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Chemical Control of the Animal Body: The Endocrine System Chapter 37 How Do Cells Communicate? • Direct communication from the inside of one cell to the next occurs through gap junctions – Ions and small molecules flow from cell to cell – Example: cardiac muscle cells – More commonly, cells release “messenger molecules” that stimulate cells if the molecules bind to receptor proteins in or on the cell • Different communication systems use different messenger molecules How Do Cells Communicate? • Paracrine communication occurs between nearby cells using local hormones that diffuse through extracellular fluid – Example: prostaglandins • Endocrine communication occurs between distant cells using hormones that are carried by the bloodstream – Example: insulin How Do Cells Communicate? • Synaptic communication occurs between neurons and their targets using neurotransmitters – Neurotransmitters are released across a small gap (synaptic cleft) between cells – Example: acetylcholine Local Hormones • Produced by most cells • Example: prostaglandins – Are modified fatty acids used for local communication • Many functions of prostaglandins, including – – – – Umbilical cord arterial constriction Stimulate uterine contraction Contribute to inflammation Stimulate pain receptors Endocrine Hormones • Endocrine hormones are released in response to stimuli inside or outside the body • Released by glands that make up the endocrine system • Travel through the circulatory system to reach target cells Endocrine Hormones • There are three classes of vertebrate endocrine hormones – Amino acid-based hormones: synthesized from single amino acids – Peptide hormones: made from chains of amino acids – Steroid hormones: made from cholesterol Target Cell Receptors • A cell will only be affected by a hormone if it has receptors specific to that hormone – If not, no effect • Different classes of hormones bind to receptors in different ways – Due to differences in chemical composition that affect their ability to penetrate the cell membrane Water-Soluble Hormones • Most peptide and amino-acid based hormones are water-soluble • Water-soluble hormones use cell membrane receptors – Cannot penetrate cell membrane • The hormone (first messenger) binds to cell membrane receptors – Receptor binding activates the formation of a second messenger (example: cAMP) in the cell – The second messenger activates enzymes that produce cell effects Lipid-Soluble Hormones • Steroid hormones are lipid soluble – Can easily penetrate cell membrane • Lipid-soluble hormones enter the cell and binds to receptors in the cytoplasm or nucleus – Forms hormone-receptor complex – The hormone-receptor complex binds to a gene on DNA – Protein synthesis is initiated for that specific gene Regulation of Hormone Release • Hormones can have powerful effects, thus secretion must be controlled • Most hormones are controlled by negative feedback, which inhibits further release – Example: antidiuretic hormone (ADH) secretion, which reduces water loss by the kidneys, declines when the body is hydrated Regulation of Hormone Release • In a few cases, positive feedback is used to amplify hormone release – Example: oxytocin secretion, which causes uterine contractions during childbirth, increases with subsequent contractions Invertebrate Hormones • Invertebrates have both peptide and steroid hormones • Many signaling mechanisms are similar to those of vertebrates – Example: estrogen and testosterone regulate sexual differentiation in snails Invertebrate Hormones • An example of hormonal regulation in insects includes the process of molting • Molting is the process in which the exoskeleton is shed to accommodate growth • Molting is regulated by the steroid hormone ecdysone The Mammalian Endocrine System • There are many animal hormones, with new ones being discovered every year • The major functions of the hormones of the vertebrate endocrine system are well established – Summarized in Table 37-3, p. 747 Exocrine and Endocrine Glands • Mammals have exocrine and endocrine glands • Exocrine glands: produce secretions that are released outside the body, or into body tubes and cavities through a duct – Examples: sweat, mammary, pancreas, sebaceous, and lacrimal glands Exocrine and Endocrine Glands • Endocrine glands: are ductless and secrete their hormones within the body • Secretions are released into interstitial spaces, enter capillaries, and are transported through the circulatory system to distant sites The Hypothalamic/Pituitary Axis • The hypothalamus controls the secretions of the pituitary gland • Both of these structures coordinate the actions of many key hormonal messaging systems • The hypothalamus of the brain contains clusters of neurosecretory cells – Release peptide hormones when stimulated The Hypothalamic/Pituitary Axis • The pituitary gland lies below the hypothalamus, connected by a stalk – Releases hormones under control of the hypothalamus • The pituitary gland has two parts – Anterior pituitary: true endocrine gland – Posterior pituitary: an outgrowth of the hypothalamus The Hypothalamic/Pituitary Axis • Hypothalamic hormones control the anterior pituitary gland • Releasing hormones – Stimulate secretions of specific hormones from the anterior pituitary • Inhibiting hormones – Inhibit secretions of specific hormones from the anterior pituitary • Hypothalamic hormones are delivered to the anterior lobe via the blood Anterior Pituitary Hormones • The anterior lobe produces peptide hormones • Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) – Are involved in the production of gametes and sex hormones Anterior Pituitary Hormones • Thyroid-stimulating hormone (TSH) – Stimulates the thyroid to release its hormones • Adrenocorticotropic hormone (ACTH) – Causes the adrenal cortex to release cortisol Anterior Pituitary Hormones • Prolactin – Helps stimulate mammary gland development during