Chapter 17 The Endocrine System and Development Copyright © 2013 by John Wiley & Sons, Inc. All rights reserved

Chapter 17

The Endocrine System and Development

Copyright © 2013 by John Wiley & Sons, Inc. All rights reserved.

The Endocrine System

• The endocrine glands and several other hormone-secreting organs and tissues together all form the endocrine system– All are connected by the cardiovascular system– All also are tied directly to the nervous system

• Endocrine glands secrete hormones directly into the bloodstream– Rather than through ducts to the surface of the gland

• Some organs and tissues also secrete hormones but are not considered glands – Kidneys, Stomach, Liver, Skin, Ovaries/Testes, Heart , Small intestine

• Organs and tissues communicate with cells using hormones as chemical messengers– The cells that hormones act upon are called their target cells– One hormone can have many different target cells– A particular hormone can have widely varying effects on different tissues

© 2013 by John Wiley & Sons, Inc. All rights reserved.

The Endocrine System


Hormones

• Hormones get to almost every cell in the body– They are chemically active compounds that are produced in one area of the

body but have their effect elsewhere – They are responsible for the many sequential changes of growth and

maturation– They are also agents of response when homeostasis is disrupted

• Specifically, hormones – Maintain fluid balance– Control calcium and glucose levels in the blood– Assist in tissue repair– Maintain basal metabolic rate– Assist in digesting food


Hormones

• There are two main classes of hormone– Steroid hormones are lipid-soluble

• Can pass directly through the phospholipid bilayer of cell membranes– Non-Steroid hormones are not lipid-soluble

• Cannot pass through the cell membrane

• The main difference between these classes is solubility– This single difference translates into completely different modes of action


Steroid Hormones

• Steroid hormones pass directly through cell and nuclear membranes of their target cells– Non-steroid hormones are lipid-soluble and thus can readily diffuse across the

phospholipid bilayer of the target cell like steroid hormones do– After crossing the cell membrane, steroid hormones can reach and bind to

their specific receptors in the cytoplasm or nucleoplasm– The receptor-hormone complexes then either act to increase or decrease the

production of specific proteins– These newly made proteins in turn alter the activities of the target cell



Steroid Hormones

Steroid Hormones


Non-Steroid Hormones

• Non-steroid hormones cannot pass through cell and nuclear membranes of their target cells– Non-steroid hormones are water-soluble and thus cannot diffuse across the

phospholipid bilayer of the target cell like steroid hormones do• To overcome this obstacle

– Non-steroid hormones bind to their specific surface receptors on target cells– These receptors have an inactive molecule attached to their cytoplasmic side– Hormone binding to the surface receptor activates the associated inactivated

cytoplasmic molecule, releasing it into the cell’s interior (its cytoplasm)• The released cytoplasmic molecule, usually cyclic AMP (cAMP), becomes

a second messenger that carries information from the hormone (the first messenger) to the interior of the cell

• cAMP in turn activates an enzyme inside the cell, often a kinase, that alters the activity of another specific protein inside the cell

• Non-Steroid hormones affect target cells quickly– Because they affect proteins that are already present in those cells


Non-Steroid Hormones




Endocrine Glands and Secretion

• Endocrine glands secrete hormones directly into the bloodstream• Hormones usually get released into our bloodstreams in short bursts

– When the endocrine gland is stimulated by a signal from the nervous system or another endocrine gland

– When the stimulation stops, the concentration of the hormone in our blood drops

– This negative feedback system usually ensures that the hormone is not overproduced or underproduced

• The endocrine system has several key components– Hypothalamus, pituitary glands, adrenal glands, thyroid gland, parathyroid

glands, thymus, pineal gland, and pancreas


The Hypothalamus and Pituitary

• The endocrine system is directly tied to the nervous system through the hypothalamus

• The hypothalamus (inside the brain) secretes releasing and inhibiting factors into a portal system of capillaries– The portal system capillaries connect to the capillaries of the anterior pituitary

gland• The hypothalamic regulatory factors thus are quickly sent to the pituitary

gland• These factors then trigger the cells of the anterior pituitary gland to

release their pituitary hormones, which in turn stimulate other endocrine glands

• The pituitary gland hangs from the hypothalamus into a depression in the sphenoid bone


