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Endocrine System

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Endocrine system - glands, tissues, and cells thatsecrete hormones

Endocrinology the study of this system and the

diagnosis and treatment of its disorders Endocrine glands organs that are traditional sources of

hormones

Hormones - chemical messengers that are transported

by the bloodstream and stimulate physiological responsesin cells of another tissue or organ, often a considerabledistance away

Endocrine System Components

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Hypothalamus

Organs of the endocrine system (purple) and other organscontaining tissues that secrete hormones (tan)

Thyroid Gland

Adrenal Glands

Pancreas (Pancreatic Islets)

Organs with Secondary

Endocrine Functions

Parathyroid Glands

Pineal Gland

Pituitary Gland

Secretes hormones involved with

fluid balance, smooth muscle

contraction, and the control ofhormone secretion by the

anterior pituitary gland

Secretes multiple hormones that

regulate the endocrine activities

of the adrenal cortex, thyroid

gland, and reproductive organs,

and a hormone that stimulates

melanin production

Secretes hormones that affect

metabolic rate and calcium levels in

body fluids.

Secretes hormones involved with

mineral balance, metaboliccontrol, and resistance to stress;

the adrenal medullae release E

and NE during sympathetic

activation

Secretes hormones regulating

the rate of glucose uptake and

utilization by body tissues

Testis

Ovary

Gonads: Secrete hormones

affecting growth, metabolism,

and sexual characteristics, as

well as hormones coordinating

the activities of organs in the

reproductive system

Kidneys: Secrete hormones that

regulate blood cell productionand the rates of calcium and

phosphate absorption by the

intestinal tract

Digestive Tract: Secretes

numerous hormones involved in

the coordination of system

functions, glucose metabolism,

and appetite

Thymus: Secretes hormones

involved in the stimulation and

coordination of the immune

response

Heart: Secretes hormones

involved in the regulation of

blood volume

Secrete a hormone important

to the regulation of calcium ion

concentrations in body fluids

Secretes melatonin, which

affects reproduction function

and helps establish circadian

(day/night) rhythms

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Anatomy of Hypothalamus

Provides highest level of endocrine control Shaped like a flattened funnel Forms floor and walls of third ventricle of

the brain Regulates primitive functions of the body

from water balance and thermoregulation tosex drive and childbirth

Many of its functions carried out bypituitary gland

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The three mechanisms by which the hypothalamus integrates the activitiesof the nervous and endocrine systems

The hypothalamus func-

tions as an endocrine

organ. Hypothalamic

neurons synthesize two

hormonesantidiuretic

hormone (ADH) and

oxytocin (OXT)

andtransport them along axons

within the infundibulum.

The hypothalamus secretes

regulatory hormones,

special hormones that control

endocrine cells in the pituitary

gland. The hypothalamic

regulatory hormones control

the secretory activities ofendocrine cells in the anterior

lobe of the pituitary gland.

The hypothalamus contains

autonomic centers that

exert direct neural control

over the endocrine cells of

the adrenal medullae. When

the sympathetic division is

activated, the adrenalmedullae are stimulated

directly and immediately.

Preganglionic

motor fibers

Adrenal gland

HYPOTHALAMUS

Adrenal cortex

Adrenal medulla

Posterior lobe

of pituitary gland

Infundibulum

Anterior lobeof pituitary gland

Hormones released

control the activities of

endocrine cells in the

thyroid gland, adrenal

cortex, and reproductive

organs.

ADH and oxytocin

are released into

the circulation.

Upon direct neuronal

stimulation, the

adrenal medullae

secrete epinephrine

and norepinephrine

into the circulation.

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Once within the anterior lobe, these

vessels form a second capillary

network that branches among the

endocrine cells.

The vessels between the median

eminence and the anterior lobe

carry blood from one capillary

network to another. Blood vessels

that link two capillary networks are

called portal vessels; in this case,

they have the histological structure

of veins, so they are called portalveins.

The capillary networks in the

median eminence are supplied by

the superior hypophyseal artery.

Before leaving the hypothalamus,

the capillary networks unite to form

a series of larger vessels that spiral

around the infundibulum to reach

the anterior lobe.

