The Endocrine System

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Objectives

• Be able to define hormone

• Know the three major categories of hormones

• Know the major endocrine glands, the hormones they secrete and their actions

Endocrine System: Hormones

• Endocrine Glands: ductless organs

• Hormones:– Chemical messengers

– Circulate in the bloodstream

– Stimulate physiological response

Characteristics• Access to every cell

• Each hormone acts only on specific cells (target cells)

• Endocrine control slower than nervous system

• Endocrine and nervous systems interact

• Three chemical classes– Steroids– Peptides– monoamines

Hormone Chemistry• Steroids

• derived from cholesterol• secreted by gonads and adrenal glands

• Peptides and glycoproteins• created from chains of amino acids• secreted by pituitary and hypothalamus

• Monoamines• derived from amino acids• secreted by adrenal, pineal, and thyroid glands

• all hormones made from either cholesterol or amino acids with carbohydrate added to make glycoproteins.

S

Thr

S

CH3

Testosterone

O

Angiotensin II

Insulin

(c) Peptides

(a) Steroids (b) Monoamines

CH3

OH

Estradiol

HO

I

ICH3

CH CH2

OH

CH2

C

H

COOHH2N

O

OH

HO

HO

Thyroxine

Epinephrine

CH2NH

OH

SS

S

SLys

ProThr Tyr

Phe

Phe

Gly

Arg

Glu

Gly

Cys

Val

Leu

Tyr

LeuAla

GluVal

Leu His Ser Gly Cys LeuHis

Gln

AsnVal

Phe

AsnCys Tyr Asn Glu

LeuGln

Tyr

Leu

Ser

Cys

lle

Ser Thr CysCys

Gln

Glu

Val

lle

Gly

Asp

ArgVal Tyr

lleHis

Pro Phe

I

I

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Major Endocrine Organs

Hypothalamus and Hypophysis

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Adenohypophysis & Neurohypophysis• adenohypophysis constitutes anterior three-quarters of pituitary

– two segments:• anterior lobe (pars distalis)• pars tuberalis small mass of cells adhering to stalk

– linked to hypothalamus by hypophyseal portal system • primary capillaries in hypothalamus connected to secondary capillaries in

adenohypophysis by portal venules• hypothalamic hormones regulate adenohypophysis cells

• neurohypophysis constitutes the posterior one-quarter of the pituitary– has 3 parts:

• median eminence, infundibulum, and the posterior lobe (pars nervosa)

– nerve tissue, not a true gland• nerve cell bodies in hypothalamus pass down the stalk as hypothalamo-

hypophyseal tract and end in posterior lobe• hypothalamic neurons secrete hormones that are stored in neurohypophysis

until released into blood

Hypophyseal Portal System

• hypothalamic releasing and inhibiting hormones travel in hypophyseal portal system from hypothalamus to anterior pituitary

• hormones secreted by anterior pituitary

(b)

Portal venules

Posterior lobe

Anterior lobe

Hypothalamic hormones

Anterior lobe hormones

Primary capillariesGonadotropin-releasing hormoneThyrotropin-releasing hormoneCorticotropin-releasing hormoneProlactin-inhibiting hormoneGrowth hormone–releasing hormoneSomatostatin

Follicle-stimulating hormoneLuteinizing hormoneThyroid-stimulating hormone (thyrotropin)Adrenocorticotropic hormoneProlactinGrowth hormone

Axons toprimarycapillaries

Neuroncell body

Hypophysealportal system:

Secondarycapillaries

Superior hypophysealartery

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17-9

Hypothalamic Hormones• eight hormones produced in hypothalamus

– six regulate the anterior pituitary– two are released into capillaries in the posterior pituitary when

hypothalamic neurons are stimulated (oxytocin and antidiuretic hormone)

• six releasing and inhibiting hormones stimulate or inhibit the anterior pituitary– TRH, CRH, GnRH, and GHRH are releasing hormones that affect

anterior pituitary secretion of TSH, PRL, ACTH, FSH, LH, and GH– PIH inhibits secretion of prolactin, and somatostatin inhibits

secretion growth hormone & thyroid stimulating hormone by the anterior pituitary

Hypothalamic Hormones

• two other hypothalamic hormones are oxytocin (OT) and antidiuretic hormone (ADH)

– both stored and released by posterior pituitary

– right and left paraventricular nuclei produce oxytocin (OT)

– supraoptic nuclei produce antidiuretic hormone (ADH)

– posterior pituitary does not synthesize them

Histology of Pituitary Gland

Chromophobe

Basophil

Acidophil

(b) Posterior pituitary

Unmyelinatednerve fibers

Glial cells(pituicytes)