The Hypothalamus and Pituitary


The Posterior Pituitary

• The posterior pituitary gland is composed of neuroendocrine neurons– These neurons carry nerve impulses– They also produce two hormones for release into the bloodstream

• Oxytocin – Has important roles in childbirth and lactation

• Antidiuretic hormone or ADH – Affects nephrons of the kidney


The Anterior Pituitary

• The anterior pituitary gland is composed of glandular tissue• It produces hormones that stimulate growth, metabolic rate, milk

production, and glucocorticoid production– Glucocorticoids are steroid hormones that maintain mineral balance and

control inflammation and stress

• Four anterior pituitary hormones are messenger hormones that cause target cells (elsewhere) to secrete other hormones– ACTH– FSH– LH– TSH


The Hormones of the Anterior Pituitary Gland

• ACTH – Stimulates the adrenal cortex (of the adrenal gland) to produce glucocorticoids

and mineralocorticoids • Follicle-stimulating hormone (FSH) and luteinizing hormone (LH)

– Both are gonadotropins that stimulate the growth and functioning of the ovaries and testes, which in turn produce estrogen and testosterone

• Thyroid-stimulating hormone (TSH) – Activates the thyroid to produce T3 (triiodothyronine) and T4 (thyroxin) - both

involved in maintaining the basal metabolic rate• Prolactin (PRL) and human growth hormone (hGH) act directly on target

tissue instead of serving as messenger hormones– Prolactin (PRL) stimulates milk production in females– Human growth hormone (hGH) stimulates the growth of muscle, cartilage, and

bone, and causes many cells to speed up protein synthesis, cell division, and the burning of fats for energy


Gigantism

• If growth hormone is produced in large amounts prior to puberty, the bones and muscles will continue to grow, causing gigantism


Acromegaly

• Acromegaly is the secretion of excess growth hormone after the closure of the epiphyseal plates, when further increase in height is impossible

• Acromegaly typically enlarges cartilage, causing an enlarged chin and accelerated growth of the nose, ears, and voice box, as well as a coarsening of the skin and an enlargement of the hands and feet


The Adrenal Glands

• Located “on top” of the kidneys

• The adrenal cortex secretes – Glucocorticoids,

mineralocorticoids, and small amounts of estrogen and testosterone

• The adrenal medulla secretes – Epinephrine and

norepinephrine


The Hormones of the Adrenal Glands

• Glucocorticoids are a group of hormones involved in glucose metabolism– ACTH - stimulates secretion of glucocorticoids– Cortisol - similar to glucagon in that it promotes the use of fats and proteins as

energy sources• Mineralocorticoids are hormones that monitor and maintain ion balance

– Aldosterone - regulates sodium and potassium, which also affects water balance

• Sex hormones– Testosterone, estrogen - help maintain secondary male and female sex

characteristics

• Non-steroid– Epinephrine, norepinephrine – flight or fight


Adrenal Gland - Diseases

• Cushing’s syndrome– Caused by hypersecretion of the adrenal cortex, which puts excess cortisol in

the blood– The cortisol breaks down muscle proteins and redistributes body fat

• Causing the typical round, flushed “moon face”, a deposit of fat at the back of the neck, and thin arms and legs

– Also causes blood chemistry imbalances• Primarily excess glucose, hypertension, and mood swings

– Bones become weak• Addison’s disease

– Caused by hyposecretion of glucocorticoids and aldosterone, usually due to autoimmune destruction of the adrenal cortex• The resultant lack of glucocorticoids causes mental slowness, anorexia,

weight loss, and a bronzing of the skin


The Thyroid Gland – Hormones and Diseases

• T3 and T4– Involved in regulating the basal metabolic rate and help determine how

quickly and efficiently you use energy– Thyroxin (T4) is responsible for the cellular conversion of glucose to ATP