Hypophyseal Portal System

Control of the production of anterior

pituitary hormones by hypothalamic

regulatory hormones

Neurons of these structures manufacture

antidiuretic hormone and oxytocin,

respectively, which are released by

synaptic terminals at capillaries in the

posterior lobe of the pituitary gland

Supraoptic

nuclei

Paraventricular

nucleiNeurosecretory

neurons

HYPOTHALAMUS

Superior

hypophyseal artery

Infundibulum

Inferior

hypophyseal artery

Posterior lobe of

pituitary gland

Endocrine cells

Anterior lobe of

pituitary gland

Hypophyseal veins

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Pituitary Gland (Hypophysis)

Suspended from hypothalamus by a stalk

infundibulum Location and size

Housed in a depression of the sphenoid bonecalled sella turcica

Size and shape of kidney bean 1.3 cm wide

Composed of two structures withindependent origins and separate functions adenohypophysis (anterior or glandular pituitary)

- three-quarters of pituitary Fleshy and glandular

neurohypophysis (posterior or neural pituitary) one quarter of pituitary

Down growth from brain Neural composition

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Pituitary Gland

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Histology of Pituitary Gland

Chromophobe

Basophil

Acidophil

(b) Posterior pituitary

Unmyelinatednerve fibers

Glial cells(pituicytes)

(a) Anterior pituitary

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Histology of Anterior Pituitary Gland

Acidophilshave cytoplasm that stains red or orange

Cells that contain the polypeptide hormones : growthhormone and prolactin

Basophils have cytoplasm that stains a bluish colour

Cells that contain the glycoprotein hormones :thyroid stimulating hormone, luteinizing hormone,follicle stimulating hormone and adrenocorticotrophichormones

Chromophobes have cytoplasm that stains very poorly

These are cells that have minimal or no hormonalcontent. Some chromophobes may also representstem cells that have not yet differentiated intohormone-producing cells

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Hypothalamic Hormones

Eight hormones produced in hypothalamus Six regulate the anterior pituitary Two produced in hypothalamus and via axoplasmic transport

are stored in the posterior pituitary from where they arereleased into the blood circulation (Oxytocin andAntidiuretic Hormone)

Six releasing and inhibiting hormones stimulate orinhibit 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 SOMATOSTATIN (growth hormone-inhibiting hormone

(GHIH) or somatotropin release-inhibiting factor (SRIF)inhibits secretion of growth hormone & thyroid stimulatinghormone by the anterior pituitary

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Hypophyseal Portal System

Portal venules

Posterior lobe

Anterior lobe

Hypothalamic hormones

Anterior lobe hormones

Primary capillariesGonadotropin-releasing hormoneThyrotropin-releasing hormone

Corticotropin-releasing hormoneProlactin-inhibiting hormoneGrowth hormonereleasing hormoneSomatostatin

Follicle-stimulating hormoneLuteinizing hormoneThyroid-stimulating hormone(thyrotropin)

Adrenocorticotropic hormoneProlactinGrowth hormone

Axons toprimarycapillaries

Neuroncell body

Hypophysealportal system:

Secondarycapillaries

Superior hypophysealartery

Hypothalamic releasing and inhibiting

hormones travel in hypophyseal portalsystem (system of blood vesselsthat link the hypothalamus and theanterior pituitary in the brain) fromhypothalamus to anterior pituitary Hormones secreted by anteriorpituitary

Portal venules

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Hypothalamic Hormones

Two other hypothalamic hormones are oxytocin(OT) and antidiuretic hormone (ADH) secreted byposterior pituitary gland

Produced by neurosecretory neurons

Transported to posterior pituitary by axoplasmictransport 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 but merely

stores these hormones

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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

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Anterior lobe of the pituitary synthesizesand secretes six principal hormones Two gonadotropin hormones that target

gonads FSH (follicle stimulating hormone)

stimulates secretion of ovarian sex hormones,development of ovarian follicles, and spermproduction

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 whichincreases metabolic rate, stimulates appetite, increasegrowth of hair, skin, teeth, increase respiratory rate,increase heart rate

Anterior Pituitary Hormones

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Anterior Pituitary Hormones

ACTH (adrenocorticotropic hormone) stimulates adrenal cortex to secrete

glucocorticoids e.g. cortisol which stimulates fatand protein metabolism, gluconeogenesis, helps to

repair damaged tissues and has an anti-inflammatory effect PRL (prolactin)

after birth stimulates mammary glands tosynthesize milk, enhances secretion of

testosterone by testes GH (growth hormone)

stimulates mitosis and cellular differentiation

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Hypothalamo-Pituitary-Target Organ Relationships

Figure 17.6

PRL

GH

ACTHTSH

Liver

TRHGnRHCRH

Hypothalamus

Adrenal cortex

OvaryTestis

Thyroid

IGF

GHRH

Mammarygland

Fat,muscle,bone

LHFSH

Principle hormones and target organs

PRL

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An overview of the relationships between hypothalamic and pituitaryhormones, and some effects of pituitary hormones on target tissues

Hypothalamus

Indirect Control through Releaseof Regulatory Hormones

Direct Releaseof Hormones

Corticotropin-releasinghormone

(CRH)

Thyrotropin-releasinghormone

(TRH)

Growthhormone-releasinghormone(GH-RH)

Growthhormone-inhibitinghormone(GH-IH)

Prolactin-releasing

factor(PRF)

Prolactin-inhibitinghormone

(PIH)

Gonadotropin-releasinghormone(GnRH)

Sensorystimulation

Osmoreceptorstimulation

Regulatory hormones are released intothe hypophyseal portal system for delivery

to the anterior lobe of thepituitary gland.

