The Endocrine System
General Function and Organization
•Bethold found that a rooster's
comb is an androgen-dependent
structure. Following castration, the
comb atrophies, aggressive male
behavior disappears, and interest in
the hens is lost.
•Importantly, Berthold also found
that these castration-induced
changes could be reversed by
administration of a crude testicular
extract (or prevented by
transplantation of the testes).
I. Organization of Endocrine System
The functions of the body are regulated by the
nervous and the endocrine system.
The endocrine system consists of endocrine
glands and cells that secrete hormones in
various tissues.
Endocrine System
• Regulates long-term processes:
– growth
– development
– Reproduction
• Uses chemical messengers to relay
information and instructions between
cells
Endocrine glands: Glands that do not use ducts
to convey the secretion to a neighboring target,
they are also called ductless glands.
The secretions, known hormones, circulate all
over the body in the blood but may produce
effects only in selected sites.
The target organ(s) may or may not be near the
site of production of the hormone.
A hormone –
--chemical substance
--is secreted into the internal body fluids
by one specialized cell or a group of cells
and
--has a physiological control effect on
other cells of the body.
Endocrine vs. Nervous System
Major communication systems in the body
 Integrate stimuli and responses to changes
in external and internal environment
 Both are crucial to coordinated functions of
highly differentiated cells, tissues and
organs
 Unlike the nervous system, the endocrine
system is anatomically discontinuous.

What are the modes of intercellular
communication used by the
endocrine and nervous systems?
What is the functional significance
of the differences between the two
systems?
• Endocrine System
- Is unable to handle split-second responses
• Nervous System
- Handles crisis management
Are similarly organized:
1. rely on release of chemicals
2. share many chemical messengers
3. are regulated primarily by negative feedback
4. share a common goal: to preserve homeostasis
Hormones travel via the bloodstream
to target cells
•The endocrine system broadcasts its
hormonal messages to essentially all
cells by secretion into blood and
extracellular fluid.
•Like a radio broadcast, it requires a
receiver to get the message –
•in the case of endocrine messages,
cells must bear a receptor for the
hormone being broadcast in order to
respond.
Transportation of Hormones
1. Endocrine: glands or specialized cells release
hormones into the circulating blood that
influence the function of cells at another
location in the body.
Transportation of Hormones
2, Neuroendocrine:
neurons secrete
substances
(neurohormones) that
reach the circulating
blood and influence the
function of cells at
another location of the
body.
Transportation of Hormones
3. Paracrine, in which cells secret substances
that diffuse into the extracellular fluid and
affect neighboring cells.
What are the major structural
classes of hormones?
Chemical Nature of Hormones
• Can be divided into 3 groups:
– Derivatives of amino acid
– Proteins and polypeptide hormones
– lipid derivatives (steroids)
Why is it important to know the chemical
structures of different hormones?
To determine:1. Synthesis , storage and release
2. The way a hormone is transported
3. Its half life and mode of clearance
4. Its medium of action (Receptors)
Steroids can not be
stored while Peptides
are stored in
secretory vesicles
Derivatives of Amino Acid Tyrosine
• Small molecules structurally related to amino
acids
• Synthesized from the amino acids tyrosine
Tyrosine Derivatives
• Iodothyronines (Thyroid hormones)
• Catecholamines (water soluble):
– epinephrine (E)
– norepinephrine (NE)
Peptide Hormones
• Chains of amino acids
• Synthesized as prohormones:
– inactive molecules converted to active hormones
before or after secretion
• Water Soluble
• Stored in the membrane bound secretory vesicles
• Circulate in blood freely unbound
• Usually administered by injection
• Hydrophilic
• Short half life
• Signal through membrane receptors
2 Groups of Peptide Hormones
• Group 1:
–glycoproteins:
• more than 200 amino acids long, with
carbohydrate side chains:
–thyroid-stimulating hormone (TSH)
–luteinizing hormone (LH)
–follicle-stimulating hormone (FSH)
2 Groups of Peptide Hormones
• Group 2:
–all hormones secreted by:
• hypothalamus
• heart
• thymus
• digestive tract
• pancreas
• posterior lobe of pituitary gland
• anterior lobe of pituitary gland
Synthesis, Storage and Release of
Peptide Hormones
Stimulus
Lipid Derivatives (Steroids)
Made by:Adrenal Cortex
Ovaries
Testes
Placenta
Kidneys
Steroid Hormones
• Synthesized from cholesterol
• Can not be stored
• Released by:
– reproductive organs (androgens by testes,
estrogens, and progestins by ovaries)
– adrenal glands (corticosteroids)
– kidneys (calcitriol)
Steroid Hormones
• Remain in circulation longer than peptide
hormones
• Are absorbed gradually by liver
• Are excreted in bile or urine
Hormones
• Circulate freely or bound to transport proteins
Free Hormones
• Remain functional for less than 1 hour:
– diffuse out of bloodstream:
• bind to receptors on target cells
– are absorbed:
• broken down by cells of liver or kidney
– are broken down by enzymes:
• in plasma or interstitial fluids
Thyroid and Steroid Hormones
• Remain in circulation much longer
• Enter bloodstream:
– more than 99% become attached to special
transport proteins
Bloodstream
• Contains substantial reserve of bound
hormones