• Diseases– Hypothyroidism occurs when the thyroid secretes too little T3 and T4

• Congenital hypothyroidism – Occurs from birth and can lead to mental retardation and stunted

bone growth unless treated immediately• Myxedema

– Results when the thyroid works normally at birth but fails to secrete enough hormones in adult life

– Causing slow heart rate, low body temperature, dry hair and skin, muscular weakness, general tiredness, and tendency to gain weight


The Thyroid Gland – Hormones and Diseases

• Diseases– Hyperthyroidism occurs when the thyroid over-secretes the thyroid

hormones• The metabolic rate can be 60 to 100 percent above normal

– Exophthalmos• Fluid builds up behind the eyes and may cause the eyes to “pop” from

their sockets• Thus making the whites of the eyes visible all around the iris

– Graves disease • Occurs more often in females than in males• Increased production of TSH causes the thyroid to enlarge and becomes

visible over the surface of the larynx, a condition called goiter• Goiter can be prevented by adding iodine to the diet, so T3 and T4

formation can be completed


Hyperthyroidism


Exophthalmos Goiter

The Thyroid Gland - Calcium

• Calcitonin– Stimulates calcium uptake by osteoblasts

• Thus depositing calcium into the bones– Calcitonin also inhibits osteoclasts

• Thus preventing bone from being destroyed – Calcitonin causes increased bone mass


Thyroid and Parathyroid Glands


Thyroid and Parathyroid Glands


The Parathyroid Glands

• The parathyroid glands secrete parathyroid hormone (PTH)– PTH - removes calcium and phosphate from bones, stimulates uptake of

calcium from the digestive tract, and prevents loss of calcium in the kidneys (where calcium is exchanged for phosphate)• PTH is present throughout life and is the major force in maintaining blood

calcium levels in adults

• Hypoparathyroidism occurs when there is too little PTH in the blood, which causes blood calcium levels to drop– Which causes nerves to depolarize and muscle cells to contract, resulting in

twitches, spasms, and tetany (continuous contraction)

• Hyperparathyroidism occurs when there is too much PTH in the blood, which causes blood calcium levels to rise– Which causes bones to lose calcium, making them soft and prone to damage– May lead to the formation of kidney stones, fatigue, and personality changes


The Thymus and Pineal Glands

• Thymus Gland– Located in the anterior mediastinum– Important during infancy and childhood, when the immune system is

developing – Secretes hormones for lymphatic cell maturation

• Thymosin and thymopoietin– Decline in thymic function compromises the overall functioning of the

immune system• Pineal Gland

– Located in the brain– Like the thymus, is more active during infancy and childhood– Secretes the hormone melatonin– Indirectly sensitive to light as they react to nerve impulses carried on the

adjacent optic nerve – Seems to be involved in sleep patterns and circadian rhythms


The Pancreas – Two Roles

• As an exocrine gland – Carried out by the acini

– secrete digestive enzymes through ducts

• As an endocrine gland – Carried out by the islets

of Langerhans – secrete several hormones involved in maintaining blood glucose levels


The Pancreas - Hormones

• The islets of Langerhans contain three kinds of secreting cells1. Alpha cells - secrete glucagon to raise blood glucose

• Glucagon– Stimulates liver and muscle cells to break down glycogen into

glucose, thus increasing blood glucose levels– Glycogen – long chains of bonded glucose molecules

2. Beta cells – secrete insulin to lower blood glucose• Insulin

– Stimulates liver, muscle, and fat cells to take up glucose from the blood, thus lowereing blood glucose levels

– Delta cells – secrete somatostatin• Somatostatin

– Inhibits the production of insulin, glucagon, human growth hormone, and several other hormones from other glands


Diseases of the Pancreas - Diabetes Mellitus

• Type I diabetes– Used to be called “juvenile diabetes”

because of its early onset– Usually appears before age 25

• When the pancreas suddenly stops making functional insulin

– An autoimmune disease• Immune system destroys its own cells

within the islet of Langerhans• Insulin production stops

– Treatment involves monitoring blood glucose levels and injecting insulin when glucose levels are high• Insulin stimulates liver, muscle, and fat

cells to take up glucose from the blood• Thus lowering blood glucose levels


Diseases of the Pancreas - Diabetes Mellitus

• Type II diabetes– Used to be called “adult-onset diabetes” because it appears during adulthood– Even though the blood contains abnormally high levels of insulin