Posterior lobeof pituitary gland

Kidneys

Males: Smoothmuscle in ductusdeferens andprostate gland

Females: Uterinesmooth muscle andmammary glands

Melanocytes (uncertainsignificance in healthyadults)

Ovariesof femaleTestes

of maleMammaryglands

Bone, muscle,other tissues

Thyroidgland

Adrenalglands

Adrenal cortex Anterior lobe ofpituitary gland

ACTH

TSHGH

PRLFSH LH

MSH

OXT

ADH

Liver

Somatomedins

Glucocorticoids(steroid

hormones)

Thyroidhormones

Inhibin Testosterone Estrogen Progesterone Inhibin

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The control ofhypothalamic andpituitary hormone

secretion by negativefeedback

Hypothalamus

RH

Pituitary

gland

Anterior

lobe

Hormone 1

Hormone 2

Endocrine

organ

Target cells

KEY

Stimulation

Inhibition

Negative feedback

Releasinghormone(RH)

Hormone 1(frompituitary)

Hormone 2(from targetorgan)

Endocrinetargetorgan

TRH

CRH

GnRH

LH

FSH

ACTH

TSH

Testes

Ovaries

Ovaries

Testes

Adrenalcortex

Thyroidgland

Thyroidhormones

Gluco-corticoids

Inhibin

InhibinEstrogens

ProgestinsEstrogens

Androgens

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The secretion of growth hormone(GH), involves both releasing

and inhibiting hormones. Thisprovides a more rapid and preciseregulation of GH.

Stimulation

Inhibition

Stimulates growth of skeletal muscle,cartilage, and many other tissues

Epithelia,adipose tissue,liver

Anteriorlobe

GH

GH-IH

GH-RH

Liver

Somatomedins

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Growth Hormone

GH has widespread effects on the body tissues especially cartilage, bone, muscle, and fat

Stimulates liver cells to synthesize somatomedinswhich increase uptake of amino acids and theirincorporation into proteins by various tissues

Suppresses protein catabolism (breakdown)

Lipid metabolism increased fat catabolized byadipocytes (protein-sparing effect) provides energyfor growing tissues

In liver stimulates breakdown of glycogen

Bone growth, thickening, and remodeling influenced,especially during childhood and adolescence

GH levels decline gradually with age

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Pituitary Disorders Hypersecretion of growth hormone (GH)

Acromegaly due to hypersecretion of GH in adultscausing thickening of bones and soft tissues

especially hands, feet and face

problems in childhood or adolescence gigantism if hypersecretion (excessive production of GH in

chlidren) pituitary dwarfism if hyposecretion rare since growthhormone is now made by genetically engineered bacteria

Age 33Age 16Age 9 Age 52

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Gigantism

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PRL

PRF

PIHStimulation

Inhibition

Stimulatesmammary

glands

Anteriorlobe

The control ofprolactin (PRL)secretion by a pairof regulatoryhormones

PIH (dopamine) inhibits release ofprolactin most of the time. Each

month before menstruationbegins, secretion of PIHdiminishes and levels of prolactinincrease. During pregnancyprolactin levels rise due toincrease secretion of PRF

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Pineal Gland

Pineal Gland

Pi l Gl d

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Pineal Gland

Pineal gland - attached to roof of third ventriclebeneath the posterior end of corpus callosum

After age 7, it undergoes involution (shrinkage) down 75% by end of puberty tiny mass of shrunken tissue in adults

Produces melatonin, a hormone that affects themodulation of wake/sleep patterns and photoperiodic(seasonal) functions i.e. it regulates the gonads andthe annual breeding cycle

It regulates the hormonal changes that usher in

sexual maturity puberty during adolescence i.e. mayregulate timing of puberty in humans Today it is associated with the sixth chakra whose

awakening is linked to prophecy and increased psychicawareness as consciousness ascends.

