Introduction to the Endocrine System

Transcription

1 1 About This Chapter 2 Chapter 7a Hormones Introduction to the Endocrine System The classification of hormones Control of hormone release Hormone interactions Endocrine pathologies Hormone evolution Hormones: Function 3 Figure 7.1 An endocrine disorder in ancient art 4 Control Rates of enzymatic reactions Transport of ions or molecules across cell membranes Gene expression and protein synthesis

2 Figure ANATOMY SUMMARY Hormones 5 Figure ANATOMY SUMMARY Hormones 6 Location Hormone Primary Target(s) Main Effect(s) Location Hormone Primary Target(s) Main Effect(s) Pineal gland Melatonin [A] Brain, other tissues Circadian rhythms; immune function; antioxidant Thyroid gland Triiodothyronine and thyroxine [A] Metabolism, growth, and development Hypothalamus (N) Trophic hormones [P] (see Fig. 7.8) Anterior pituitary Release or inhibit pituitary hormones Calcitonin [P] Bone Plasma calcium levels (minimal effect in humans) Posterior pituitary (N) Oxytocin [P] Vasopressin (ADH) [P] Breast and uterus Kidney Milk ejection; labor and delivery; behavior Water reabsorption Parathyroid gland Thymus gland Parathyroid hormone [P] Thymosin, thymopoietin [P] Bone, kidney Lymphocytes Regulates plasma Ca 2+ and phosphate levels Lymphocyte development Anterior pituitary (G) Prolactin [P] Growth hormone (somatotropin) [P] Corticotropin (ACTH) [P] Thyrotropin (TSH) [P] Follicle-stimulating hormone [P] Luteinizing hormone [P] Breast Liver Adrenal cortex Thyroid gland Gonads Gonads Milk production Growth factor secretion Growth and metabolism Cortisol release Thyroid hormone synthesis Egg or sperm production; sex hormone production Sex hormone production; egg or sperm production Heart (C) Liver (C) Stomach and small intestine (C) Pancreas (G) Atrial natriuretic peptide [P] Angiotensinogen [P] Insulin-like growth factors [P] Gastrin, cholecystokinin, secretin, and others [P] Insulin, glucagon, somatostatin, pancreatic polypeptide [P] Kidneys Adrenal cortex, blood vessels GI tract and pancreas Increases Na + excretion Aldosterone secretion; increases blood pressure Growth Assist digestion and absorption of nutrients Metabolism of glucose and other nutrients KEY G = gland C = endocrine cells N = neurons KEY G = gland C = endocrine cells N = neurons P = peptide S = steroid A = amino acid derived P = peptide S = steroid A = amino acid derived Figure ANATOMY SUMMARY Hormones 7 Figure ANATOMY SUMMARY Hormones 8 Location Hormone Primary Target(s) Main Effect(s) Location Hormone Primary Target(s) Main Effect(s) Testes (male) (G) Androgens [S] Sperm production, secondary sex characteristics Adrenal cortex (G) Adrenal medulla (N) Kidney (C) Skin (C) Aldosterone [S] Cortisol [S] Androgens [S] Epinephrine, norepinephrine [A] Erythropoietin [P] 1,25 Dihydroxy-vitamin D 3 (calciferol) [S] Vitamin D 3 [S] Kidney Bone marrow Intestine Intermediate form of hormone Na + and K + homeostasis Stress response Sex drive in females Fight-or-flight response Red blood cell production Increases calcium absorption Precursor of 1,25 dihydroxy-vitamin D 3 Ovaries (female) (G) Adipose tissue (C) Placenta (pregnant females only) (C) Inhibin [P] Estrogen, progesterone [S] Inhibin [P] Relaxin (pregnancy) [P] Leptin, adiponectin, resistin Estrogen, progesterone [S] Chorionic somatomammotropin [P] Anterior pituitary Anterior pituitary Uterine muscle Hypothalamus, other tissues Inhibits FSH secretion Egg production, secondary sex characteristics Inhibits FSH secretion Relaxes muscle Food intake, metabolism, reproduction Fetal, maternal development Metabolism Chorionic gonadotropin [P] Corpus luteum Hormone secretion KEY G = gland C = endocrine cells N = neurons P = peptide S = steroid A = amino acid derived KEY G = gland C = endocrine cells N = neurons P = peptide S = steroid A = amino acid derived

