells Estrogen – maintains secondary sex characteristics and signals uterine mucosa to repair itself after menstrual period Also secrete progesterone – produced by the follicular granulosa cells and the corpus luteum and prepares the uterus for pregnancy Corpus luteum also secretes estrogen and progesterone. Other Endocrine Structures Endocrine cells occur within: The GI tract – enteroendocrine cells, scattered within the epithelial lining of the alimentary canal. Assist with digestion blood chemistry digestion, chemistry, etc etc. The placenta – sustains the fetus and secretes several steroid and protein hormones (Human chorionic gonadotropin- HCG, pregnancy tests) The kidneys – cells of the juxtaglomerular apparatus secrete renin (signals adrenal cortex to release aldosterone) 13