pregnancy • Melanocyte-stimulating hormone (MSH) – Stimulates the synthesis of skin pigment Anterior Pituitary Hormones • Growth hormone – Regulates body growth – Dwarfism results from too little growth hormone during development – Gigantism results from too much growth hormone during development Posterior Pituitary Hormones • The posterior pituitary contains hypothalamic neurosecretory cell endings • Neurosecretory cell bodies are in the hypothalamus – Hormones are made in the cell bodies and transported to cell endings in the posterior pituitary Posterior Pituitary Hormones • The posterior pituitary makes two peptide hormones • Antidiuretic hormone (ADH) – Helps prevent dehydration by increasing water permeability of nephron collecting ducts • Oxytocin – Involved in breastfeefing (“milk letdown reflex”) and childbirth, as well as some behaviors The Thyroid Gland • The thyroid gland lies at the front of the neck just below the larynx • The thyroid gland secretes two major hormones – Thyroxine – Calcitonin Thyroxine • • • • An iodine-containing hormone Regulates metabolism of all body cells Regulates development of nervous system Under-secretion during development can cause cretinism – Retardation of mental and physical development • Over-secretion during development can speed up development • Thyroxine secretion is controlled by negative feedback The Thyroid Gland • Iodine deficiencies cause an enlargement of the thyroid gland called a goiter – Iodine deficiencies reduce thyroxine production – The number of thyroxine-producing cells increase to compensate, enlarging the thyroid gland Calcitonin • Inhibits the release of calcium from the bones – Results in decreased blood calcium concentrations The Parathyroid Glands • Four disk-like parathyroid glands are embedded in the back of the thyroid gland • Secrete parathyroid hormone – Works in opposition to calcitonin to maintain calcium homeostasis – Stimulates the release of calcium from the bones, increasing calcium concentrations in the blood The Parathyroid Glands • Secrete parathyroid hormone – Causes the kidneys to reabsorb more calcium as urine is formed – Increased blood calcium levels then inhibit parathyroid hormone release in a negative feedback loop The Pancreas • The pancreas is an endocrine and exocrine gland • The exocrine portion – Synthesizes digestive secretions that are released into the small intestine • The endocrine portion – Consists of islet cell clusters that produce the hormones insulin and glucagon Insulin and Glucagon • Work in opposition to each other to regulate carbohydrate and fat metabolism • Insulin – Secreted when blood glucose levels are high – Promotes uptake of glucose by cells • Cells convert it to fat or glycogen (by liver) – End result: decreases blood glucose levels Insulin and Glucagon • Diabetes mellitus – Occurs when insulin cannot be secreted or if target cells are unresponsive – Causes circulatory problems that result in heart attacks, blindness, and kidney failure – Human insulin (synthesized by bacteria) is administered when a patient cannot produce their own Insulin and Glucagon • Glucagon – Secreted when blood glucose levels are low – Causes breakdown of fat, and glycogen into glucose, which can be used for energy – End result: increases blood glucose levels The Sex Organs Secrete Steroids • In females, ovaries secrete two types of steroid hormones – Estrogen: stimulates egg cell maturation and the development of secondary sexual characteristics – Progesterone: prepares the reproductive tract to receive and nourish the fertilized egg The Sex Organs Secrete Steroids • In males, testes produce steroid androgens – Testosterone is the most important androgen • Stimulates sperm cell production • Stimulates the development of secondary sexual characteristics during puberty The Adrenal Glands • Atop each kidney sits an adrenal gland • The adrenal glands have two parts that secrete different hormones The Adrenal Glands • The adrenal medulla: located at the center and secretes two amino acidderived hormones – Epinephrine and norepinephrine: released in response to emergency situations to prepare the body for action The Adrenal Glands • The adrenal cortex: the outer part of the gland; produces three types of steroid hormones – Glucocorticoids – Aldosterone – Testosterone The Adrenal Glands • Glucocorticoids (such as cortisol) – Release is stimulated by ACTH in response to stress – Increase blood glucose levels and suppress the immune system • Aldosterone – Secreted when blood sodium levels are too low – Causes the kidneys and sweat glands to retain sodium – Enables dietary sodium to raise blood sodium levels The Adrenal Glands • Testosterone – A small amount is produced by both males and females Other Sources of Hormones • The pineal gland – Located in between the two hemispheres of the brain above the hypothalamus – Secretes melatonin in a daily rhythm • May play a role in regulating seasonal reproductive cycles in some mammals • May influence sleep-wake cycles Other Sources of Hormones • The thymus gland – Located in the chest cavity behind the breastbone – Secretes thymosin: regulates the development of T cells of immunity Other Sources of Hormones • The kidneys: produce two hormones – Erythropoietin: increases red blood cell production – Renin: catalyzes the production of angiotensin, which constricts blood vessels and raises blood pressure Other Sources of Hormones • The heart – Secretes atrial natriuretic peptide (ANP) when blood volume is too high • Reduced blood volume by decreasing ADH and aldosterone release Other Sources of Hormones • The stomach and small intestine produce a variety of peptide hormones that help regulate digestion – Gastrin – Secretin – Cholecystokinin Other Sources of Hormones • Fat cells – Release the peptide hormone leptin • Thought to be involved in regulation of body weight and a variety of other possible functions • Mice missing the gene for leptin become obese