3 Figure ANATOMY SUMMARY Hormones 9 Hormones 10 Cell-to-cell communication molecules Chemical signals Secreted by a cell or group of cells Transported by blood Distant target tissue s Activates physiological response at low concentrations Pheromones: elicit physiological or behavioral response on other organisms of the same species Hormones Cellular mechanism of action Depends on binding to target cell s Initiates biochemical responses Half-life indicates length of activity 11 Hormones: Classification by Chemical Class Peptide or protein hormones Steroid hormones Amino acid derived or amine hormones 12

4 Table 7.1 Comparison of Peptide, Steroid, and Amino Acid-Derived Hormones 13 Hormones: Peptides or Proteins 14 Preprohormone Large, inactive precursor Prohormone Smaller, inactive Proteolytic, post-translational modification Peptide/protein hormones Bind surface membrane s Cellular response through signal transduction system Figure 7.3 ESSENTIALS Peptide Hormone Synthesis and Processing 15 Figure 7.3 ESSENTIALS Peptide Hormone Synthesis and Processing 16 Slide 1 mrna Preprohormone Ribosome Endoplasmic reticulum (ER) Messenger RNA on the ribosomes binds amino acids into a peptide chain called a preprohormone. The chain is directed into the ER lumen by a signal sequence of amino acids. Cytoplasm ECF Plasma Capillary endothelium

5 Figure 7.3 ESSENTIALS Peptide Hormone Synthesis and Processing Slide 17 Figure 7.3 ESSENTIALS Peptide Hormone Synthesis and Processing 2 18 Slide 3 mrna Preprohormone Ribosome Endoplasmic reticulum (ER) Messenger RNA on the ribosomes binds amino acids into a peptide chain called a preprohormone. The chain is directed into the ER lumen by a signal sequence of amino acids. mrna Preprohormone Ribosome Endoplasmic reticulum (ER) Messenger RNA on the ribosomes binds amino acids into a peptide chain called a preprohormone. The chain is directed into the ER lumen by a signal sequence of amino acids. Signal sequence Prohormone Enzymes in the ER chop off the signal sequence, creating an inactive prohormone. Signal sequence Prohormone Transport vesicle Enzymes in the ER chop off the signal sequence, creating an inactive prohormone. The prohormone passes from the ER through the Golgi complex. Golgi complex Cytoplasm Cytoplasm ECF ECF Plasma Capillary endothelium Plasma Capillary endothelium Figure 7.3 ESSENTIALS Peptide Hormone Synthesis and Processing Slide 19 Figure 7.3 ESSENTIALS Peptide Hormone Synthesis and Processing 4 20 Slide 5 mrna Preprohormone Ribosome Endoplasmic reticulum (ER) Messenger RNA on the ribosomes binds amino acids into a peptide chain called a preprohormone. The chain is directed into the ER lumen by a signal sequence of amino acids. mrna Preprohormone Ribosome Endoplasmic reticulum (ER) Messenger RNA on the ribosomes binds amino acids into a peptide chain called a preprohormone. The chain is directed into the ER lumen by a signal sequence of amino acids. Signal sequence Prohormone Transport vesicle Enzymes in the ER chop off the signal sequence, creating an inactive prohormone. Signal sequence Prohormone Transport vesicle Enzymes in the ER chop off the signal sequence, creating an inactive prohormone. The prohormone passes from the ER through the Golgi complex. The prohormone passes from the ER through the Golgi complex. Golgi complex Cytoplasm Secretory vesicle Active hormone Peptide fragment Secretory vesicles containing enzymes and prohormone bud off the Golgi. The enzymes chop the prohormone into one or more active peptides plus additional peptide fragments. Golgi complex Cytoplasm Secretory vesicle Active hormone Peptide fragment Secretory vesicles containing enzymes and prohormone bud off the Golgi. The enzymes chop the prohormone into one or more active peptides plus additional peptide fragments. ECF ECF Release signal The secretory vesicle releases its contents by exocytosis into the extracellular space. Plasma Capillary endothelium Plasma Capillary endothelium

