The Endocrine System

Chapter 45

• Types of chemical messengers:• Hormone• Paracrine regulators• pheromones

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Types of Chemical Messengers• Hormone

– Regulatory chemical that is secreted into extracellular fluid and carried by the blood

– Can act at a distance from source

• Endocrine system– Organs and tissues that produce hormones

• Only targets having the appropriate receptor can respond

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http://www.personal.kent.edu/~cearley/ChemWrld/drugs/drugs.htm

Types of Chemical Messengers

• Paracrine regulators do not travel in blood– Allow cells of an organ to regulate each other– Can be neurotransmitters

• Pheromones are chemicals released into the environment to communicate among individuals of a single species– Not involved in normal metabolic regulation

within an animal

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Types of Chemical Messengers

• Neurohormones– Some neurotransmitters are distributed by the

blood and act as a hormone– Norepinephrine coordinates the activity of

heart, liver, and blood vessels during stress

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Endocrine System

• The endocrine system includes all the organs that secrete hormones– Endocrine – product secreted into extracellular fluid

and carried in blood– Exocrine – secrete product into a duct

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• 2 basic hormone characteristics1. Must be sufficiently complex to convey

regulatory information to their target cells

2. Must be adequately stable to resist destruction before reaching their target

cells (having to survive in extracellular environments)

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I have to be both Tough and complex!

HORMONE

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Adenohypophysis

HypothalamusHypothalamus Pineal gland

Neurohypophysis Pituitarygland

Thymus

Thyroid gland

Pancreas

Testes(in males)

Ovaries (in females)

Pineal gland

Parathyroid glands(behind thyroid)

Adrenalglands

3 Classes of Hormones1. Peptides and proteins

– Glycoproteins

2. Amino acid derivatives– Catecholamines– Thyroid hormones– Melatonin

3. Steroids– Sex steroids– Corticosteroids

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1. Human Chorionic Gondadotrophin( a glycoprotein)

1. www.dreamstime.com

2. Catecholamines

2. themedicalbiochemistrypage.org 3. www.vanderbilt.edu3. Steroids

• Lipophilic (nonpolar) – fat-soluble– Steroid hormones and thyroid hormones– Travel on transport proteins in blood– Bind to intracellular receptors– Tend to act over brief time period

• Hydrophilic (polar) – water-soluble– All other hormones– Freely soluble in blood– Bind to extracellular receptors– Tend to have much longer active period

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Hormones may be categorized as:

• Lipophilic hormones

• Transported by proteins

• Cross cellular membrane

• Bind to receptor in nucleus, activating transcription factors

• Notice where the hormone ended up in this picture

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• Hydrophilic hormones– Bind to receptors on cell surface– Initiates signal transduction through activation

of protein kinases– Regulatory enzymes that activate intracellular proteins

through phosphorylation

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Paracrine Regulation occurs in most organs

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Hydrophilic Hormones

• Second-messenger systems– Many hydrophilic hormones work through

second messenger systems– Two have been described

• One involving cyclic adenosine monophosphate (cAMP)

• One that generates 2 lipid messengers: inositol triphosphate (IP3) and diacyl glycerol (DAG)

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The Pituitary Gland

• Also known as the hypophysis

• Hangs by a stalk from the hypothalamus

• Consists of two parts– Anterior pituitary (adenohypophysis)

• Appears glandular

– Posterior pituitary (neurohypophysis)• Appears fibrous

– Different embryonic origins– Different hormones

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The Posterior Pituitary

• Appears fibrous because it contains axons that originate in cell bodies within the hypothalamus and that extend along the stalk of the pituitary as a tract of fibers– Develops from outgrowth of the brain

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The Posterior Pituitary

• Releases 2 hormones:• ADH• Oxytocin

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The Anterior Pituitary

• Develops from a pouch of epithelial tissue of the embryo’s mouth– Not part of the nervous system

• Produces at least 7 essential hormones• Tropic hormones or tropins

– Act on other endocrine glands

• Can be categorized into three families

• 3 families of anterior pituitary hormones:• Peptide

– Corticotropin – stimulates adrenal to make cortisol– Melanocyte-stimulating hormone – stimulates dispersion of

pigment

• Protein– Growth hormone– Prolactin – stimulate mammary glands

• Glycoprotein– Thyroid-stimulating hormone – acts on thyroid– Luteinizing hormone – acts on ovaries, testes– Follicle-stimulating hormone – development of ovarian follicles,

development of sperm

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• Anterior pituitary controlled by hormones from hypothalamus

• Hypothalamus secretes neurohormones into anterior pituitary causing anterior pituitary to release hormones

20www.austincc.edu

• The hypothalamus and the anterior pituitary are partially controlled by the very hormones whose secretion they stimulate (FEEDBACK)– Negative feedback (most)– Positive feedback (few)

• Causes deviations from homeostasis• Control of ovulation and labor contractions

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The Thyroid Gland

• In humans, the thyroid gland is shaped like a bow tie, and lies just below the Adam’s apple in the front of the neck

• Secretes– Thyroid hormones

• Thyroxine• Triiodothyronine

– Calcitonin

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More Glands: Parathyroid Glands• 4 small glands attached to the thyroid• Produce parathyroid hormone (PTH) • Raises blood Ca2+ levels by…

– Pulling It out of bone (via osteoclasts)

– Stimulating the kidneys to reabsorb Ca2+ from the urine

• Vitamin D activated by a PTH controlled enzyme– Stimulates the intestinal absorption of Ca2+

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The Adrenal Glands• Medulla (inner portion)

– Stimulated by the sympathetic division of the autonomic nervous system

– Secretes the catecholamines epinephrine and norepinephrine (adrenalin and noradrenalin)

• Cortex (outer portion)– Stimulated by anterior pituitary hormone ACTH

– Corticosteroids• Glucocorticoids: maintain glucose homeostasis (e.g., cortisol)

• Aldosterone: helps regulate mineral balance

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The Pancreas

• Exocrine and endocrine glands

• Connected to the duodenum of the small intestine by the pancreatic duct

• Islets of Langerhans are scattered clusters of cells throughout the pancreas

• These govern blood glucose levels through two hormones with antagonistic functions

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The Pancreas

• Insulin – Secreted by beta () cells of the islets– Stimulates cellular uptake of blood glucose

and its storage as glycogen in the liver and muscle cells, or as fat in fat cells

• Glucagon – Secreted by alpha () cells of the islets– Promotes the hydrolysis of glycogen in the

liver and fat in adipose tissue26

The Gonads

• Ovaries and testes in vertebrates

• Produce sex steroids that regulate reproductive development

• Estrogen and progesterone– “Female” hormones

• Androgens– “Male” hormones– Testosterone and its derivatives

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The Pineal Gland

• Located in the roof of the third ventricle of the brain

• Secretes melatonin

• Functions of melatonin– Reduces dispersal of melanin granules– Synchronizes various body processes to a

circadian rhythm • Secretion of melatonin activated in the dark

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