(a) Anterior pituitary

a: © Dr. John D. Cunningham/Visuals Unlimited; b: © Science VU/Visuals Unlimited

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• Synthesizes and secretes six principal hormones

• two gonadotropin hormones – FSH (follicle stimulating hormone)

• stimulates secretion of ovarian sex hormones, development of ovarian follicles, and sperm production

– LH (luteinizing hormone)• stimulates ovulation, stimulates corpus

luteum to secrete progesterone, stimulates testes to secrete testosterone

• TSH (thyroid stimulating hormone) – stimulates secretion of thyroid hormone

• ACTH (adrenocorticotropic hormone)– stimulates adrenal cortex to secrete

glucocorticoids• PRL (prolactin)

– after birth stimulates mammary glands to synthesize milk, enhances secretion of testosterone by testes

• GH (growth hormone)– stimulates mitosis and cellular

differentiation

Anterior Pituitary Hormones

Hypothalamo-Pituitary-Target Organ Relationships

Figure 17.6

GH

ACTHTSH

Liver

TRHGnRHCRH

Hypothalamus

Adrenal cortex

OvaryTestis

Thyroid

IGF

GHRH

PRL

Mammarygland

Fat,muscle,bone

LHFSH

• principle hormones and target organs

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Posterior Pituitary Hormones

Third ventricle of brain

Median eminence

Hypothalamo–hypophyseal tract

Stalk (infundibulum)

Neurohypophysis:

Posterior lobe

Pars tuberalisAnterior lobe

Adenohypophysis:

(a)

Optic chiasm

Nuclei of hypothalamus:Paraventricular nucleusSupraoptic nucleus

Oxytocin

Antidiuretic hormone

Pineal gland

Cerebral aqueduct

Mammillary body

Anterior Posterior

Floor ofhypothalamus

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• produced in hypothalamus– transported by hypothalamo-hypophyseal tract to posterior lobe– releases hormones when hypothalamic neurons are stimulated

• ADH (antidiuretic hormone)– increases water retention thus reducing urine volume and prevents

dehydration– also called vasopressin because it can cause vasoconstriction

• OT (oxytocin)– surge of hormone released during sexual arousal and orgasm

• stimulate uterine contractions and propulsion of semen– promotes feelings of sexual satisfaction and emotional bonding

between partners– stimulates labor contractions during childbirth– stimulates flow of milk during lactation– promotes emotional bonding between lactating mother and infant

Posterior Pituitary Hormones

Control of Pituitary Secretion • Rates of secretion are not constant

– regulated by hypothalamus, other brain centers, and feedback from target organs

• Hypothalamic and Cerebral Control– anterior lobe control - releasing hormones and inhibiting hormones

from hypothalamus

– posterior lobe control - neuroendocrine reflexes • neuroendocrine reflex - hormone release in response to nervous system

signals• suckling infant stimulates nerve endings hypothalamus posterior

lobe oxytocin milk ejection

Growth Hormone• GH has widespread effects on the body tissues

– especially cartilage, bone, muscle, and fat

• induces liver to produce growth stimulants– insulin-like growth factors (IGF-I) or somatomedins (IGF-II)

• stimulate target cells in diverse tissues

Thymus• Thymus plays a role in three systems: endocrine, lymphatic, and immune

• Bilobed gland in the mediastinum superior to the heart– goes through involution after puberty

• T cell maturation

• secretes hormones (thymopoietin, thymosin, and thymulin) that stimulate development of other lymphatic organs and activity of T-lymphocytes

Thyroid

ThymusLung

Heart

Trachea

Diaphragm

Liver(a) Newborn

(b) Adult

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Thyroid Gland Anatomy• Largest true endocrine gland

• Thyroid follicles – sacs that compose most of thyroid– follicular cells – simple cuboidal

epithelium that lines follicles

– secretes thyroxine (T4) and triiodothyronine (T3)

– Increases metabolic rate, O2 consumption, heat production (calorigenic effect), appetite, growth hormone secretion, alertness and quicker reflexes

• Parafollicular (C or clear) cells secrete calcitonin with rising blood calcium – stimulates osteoblast activity and bone

formation

Inferior thyroid vein

Isthmus

(a)

Thyroidcartilage

Thyroidgland

Trachea

Superior thyroidartery and vein

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Histology of the Thyroid Gland

thyroid follicles are filled with colloid and lined with simple cuboidal epithelial cells (follicular cells).

