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Endocrine System
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Introduction to Endocrine system
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Introduction
 Endocrine
 One

system
of the system involved
In coordination of body functions
 Systems
1.
involved include
Neural system
Release chemical substances
(neurotransmitters) at axon terminals
 Act locally to control cell function

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Introduction to Endocrine system
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Introduction
2.
Endocrine system
 Glands
or specialized cells
 Release chemicals (hormones)

3.
Into blood
 Have influence on cell function at
another location
Neuro-endocrine
 Neurons
secrete substances
(neurohormone)
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Introduction
 Reach
circulating blood
 Influence cell function at another
location
4.
Paracrine
 Cells
secrete substances
 Diffuse in ECF
 Affect neighboring cells
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Introduction
5.
Autocrine
 Cells

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secrete substances
Affect the functioning of the same cell
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Hormones
 Working
definition
 Hormone is chemical substance
 Serve

as regulators & coordinators of
Various biologic functions in the body
 Highly
potent, specialized organic
molecules
Produced by endocrine cells
 In response to specific stimuli

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Hormones
 They
exert their actions on specific
target cells
 The
target cells are equipped with
 Receptor

that bind hormones
With high affinity & specificity
 When
bound hormone
 Initiate
characteristic biological
responses by target cells
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Principal Endocrine
Organs
Organ
Hormone
Pituitary gland
Tropic hormones (ACTH, GH, prolactin)
Hypothalamus
Hypophysiotropic hormones (releasing
hormones, TRH); ADH, oxytocin
Thyroid gland
Thyroxin , triiodothyronin
Adrenal glands
Mineralocorticoids (aldosterone),
glucocorticoids (cortisol), catecholamines
(epinephrine, norepinephrine)
Parathyroid gland
Parathtyroid hormone (PTH)
Gonads
Testosterone, estrogen, progesterone
Pancreatic islets
Insulin, glucagon, somatostatin
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Principal Endocrine
Organs
Other organs
Hormone
Heart
Atrial natriuretic peptide (ANP)
Kidney
1,25-dihydroxycholecalciferol (calcitriol),
erythropoeitin
Liver
25-hydroxycholecalciferol (calcidiol),
somatomedin
Pineal gland
Melatonin
Skin
Calciferol (vitamin D)
GIT
Gastrin, cholecyctokinin (CCK), secretin,
vasoactive intestinal peptide (VIP) etc
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Structure & Synthesis
3
1.
general classes of hormone
Protein & polypeptides
 Include
hormones secreted by
Anterior pituitary
 The pancreas (insulin, glucagon)
 Parathyroid gland (PTH)

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Structure & Synthesis
2.
Steroids, include hormones secreted
by

Adrenal cortex


Ovaries


Oestrogen, progesterone
Testes


Cortisol, aldosterone
Testosterone
Placenta

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Oestrogen, progesterone
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Structure & Synthesis
3.
Derivatives of amino acid
tyrosine
 Thyroid

Thyroxin, tri-iodothyronin
 Adrenal

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gland
medulla
Epinephrine & norepinephrine
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Peptide Hormones
Preprohormone
Synthesis in
RER
Cleaved into
prohormone in
ER
Prohormone
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Polypeptides and
protein hormone


Packaging in
Golgi, cleaved
into active
hormone
Hormone &
Inactive fragments

Synthesized on
 Rough endplasmic
reticulum of the
endocrine cells
Usually synthesized as
preprohormone
 Larger protein
 Has no biological
activity
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Peptide Hormones
Preprohormone
Synthesis in
RER

This is then cleaved

Cleaved into
prohormone in
ER
To a prohormone

In endoplasmic
reticulum
Prohormone
Packaging in
Golgi, cleaved
into active
hormone
Hormone &
Inactive fragments
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Peptide Hormones
Preprohormone
Synthesis in
RER


Cleaved into
prohormone in
ER
Packaging in
Golgi, cleaved
into active
hormone
Hormone &
Inactive fragments
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

Prohormone
The prohormone is
then
Transferred to Golgi
apparatus
For packaging
In the process enzymes
cleave it into
 An active hormone &
 A larger inactive
fragment
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Peptide Hormones
Ca++

Secretion of peptide
hormones

 Ca++

Occur by exocytosis
when

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Active hormone &
inactive fragment
secretory vesicles
fuse with cell
membrane
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Peptide Hormones

Ca++
 Ca++

a)
Stimulation
In many cases the
stimulus for the
exocytosis is

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Granular content are then
extruded into
 Interstitial fluid
 Blood stream
 Ca++ in cytosol as a
result of
 Depolarization of
plasma membrane
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Peptide Hormones
Activation
Receptor
2.
 cAMP
Prot kinase
In other cases

Stimulation of the
endocrine cell
surface receptors



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 cAMP
Activation of protein
kinase
Initiate release of the
hormone
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Peptide Hormones
Activation
Receptor

 cAMP
Prot kinase
Peptide hormone
are water soluble


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Enter the circulatory
system easily
Carried to target
tissues
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Steroid Hormones
 Usually
synthesized from
cholesterol
 Chemical structure
 Similar
 They
to that of cholesterol
are lipid soluble,consist of
3
cyclohexyl ring and
 1 cyclopentyl ring
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Structure of Steroid
Hormones
Derivative of cholesterol
Have 3 cyclohexyl rings( A, B, C)
One cyclopentyl ring (D)
18
19
2
10
A
4
12
5
17
16
14
9
B
13
D
C
1
3
11
8
15
7
6
Cyclopentanoperhydrophenanthrene
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Steroid Hormones
 There
 In
 But
is little hormone stored
steroid forming cells
there are large stores
 Of
cholesterol esters
 Can be rapidly mobilized for steroid
synthesis
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Steroid Hormones

Most of the cholesterol
 Come
from plasma
 But cells have the ability to
synthesize it
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Steroid Hormones
 Steroid
hormones are lipid
soluble
 Once
they are synthesized
 They diffuse across cell membrane

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Into interstitial fluid
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Amine Hormones
 Derivatives
of tyrosine
 From the thyroid & adrenal
medulla glands
 Formed by the action
 Of
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cytoplasmic enzymes
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Amine Hormones

Thyroid hormones

Synthesized and stored in thyroid gland



Incorporated in thyroglobulin
Stored in large follicle within the gland
Secretion occurs when

The amines are split from thyroglobulin



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Free hormone released in blood
Bound to thyroxine binding globulin (TBG)
Which slowly releases the hormone to target
tissues
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Amine Hormones
Epinephrine & norepinephrine
 Formed in the adrenal medulla



Stored in vesicles


Secrete 4 times more epinephrine than
norepinephrine
Secreted by exocytosis
In plasma can exist

Free form or in conjugation with other
substances
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