Download Endocrine System

Endocrine System
Introduction:
• Animal hormones are usually secreted by
organs specialized for their production and
transported through the circulatory system
to a very specific target tissue
• Overall Function – guide growth and
development, regulate metabolism and
maintain general homeostasis
Hormones in Invertebrates
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Insects – V.B. Wigglesworth (1930’s) did
work on insect growth hormones by
studying the metamorphosis of insects
Insects show a pattern of growth different
to that of vertebrates because of
exoskeleton (limits size)
Growth exerts pressure against inner
surface of exoskeleton until it is shed
Wigglesworth performed experiments on bloodsucking insect
– Rhodnius
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Goes through 5 nymphal stages, each separated by a
molt
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must obtain a blood meal during each nymphal stage –
stimulates release of hormones that cause molting at the
end of a definite time interval following meal
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last molt occurs approx. 28 days after blood meal
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if decapitated during days 1 – 7 after meal – no molting
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if decapitated 8 days or after – molts in 28 days and
produces a headless moth
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if decapitated 8 days after and joined to a another
Rhodnius (decapitated during days 1 – 7) – both insects
molt into adults
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therefore, some stimulus passes via the blood from one
insect to another and induces molting
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Stimulus must be a hormone whose
secretion by the head begins about 8 days
after the blood meal
Brain Hormone (BH) – stimulates glands in
the prothorax
Prothoracic Gland – secretes a 2nd
hormone, Ecdysone, which induces molting
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Wigglesworth then experimented to find out
whether a molt will result in an adult or
another nymphal stage
found a 3rd hormone – Juvenile Hormone
(JH) – produced by a pair of glands (corpus
allatum) located behind brain
when JH is present in high concentrations
at time of molting – another nymphal stage
follows molt
Hormones in Vertebrates
Hormones are divided into four chemical
groups:
1. Peptide hormones – composed of chains
of amino acids (peptide refers to
comparatively short amino acid chains –
longer chains are proteins)
– both peptides and proteins can function as
hormones – all are referred to as peptide
hormones
– largest group of hormones, water soluble, ex.
Oxytocin and antidiuretic hormone (ADH)
2. Amino-acid derivatives – simplest
hormones, simple molecules made from
single amino acids ex. tyrosine is used to
make epinephrine (adrenaline)
3. Steroid hormones – made from cholesterol,
structure resembles cholesterol
– secreted by ovaries, placenta, testes, and
adrenal cortex
4. fatty acid derivatives – also called
prostaglandins
– produced by nearly every type of cell in
body, and affect variety of body
processes
– referred to as “local hormones” because
act on cells in their immediate
environment
• modify cyclic AMP levels, interact with
other hormones to regulate various
metabolic activities
Hormones function by binding to specific receptors on
and in target cells
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some hormones bind to surface receptors
(peptide and amino acid-based hormones are
water soluble and cannot pass through cell
membrane)
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react with protein receptors on target cell’s
plasma membrane – may cause the following
affect:
• surface receptor may be linked directly to
channels in membrane that open when
hormone binds
• ex. binding of epinephrine to heart muscle
cells opens calcium channels increasing
strength of contraction
Second messenger system
• most common
• hormone is the first messenger (binds to
receptor)
• triggers release inside the cell of a
chemical (second messenger) – this
initiates a series of biochemical reactions
• usually, cyclic AMP (cAMP)is the second
messenger
• binding of hormone activates an enzyme
that converts ATP to cAMP
• cAMP then initiates a chain of reactions
Second Messenger system
Some hormones bind to intracellular receptors
• Steroids are lipid soluble and pass through
the cell membrane
• hormones then bind to receptors inside the
cell (usually protein receptors in nucleus)
• receptor-hormone complex binds to DNA
and stimulates certain genes to transcribe
mRNA (resulting in protein synthesis)
• steroid and thyroid hormones work by
stimulating the expression of genes
Steroid Hormones
Hormones are regulated by feedback mechanisms
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negative feedback usually controls hormones in
animals – the secretion of the hormone
stimulates a response in target cell that inhibits
further secretion of the hormone
few cases are controlled by positive feedback –
end result stimulates release of more hormone
–
ex. oxytocin stimulates contractions of uterus in
childbirth – contractions stimulates further release of
oxytocin
The Endocrine System
Mammals have types
of glands:
• exocrine glands –
have ducts and
produce secretions
that are released
outside the body or
into digestive
system
• endocrine glands –
no ducts, release
hormones into
bloodstream
Hypothalamus
• part of the brain that contains clusters of special
nerves cells called neurosecretory cells
• neurosecretory cells form peptide hormones,
store them, and release them when stimulated
• hypothalamus controls the pituitary gland
• hypothalamus and pituitary gland together
coordinate action of many key hormonal