6 Figure 7.3 ESSENTIALS Peptide Hormone Synthesis and Processing mrna Preprohormone Ribosome Endoplasmic reticulum (ER) Messenger RNA on the ribosomes binds amino acids into a peptide chain called a preprohormone. The chain is directed into the ER lumen by a signal sequence of amino acids. Slide 21 Figure 7.3a ESSENTIALS Peptide Hormone Synthesis and Processing 6 Preprohormones PreproTRH (thyrotropin-releasing hormone) has six copies of the 3-amino acid hormone TRH. 22 Signal sequence Prohormone Transport vesicle Enzymes in the ER chop off the signal sequence, creating an inactive prohormone. Preprohormone Golgi complex Cytoplasm Secretory vesicle Active hormone Peptide fragment The prohormone passes from the ER through the Golgi complex. Secretory vesicles containing enzymes and prohormone bud off the Golgi. The enzymes chop the prohormone into one or more active peptides plus additional peptide fragments. PreproTRH (242 amino acids) processes to 6 TRH (3 amino acids each) ECF Release signal The secretory vesicle releases its contents by exocytosis into the extracellular space. Other peptide fragments Plasma Capillary endothelium To target The hormone moves into the circulation for transport to its target. Signal sequence Figure 7.3b ESSENTIALS Peptide Hormone Synthesis and Processing Prohormones Prohormones, such as proopiomelanocortin, the prohormone for ACTH, may contain several peptide sequences with biological activity. 23 Figure 7.3c ESSENTIALS Peptide Hormone Synthesis and Processing Prohormones Process to Active Hormone Plus Peptide Fragments 24 Pro-opiomelanocortin processes to The peptide chain of insulin s prohormone folds back on itself with the help of disulfide (S S) bonds. The prohormone cleaves to insulin and C-peptide. Proinsulin processes to Insulin C-peptide ACTH γ lipotropin β endorphin Peptide fragment

7 Endocrine System Review 25 Figure 7.4 Membrane s and signal transduction for peptide hormones Peptide hormones (H) cannot enter their target cells and must combine with membrane s (R) that initiate signal transduction processes. 26 H H R R Interactive Physiology Animation: Endocrine System: Endocrine System Review G AE TK Opens ion channel Second messenger systems phosphorylate KEY TK = Tyrosine kinase AE = Amplifier enzyme G = G protein Proteins Cellular response Hormones: Steroid 27 Figure 7.5a ESSENTIALS Steroid Hormones 28 Cholesterol-derived Lipophilic and easily cross membranes Bind carrier proteins in blood Longer half-life Cytoplasmic or nuclear s Genomic effect to activate or repress genes for protein synthesis Slower acting Cell membrane s Nongenomic responses Cholesterol is the parent compound for all steroid hormones. DHEA = dehydroepiandrosterone = intermediate compounds whose names have been omitted for simplicity. Cholesterol DHEA Androstenedione Dihydrotestosterone (DHT) aromatase Testosterone aromatase 21-hydroxylase 21-hydroxylase Progesterone Estradiol Estrone Cortisol Ovary Corticosterone Adrenal cortex Aldosterone Each step is catalyzed by an enzyme, but only two enzymes are shown in this figure.

8 Figure 7.5b ESSENTIALS Steroid Hormones 29 Figure 7.5b ESSENTIALS Steroid Hormones 30 Slide 1 Steroid hormones act primarily on intracellular s. Blood vessel Steroid hormone Cell surface Most hydrophobic steroids are bound to plasma protein carriers. Only unbound hormones can diffuse into the target cell. Blood vessel Most hydrophobic steroids are bound to plasma protein carriers. Only unbound hormones can diffuse into the target cell. Rapid responses Protein carrier Interstitial fluid Cell membrane Endoplasmic reticulum Cytoplasmic Nuclear Nucleus DNA Transcription produces mrna Steroid hormone s are in the cytoplasm or nucleus. Some steroid hormones also bind to membrane s that use second messenger systems to create rapid cellular responses. The -hormone complex binds to DNA and activates or represses one or more genes. Activated genes create new mrna that moves back to the cytoplasm. Protein carrier Interstitial fluid Cell membrane Nucleus New proteins Translation Translation produces new proteins for cell processes. Figure 7.5b ESSENTIALS Steroid Hormones Slide 31 Figure 7.5b ESSENTIALS Steroid Hormones 2 32 Slide 3 Blood vessel Steroid hormone Most hydrophobic steroids are bound to plasma protein carriers. Only unbound hormones can diffuse into the target cell. Blood vessel Steroid hormone Cell surface Most hydrophobic steroids are bound to plasma protein carriers. Only unbound hormones can diffuse into the target cell. Rapid responses Protein carrier Nucleus Steroid hormone s are in the cytoplasm or nucleus. Protein carrier Nucleus Steroid hormone s are in the cytoplasm or nucleus. Cytoplasmic Nuclear Cytoplasmic Nuclear Some steroid hormones also bind to membrane s that use second messenger systems to create rapid cellular responses. Interstitial fluid Interstitial fluid Cell membrane Cell membrane