• Basically – the cells cannot absorb glucose properly• Leaving high levels of blood glucose

– Strongly associated with a family history of diabetes, older age, obesity, and lack of exercise• More common in women

– Especially those with a history of gestational diabetes (diabetes during pregnancy)

• Also common among Hispanic, Native American, and African American populations


Regulation of Blood Glucose Levels

• Rising blood glucose levels stimulate the production and secretion of insulin– As insulin circulates in blood– It prompts cells to absorb

glucose from the blood– Thus lowering blood glucose

• When blood glucose levels are lowered, glucagon is secreted and insulin secretion stops– Glucagon triggers glucose

release from glycogen stores – Thus increasing blood glucose


Other Organs Also Secrete Hormones

• Hormones secreted by the kidney– Erythropoietin - stimulates the production of red blood cells, thereby

increasing blood volume and blood pressure– Calcitriol - involved in calcium ion homeostasis by stimulating the absorption

of calcium and phosphate along the digestive tract– Renin - involved in blood pressure and blood volume

• Lowered renal blood causes the kidney cells to secrete renin• Causing the secretion of aldosterone, an increase in thirst, and water

retention• As a result

– Blood volume increases, renal blood flow increases, and renin secretion stops


Other Organs Also Secrete Hormones

• Hormones secreted by the heart– Atrial natriuretic peptide (ANP) - decreases blood volume by causing water

loss to the urine and suppression of thirst• Occurs in response to the stretching of cells within the atria of heart

– The extra stretching is caused by the increased blood volume• Opposite function of renin

• Hormones secreted by the intestines– Gastrin - stimulates stomach secretions– Cholecystokinin - causes release of bile– Secretin - causes pancreas to secret digestive enzymes into duodenum


Paracrines

• Paracrines – Compounds produced by one cell and released into the local environment– They affect only surrounding cells– Histamines and prostaglandins - examples of paracrine secretions

• Both cause localized inflammation and fluid leakage• They become inactive a short distance from their point of secretion


Development

• Endocrine gland hormones direct growth and maturation– From the neonatal period and infancy, through childhood, adolescence, and

adulthood– Senescence, or aging, also has hormonal controls

• The endocrine system directs– The patterned growth of bones, muscles, and nervous tissue through to the

end of childhood– Also, maintains a healthy metabolic rate, monitors sleep patterns, maintains

ion and water balance, and regulates blood levels of calcium and glucose

• At puberty, the endocrine system stimulates – The appearance of secondary sexual characteristics – And the production of eggs or sperm


Development

• Between ages 12 and 17, a final growth spurt occurs– Human growth hormone causes a rapid increase in size of the muscular and

skeletal systems– The internal organs also grow

• The lungs, stomach, and kidneys double in size• The brain increases by approximately 5 percent


Development

• During puberty, the reproductive organs begin to function– Puberty is defined as

• The onset of the menstrual cycle in females (menarche) • The appearance of nocturnal emissions in males

– Puberty is directed by the production of gonadotropin-releasing hormone (GnRH) from the hypothalamus

– GnRH causes the release of FSH and LH by the anterior pituitary gland• In females - the ovaries respond by maturing egg follicles and producing

estrogen, causing the appearance of secondary sex characteristics• In males – the testes respond by producing sperm and testosterone

• Adulthood begins after puberty is reached– this is a long stage, lasting from approximately age 15 to 18 until death


Aging

• There are three explanations for aging 1. Limits on cellular division

– Normal cells go through a predetermined number of mitotic cycles– After this number of cell divisions, cells stop dividing and die

2. Accumulated cellular damage– Inability to repair damaged DNA

3. The demise of organ systems– Weakening organ structure and function




Documents

Chapter 17 The Endocrine System and Development Copyright © 2013 by John Wiley & Sons, Inc. All rights reserved