messaging systems
• hypothalamus produces at least 9 different
peptide hormones that control the anterior
pituitary - releasing hormones or inhibiting
hormones – stimulate or inhibit release of
pituitary hormones
Pituitary Gland
consists of two lobes: anterior and posterior pituitary
(both parts controlled by hypothalamus)
1. anterior pituitary – produces several peptide
hormones (1st four of these control hormone
production in other endocrine glands):
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Follicle-Stimulating Hormone (FSH) and Luteinizing
Hormone (LH) – stimulate production of sperm and
testosterone in males and of eggs, estrogen, and
progesterone in females
Thyroid-Stimulating Hormone (TSH) – stimulates
thyroid gland to release its hormones
Adrenocorticotropic Hormone (ACTH) – causes release
of hormones from adrenal cortex
remaining hormones do not act on other
endocrine glands
– prolactin – stimulates development of
mammary glands (exocrine glands)
during pregnancy
– endorphins – inhibit the perception of
pain by binding to receptors in brain
– melanocyte-stimulating hormone (MSH)
– stimulates synthesis of skin pigment,
melanin
– human growth hormone (GH) –
regulates body’s growth
2. posterior pituitary – releases two hormones
actually produced in hypothalamus:
– Antidiuretic hormone (ADH)
• prevent dehydration
• increases permeability to water of collecting
ducts of nephrons
• causes water to be reabsorbed from urine
and retained in body
– Oxytocin – causes contraction of uterus
during childbirth and triggers “milk letdown
reflex” in nursing mothers
Thyroid Gland
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lies at the front of the neck around larynx
Produces:
– thyroxine – iodine-containing modified amino
acid that raises metabolic rate of most body
cells – levels are finely tuned by negative
feedback
– release of thyroxine is stimulated by thyroidstimulating hormone (TSH) from anterior
pituitary (which is stimulated by a releasing
hormone from hypothalamus)
– calcitonin – peptide important in calcium
metabolism
Negative
Feedback
in Thyroid
Gland
Parathyroid glands
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four small disks embedded in the back of
the thyroid gland
secrete parathyroid hormone (PTH) –
controls concentration of calcium in blood
and body fluids along with calcitonin
(together they regulate calcium absorption
and release by the bones)
PTH stimulates release of calcium
calcitonin inhibits release of calcium from
bones
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Pancreas
acts as both an exocrine and endocrine gland
exocrine portion releases digestive secretions to digestive
system through pancreatic duct
endocrine portion is made up of clusters of cells called islets of
Langerhans cells – produce peptide hormones
beta cells produce insulin
alpha cells produce glucagon
work antagonistically to each other to control carbohydrate and
fat metabolism, and control blood glucose levels
insulin promotes storage of fuel molecules
stimulates tissues (including liver, muscle, and fat cells) to take
up glucose from blood by facilitated diffusion
glucose is either immediately used for fuel or stored as glycogen
also inhibits liver cells from releasing glucose
overall effect is to lower glucose level in blood
effect of glucagon is opposite
raises blood glucose level by stimulating liver cells to convert
glycogen to glucose
also mobilizes fatty acids and amino acids
Gonads (testes and ovaries)
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testes produce androgens (steroid
hormones including testosterone)
ovaries produces estrogen and
progesterone (both steroids)
sex hormones play key role in puberty and
development of secondary sex
characteristics
FSH and LH stimulate the gonads to
produce sex hormones
Adrenal glands
made up of two glands: 1) adrenal medulla and 2)
adrenal cortex
1. adrenal medulla is located in the center of each
gland and is derived during development from
nervous tissue
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hormone production is controlled directly by nervous
system (sympathetic NS)
produces epinephrine (adrenaline) and
norepinephrine (noradrenaline) in response to stress
(prepare body for emergency)
increase heart and respiratory rates, cause blood
glucose levels to rise, and directs blood flow to brain
and muscles
2. adrenal cortex is the outer layer of glands
• secretes three types of steroid hormones called
glucocorticoids made from cholesterol
– glucocorticoid release is stimulated by ACTH (from
pituitary gland which is stimulated by hypothalamus
releasing hormones)
– act similar to glucagon – raise blood glucose levels and
promotes use of fats for energy instead of glucose
• secretes aldosterone – regulates sodium content of
the blood (stimulates kidneys and sweat glands to
retain sodium)
• also produces testosterone in low amounts (both
males and females)
Other body tissues act as endocrine glands
1. pineal gland – located between two
hemispheres of brain – secretes melatonin
which affects daily rhythms – poorly
understood
2. thymus – located behind breastbone –
secretes thymosin which affects immune
system
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Also where T lymphocytes of immune system
mature
3. stomach and small intestine – digestion is
controlled by both nerves and hormones
– endocrine cells in certain regions of digestive
tract produce at least 4 hormones to regulate
digestion
• gastrin (stimulates production of acid in
stomach)
• secretin (stimulates pancreas to release
sodium bicarbonate)
• cholecystokinin (stimulates pancreas to
release enzymes)
• gastric inhibitory peptide (inhibits acid
production and peristalsis to slow down rate
of passage of chyme into small intestine to
allow more time for digestion and
absorption