Th

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Thymus Bilobed gland in the mediastinum superior to the heart

goes through involution after puberty

Site of maturation of T lymphocytes important in immune defense

Secretes hormones (thymopoietin, thymosin, and thymulin) thatstimulate development of other lymphatic organs and activity of T-lymphocytes

Thymus

Thyroid

Trachea

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Thyroid Gland Largest endocrine gland

composed of two lobes and an isthmusbelow the larynx

dark reddish brown color due to richblood supply

Secretes thyroxine (T4 because of 4iodine atoms) and triiodothyronine(T3) T4 which is converted to T3

increases metabolic rate, O2consumption, heat production(calorigenic effect), appetite, growthhormone secretion, alertness andquicker reflexes

Parafollicular (C or clear) cellssecrete calcitonin with rising bloodcalcium stimulates osteoblast activity and

bone formation

Inferior thyroid vein

Isthmus

Thyroidcartilage

Thyroidgland

Trachea

Superior thyroidartery and vein

l f P

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Control of Pituitary:Feedback from Target Organs

Negative feedback -increased target organhormone levels inhibits

release of hormones+

+

+

+

TRH

TSH

Target organs

Thyroid hormone

Stimulatory effect

Inhibitory effect

2

3

4

1

5

6Negative feedbackinhibition

-

-

Th id Gl d Di d

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Thyroid Gland Disorders

Goiter any pathological enlargement of the thyroid

gland- Hashimoto's thyroiditis or chronic lymphocytic thyroiditis

is an autoimmune disease in which the thyroid gland isgradually destroyed by a variety of cell and antibody mediated

immune processes Endemic goiter

dietary iodine deficiency, no TH, no - feedback,increased TSH stimulates hypertrophy

Toxic goiter (Graves disease) autoantibodies mimic the effect of TSH on the thyroid

causing hypersecretion

overgrown thyroid produces functional TH (increasedlevels)

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Goiter

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Parathyroid Glands

Usually four glands partially

embedded in posteriorsurface of thyroid gland can be found from as high as

hyoid bone to as low as aorticarch

Secrete parathyroidhormone (PTH) increases blood Ca2+ levels

promotes synthesis ofcalcitriol

increases absorption of Ca2+

decreases urinary excretion increases bone resorption increases osteoclast

activity

Thyroid gland

Esophagus

Trachea

Pharynx(posterior view)

Parathyroidglands

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Parathyroid Disorders

Hypoparathyroidism surgical excision during thyroid surgery

fatal tetany in 3 - 4 days due to rapid decline in

blood calcium level Hyperparathyroidism - excess PTH

secretion

parathyroid tumor bones become soft, fragile, and deformed

Ca2+ and phosphate blood levels increase

promotes renal calculi formation

l l

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(a)

(b)

Adrenal gland

Kidney

Adrenal cortex

Adrenal medulla

Connectivetissue capsule

Adrenal cortex

Adrenal medulla

Zonaglomerulosa

Zona

fasciculata

Zonareticularis

Suprarenal vein

Adrenal Gland

Small gland that sits on top of each kidney

They are retroperitoneal like the kidney

Ad l M d ll

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Adrenal Medulla Adrenal medulla inner core, 10% to 20% of gland which is

surrounded by a much thicker adrenal cortex constituting 80% -

90% Consists primarily of modified sympathetic neurons calledchromaffin cells (neuroendocrine cells) which have no dendrites oraxons but receive sympathetic input and release their productsdirectly into the blood stream. When stimulated releasecatecholamines (epinephrine and norepinephrine)

Effect is longer lasting than neurotransmitters increases alertness (fight and flight) and prepares body for

physical activity mobilize high energy fuels, lactate, fatty acids, and glucose glycogenolysis and gluconeogenesis boost glucose levels glucose-sparing effect because inhibits insulin secretion

muscles use fatty acids saving glucose for brain increases blood pressure, heart rate, blood flow to muscles,

pulmonary air flow and metabolic rate decreases digestion and urine production

Ad l C t

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Adrenal Cortex Surrounds adrenal medulla and produces major

steroid hormones called corticosteroids Secretes 3 major categories of steroid hormones

from three layers of glandular tissue zona glomerulosa (thin, outer layer)

cells are arranged in rounded clusters

secretes mineralocorticoids e.g. aldosterone regulate thebodys electrolyte balance i.e. sodium retention

zona fasciculata (thick, middle layer) cells arranged in fascicles separated by capillaries secretes glucocorticoids e.g. cortisol regulate metabolism

of glucose, suppresses immune system, helps body adapt tostress and repair tissues

zona reticularis (narrow, inner layer) cells in branching network secretes sex steroids e.g. androgens, estrogen (estradiol).

DHEA is converted to testosterone in other tissues.

Adrenal Disorders

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Adrenal Disorders Cushing syndrome - excess cortisol secretion

hyperglycemia, hypertension, weakness, edema

rapid muscle and bone loss due to protein catabolism

abnormal fat deposition moon face and buffalo hump

Adrenogenital syndrome (AGS)

adrenal androgen hypersecretion (accompanies Cushing)

enlargement of external sexual organs in children and early onset of

puberty masculinizing effects on women

increased body hair, deeper voice and beard growth