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
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
(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
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