9 Figure 7.5b ESSENTIALS Steroid Hormones Slide 33 Figure 7.5b ESSENTIALS Steroid Hormones 4 34 Slide 5 Blood vessel Steroid hormone Cell surface Most hydrophobic steroids are bound to plasma protein carriers. Only unbound hormones can diffuse into the target cell. Blood vessel Steroid hormone Cell surface Most hydrophobic steroids are bound to plasma protein carriers. Only unbound hormones can diffuse into the target cell. Rapid responses Rapid responses Protein carrier Nucleus Steroid hormone s are in the cytoplasm or nucleus. Protein carrier Nucleus Steroid hormone s are in the cytoplasm or nucleus. Cytoplasmic Nuclear Some steroid hormones also bind to membrane s that use second messenger systems to create rapid cellular responses. Cytoplasmic Nuclear Some steroid hormones also bind to membrane s that use second messenger systems to create rapid cellular responses. DNA DNA Interstitial fluid The -hormone complex binds to DNA and activates or represses one or more genes. Interstitial fluid The -hormone complex binds to DNA and activates or represses one or more genes. Cell membrane Cell membrane Transcription produces mrna Activated genes create new mrna that moves back to the cytoplasm. Translation Figure 7.5b ESSENTIALS Steroid Hormones 35 Slide 6 Hormones: Amino Acid Derived, or Amine 36 Blood vessel Protein carrier Steroid hormone Cytoplasmic Cell surface Rapid responses Nuclear Nucleus Most hydrophobic steroids are bound to plasma protein carriers. Only unbound hormones can diffuse into the target cell. Steroid hormone s are in the cytoplasm or nucleus. Some steroid hormones also bind to membrane s that use second messenger systems to create rapid cellular responses. Derived from one of two amino acids Tryptophan Tyrosine Ring structure DNA Interstitial fluid Cell membrane Endoplasmic reticulum Transcription produces mrna The -hormone complex binds to DNA and activates or represses one or more genes. Activated genes create new mrna that moves back to the cytoplasm. New proteins Translation Translation produces new proteins for cell processes.

10 Amine Hormones: Examples Melatonin Catecholamines Epinephrine Norepinephrine Dopamine Thyroid hormones 37 Figure 7.6 Most amine hormones are derived from the amino acid tyrosine. Catecholamines are made by modifying the side groups of tyrosine. Dopamine Tyrosine is the parent amino acid for catecholamines and thyroid hormones. FIGURE QUESTION Determine how each catecholamine molecule differs from the tyrosine molecule. Thyroid hormones are synthesized from two tyrosines and iodine (I) atoms. Thyroxine (Tetraiodothyronine, T 4 ) 38 Norepinephrine Epinephrine Triiodothyronine (T 3 ) Biochemistry, Secretion, and Transport of Hormones Interactive Physiology Animation: Endocrine System: Biochemistry, Secretion, and Transport of Hormones 39 Endocrine Reflex Pathways Stimulus Sensor Input signal Integration Output (efferent) signal (hormone in blood) Targets Response physiological action Negative feedback 40

11 Figure 7.7a Examples of simple endocrine pathways (1 of 2) A Simple Endocrine Reflex: Parathyroid Hormone 41 Figure 7.7b Examples of simple endocrine pathways (2 of 2) Multiple Pathways for Insulin Secretion 42 Low plasma [Ca 2+ ] Blood glucose Eat a meal Glucose in lumen Stretch in digestive tract Parathyroid cell Sensory neuron Parathyroid hormone CNS Endocrine cells in small intestine Efferent neuron Negative feedback Negative feedback Pancreas Insulin GLP-1 Bone and kidney Bone resorption Kidney reabsorption of calcium Plasma [Ca 2+ ] Production of calcitriol leads to intestinal absorption of Ca 2+ ] KEY Stimulus Receptor Hormone Target Tissue response Sensory neuron Efferent neuron Integrating center Systemic response Target tissues Glucose uptake and utilization Blood glucose KEY Stimulus Receptor Hormone Target Tissue response Sensory neuron Efferent neuron Integrating center Systemic response The Actions of Hormones on Target Cells Chapter 7b Interactive Physiology Animation: Endocrine System: The Actions of Hormones on Target Cells Introduction to the Endocrine System