Follicle

(b)

Follicularcells

Colloid ofthyroglobulin

C (parafollicular)cells

© Robert Calentine/Visuals Unlimited

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Parathyroid Glands• Secrete parathyroid hormone (PTH)

–increases blood Ca2+ levels• promotes synthesis of calcitriol• increases absorption of Ca2+

• decreases urinary excretion• increases bone resorption Thyroid gland

Esophagus

Trachea

(a)

Pharynx(posterior view)

Parathyroidglands

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Adipose tissue

Adipocytes

(b)

Parathyroidcapsule

Parathyroid glandcells

© John Cunningham/Visuals Unlimited

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Adrenal Medulla

• adrenal medulla – inner core, 10% to 20% of gland

• Neuroendocrine gland– innervated by sympathetic preganglionic

fibers– Chromaffin cells– when stimulated release catecholamines

and a trace of dopamine directly into the bloodstream

– increases alertness and prepares body for physical activity

– decreases digestion and urine production

Adrenal Cortex• surrounds adrenal medulla and produces more than 25

steroid hormones called corticosteroids or corticoids

• secretes 5 major steroid hormones from three layers of glandular tissue– zona glomerulosa (thin, outer layer)

• cells are arranged in rounded clusters• secretes mineralocorticoid – regulate the body’s electrolyte balance• aldosterone

– zona fasciculata (thick, middle layer) • cells arranged in fascicles separated by capillaries• secretes glucocorticoids• cortisol

– zona reticularis (narrow, inner layer)• cells in branching network• secretes sex steroids

Bile duct

Duodenum

Tail of pancreas

Head ofpancreas

(a)

(b) Pancreatic islet

Pancreaticducts Beta cell

Delta cellAlpha cell

(c) Pancreatic isletExocrine acinus

c: © Ed Reschke

Pancreas

• exocrine digestive gland and endocrine cell clusters (pancreatic islets) found retroperitoneal, inferior and posterior to stomach.

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Pancreatic Hormones• 1-2 million pancreatic islets (Islets of Langerhans)

produce hormones– other 98% of pancreas cells produces digestive enzymes

• insulin secreted by B or beta () cells– secreted during and after meal when glucose and amino acid

blood levels are rising– stimulates cells to absorb these nutrients and store or

metabolize them lowering blood glucose levels– insufficiency or inaction is cause of diabetes mellitus

Pancreatic Hormones• glucagon – secreted by A or alpha () cells

– released between meals when blood glucose concentration is falling– in liver, stimulates gluconeogenesis, glycogenolysis, and the release of glucose

into the circulation raising blood glucose level

• somatostatin secreted by D or delta () cells– partially suppresses secretion of glucagon and insulin – inhibits nutrient digestion and absorption which prolongs absorption of nutrients

• pancreatic polypeptide secreted by PP cells or F cells)– inhibits gallbladder contraction and secretion pancreatic digestive enzymes

The Gonads• ovaries and testes are both endocrine and exocrine

– exocrine product – whole cells - eggs and sperm (cytogenic glands)

– endocrine product - gonadal hormones – mostly steroids

• ovarian hormones– estradiol, progesterone, and inhibin

• testicular hormones– testosterone, weaker androgens, estrogen and inhibin

Histology of Ovary and Testis

follicle - egg surrounded by granulosa cells and a capsule (theca)

Granulosa cells(source ofestrogen)

Egg nucleus

Egg

Ovary

(a)

Theca

100 µm

© Manfred Kage/Peter Arnold, Inc.

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(b)

Interstitial cells(source oftestosterone)

Connective tissuewall of tubule

Germ cells

Seminiferoustubule

Blood vessels

Testis

Sustentacularcells

50 µm

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© Ed Reschke

Endocrine Functions of Other Organs• skin

– keratinocytes convert a cholesterol-like steroid into cholecalciferol

• liver – involved in the production of at least five hormones– converts cholecalciferol into calcidiol– secretes angiotensinogen (a prohormone)

– secretes 15% of erythropoietin– hepcidin – promotes intestinal absorption of iron– source of IGF-I

• kidneys – plays role in production of three hormones– converts calcidiol to calcitriol, active form of vitamin D

• increases Ca2+ absorption by intestine and inhibits loss in the urine– secrete renin that converts angiotensinogen to angiotensin I

• angiotensin II created by converting enzyme in lungs– constricts blood vessels and raises blood pressure

– produces 85% of erythropoietin

Endocrine Functions of Other Organs • heart

– cardiac muscle secretes ANP and BNP in response to an increase in blood pressure

– decreases blood volume and pressure – opposes action of angiotensin II

• stomach and small intestine secrete at least ten enteric hormones secreted by enteroendocrine cells– coordinate digestive motility and glandular secretion – cholecystokinin, gastrin, Ghrelin, and peptide YY

• adipose tissue secretes leptin– slows appetite

• placenta– secretes estrogen, progesterone and others

• regulate pregnancy, stimulate development of fetus and mammary glands

Endocrine Disorders

• Gigantism, Acromegaly, Pituitary dwarfism

• Congenital hypothyroidism, myxedema

• Cushing syndrome

• Diabetes mellitus