12 Neurohormones: Major Groups 45 Figure 7.8a ESSENTIALS The Pituitary Gland 46 Adrenal medulla Catecholamines Hypothalamus Posterior pituitary is neural tissue Anterior pituitary is endocrine tissue The pituitary gland sits in a protected pocket of bone, connected to the brain by a thin stalk. HYPOTHALAMUS Infundibulum is the stalk that connects the pituitary to the brain. Sphenoid bone Posterior pituitary is an extension of the neural tissue. ANTERIOR POSTERIOR Anterior pituitary is a true endocrine gland of epithelial origin. Figure 7.8c ESSENTIALS The Pituitary Gland The posterior pituitary is an extension of the brain that secretes neurohormones made in the hypothalamus. HYPOTHALAMUS 47 Figure 7.8c ESSENTIALS The Pituitary Gland HYPOTHALAMUS 48 Slide 1 Neurohormone is made and packaged in cell body of neuron. Vesicles are transported down the cell. Neurohormone is made and packaged in cell body of neuron. Vesicles containing neurohormone are stored in posterior pituitary. POSTERIOR PITUITARY Vein Neurohormones are released into blood. Oxytocin Vasopressin Ile Gln Phe Gln Tyr Asp Tyr Asp Cys Cys Pro Leu Gly Cys Cys Pro Arg Gly POSTERIOR PITUITARY Mammary glands and uterus Kidneys

13 Figure 7.8c ESSENTIALS The Pituitary Gland Slide 49 Figure 7.8c ESSENTIALS The Pituitary Gland 2 50 Slide 3 HYPOTHALAMUS HYPOTHALAMUS Neurohormone is made and packaged in cell body of neuron. Neurohormone is made and packaged in cell body of neuron. Vesicles are transported down the cell. Vesicles are transported down the cell. Vesicles containing neurohormone are stored in posterior pituitary. POSTERIOR PITUITARY POSTERIOR PITUITARY Figure 7.8c ESSENTIALS The Pituitary Gland HYPOTHALAMUS Slide 51 Figure 7.8b ESSENTIALS The Pituitary Gland 4 The anterior pituitary is a true endocrine gland that secretes six classic hormones. Neurohormones from the hypothalamus control release of the anterior pituitary hormones. The hypothalamic hormones reach the anterior pituitary through a specialized region of the circulation called a portal system. 52 Neurohormone is made and packaged in cell body of neuron. Neurons synthesizing trophic neurohormones release them into capillaries of the portal system. HYPOTHALAMUS Capillary bed Vesicles are transported down the cell. Portal vessels carry the trophic neurohormones directly to the anterior pituitary, where they act on the endocrine cells. Artery POSTERIOR PITUITARY Vesicles containing neurohormone are stored in posterior pituitary. Endocrine cells release their peptide hormones into the second set of capillaries for distribution to the rest of the body. Capillary bed ANTERIOR PITUITARY Veins TO TARGET ORGANS POSTERIOR PITUITARY Prolactin GH TSH ACTH Gonadotropins (LH & FSH) Vein Neurohormones are released into blood. Mammary glands Musculoskeletal system Thyroid gland Adrenal cortex Ovary Gonads Testis

14 Figure 7.9 ESSENTIALS Hormones of the Hypothalamic Anterior Pituitary Pathway 53 Endocrine Control 54 A trophic hormone controls the secretion of another hormone Hypothalamic-hypophyseal portal system Three integrating centers Hypothalamic stimulation from CNS Anterior pituitary stimulation from hypothalamic trophic hormones Endocrine gland stimulation from anterior pituitary trophic hormones (except prolactin) Figure 7.10 The Growth Hormone Pathway Hypothalamic growth hormone releasing hormone (GHRH) 55 Figure 7.11a (1 of 2) In complex endocrine pathways, the hormones of the pathway serve as negative feedback signals. 56 stimulates growth hormone (GH) secretion. Growth hormone acts directly on many body tissues but also influences liver production of insulin-like growth factors (IGFs or somatomedins), another group of hormones that regulate growth. Stimulus HYPOTHALAMUS Hypothalamus ANTERIOR PITUITARY GHRH GH cells in anterior pituitary Short-loop negative feedback Hypothalamus (IC 1 ) Trophic hormone (H 1 ) Anterior pituitary (IC 2 ) Trophic hormone (H 2 ) Long-loop negative feedback GH Endocrine gland (IC 3 ) Liver Hormone (H 3 ) IGFs Target tissue T T Bone and soft tissue Growth Response

15 Figure 7.11b (2 of 2) Control Pathway for Cortisol Secretion Cortisol is a steroid hormone secreted by the adrenal cortex. ACTH = corticotropin or adrenocorticotropic hormone; CRH = corticotropin-releasing hormone. 57 Hormone Interactions 58 Hypothalamus Synergism CRH Combined effect is greater than the sum of individual effects To target tissue Anterior pituitary ACTH Adrenal cortex Cortisol Target tissue Long-loop negative feedback Permissiveness Need second hormone to get full effect Antagonism One substance opposes the action of another Competitive inhibitors vs. functional antagonism Glucagons oppose insulin FIGURE QUESTION Draw in the short-loop negative feedback for this pathway. Response Figure 7.12 Synergism 250 Glucagon + Epinephrine + Cortisol 59 Endocrine Pathologies 60 Hypersecretion: excess hormone Blood glucose (mg/dl) Glucagon + Epinephrine Caused by tumors or exogenous iatrogenic treatment Negative feedback Hyposecretion: deficient hormone Caused by decreased synthesis materials or atrophy Absence of negative feedback Epinephrine 100 Glucagon Cortisol Time (hours)

16 Figure 7.13 Exogenous hormone has the same negative feedback effect as endogenous hormone. 61 Pathologies: Abnormal Receptors 62 Exogenous cortisol CRH ACTH Cortisol (Hypothalamus) (Anterior pituitary) (Adrenal cortex) Down-regulation Decreased number of s Hyperinsulinemia Receptor and signal transduction abnormalities Testicular feminization syndrome Pseudohypothyroidism Target tissue Response Figure 7.14 Primary and secondary hypersecretion of cortisol Primary Hypersecretion Due to Problem with Adrenal Cortex Secondary Hypersecretion Due to Pituitary Problem Secondary Hypersecretion Due to Hypothalamic Problem 63 Figure 7.15 Patterns of hormone secretion in hypocortisolism Hyposecretion from Damage to the Pituitary Hyposecretion from Atrophy of the Adrenal Cortex 64 Hypothalamus CRH Hypothalamus CRH HYPERSECRETING TUMOR IN CRH HYPOTHALAMUS Hypothalamus CRH Hypothalamus CRH Anterior pituitary ACTH PATHOLOGY IN ANTERIOR PITUITARY ACTH Anterior pituitary ACTH Anterior pituitary ACTH Anterior pituitary ACTH Adrenal cortex Cortisol Adrenal cortex Cortisol PATHOLOGY IN ADRENAL CORTEX Cortisol Adrenal cortex Cortisol Adrenal cortex Cortisol Negative feedback fails Symptoms of deficiency Symptoms of deficiency Symptoms of excess CRH levels low ACTH levels low Cortisol levels high Symptoms of excess CRH levels low ACTH levels high Cortisol levels high Symptoms of excess CRH levels high ACTH levels high Cortisol levels high FIGURE QUESTION For each condition, use arrows to indicate whether levels of the three hormones in the pathway will be increased, decreased, or unchanged. Draw in negative feedback loops where functional.

17 The Hypothalamic-Pituitary Axis 65 Hormone Evolution 66 Evolutionary conservation of hormone function Proteomics Interactive Physiology Animation: Endocrine System: The Hypothalamic- Pituitary Axis Calcitonin gene-related peptide example Vestigial Melanocyte-stimulating hormone example Comparative endocrinology Pineal gland and melatonin example Figure 7.16 FOCUS ON... The Pineal Gland 67 Figure 7.16 FOCUS ON... The Pineal Gland Corpus callosum Thalamus The pineal gland is a pea-sized structure buried deep in the brain of humans. Nearly 2000 years ago, this seat of the soul was thought to act as a valve that regulated the flow of vital spirits and knowledge into the brain. By 1950, however, scientists had decided that it was a vestigial structure with no known function. Melatonin (pg/ml plasma) Noon 6 P.M. Midnight 6 A.M. Noon 3 P.M. Melatonin is the darkness hormone, secreted at night as we sleep. It is the chemical messenger that transmits information about light-dark cycles to the brain center that governs the body s biological clock. (Adapted from J. Arendt, Clin. Endocrinol. 29: , 1988.)

18 Figure 7.16 FOCUS ON... The Pineal Gland 69 Summary 70 Hormones The classification of hormones Control of hormone release Hormone interactions Endocrine pathologies Melatonin is an amino acid derived hormone made from tryptophan. Hormone evolution