Glands

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Glands Hormones
oestrogen
glucagon
testosterone
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Syllabus reference
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• This symbol in the corner of a slide indicates a
  picture, diagram or table taken from your text
  book

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Glands
simple
stratified
Types of epithelial cells
cuboidal
squamous
columnar
Covering and lining most surfaces in the body are
epithelial cells. Glands are structures formed
from epithelial cells
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Some epithelial cells are specialized by having
cilia others are capable of secreting complex
molecules.
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Glands of the body are classified as either
exocrine or endocrine types.
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Glands summary so far

• Glands are multicellular epithelial structures
  that specialize in synthesizing and secreting
  complex molecules including hormones and enzymes.
  
• Glands are classified as either exocrine or
  endocrine glands.
• Exocrine glands have ducts to body surfaces or
  body cavities.  
• Endocrine glands are small isolated blocks of
  tissues with no ducts to surfaces.  They are
  therefore referred to as "ductless" glands.
•  
• The products of exocrine glands collect in the
  duct of the gland and flow toward the surface
  through the duct.  
• Since endocrine glands lack ducts, their product
  is released across the cell membrane into
  interstitial spaces around the cells.  They then
  diffuse into capillaries.

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Types of gland
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Exocrine Glands

• Most glands of the body are exocrine glands with
  ducts connecting to body surfaces or body
  cavities.
• For example, your salivary glands open into the
  oral cavity and sweat glands deposit their
  product on the skin surface.  

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Exocrine Glands
Exocrine glands can also be classified by their
secretion mechanism.
1. Merocrine glands these secrete substances
by the process of exocytosis. Vesicles fuse with
the plasma membrane resulting in the release of
their contents into the extracellular space.
Examples include most sweat glands, the
salivary glands and the goblet cells lining of
the respiratory and digestive tracts.
secretory product
Most exocrine glands release their products in
this way.
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Example of a merocrine gland sweat glands
Section through a sweat gland of the skin a
coiled tube exocrine gland
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Example of a merocrine gland salivary glands
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Exocrine Glands
Exocrine glands can also be classified by their
secretion mechanism.
2. Apocrine glands Glands that secrete by
shedding a portion of their cytoplasm, containing
the secretory product, into a duct
pinched off portion of the cell
Examples include the mammary glands and sweat
glands in the armpits.
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Example of an apocrine gland mammary glands
Simplified diagram of a mammary gland Each one is
a functional unit alveolus of epithelial cells
that secretes milk into its centre.
Normal alveoli in lactating mammary gland look
like bunches of grapes.
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Exocrine Glands
Exocrine glands can also be classified by their
secretion mechanism.
3. Holocrine glands Glands that secrete by
shedding entire cells from the lining of a duct.
disintegrating cell and its released contents
Examples include the sebaceous glands associated
with hair follicles.
mitotic divisions to replace lost cells
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Example of a holocrine gland Sebaceous glands
Sebaceous gland associated with a hair follicle
a branched exocrine gland
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Endocrine Glands
NOTE the pancreas has both exocrine and
endocrine gland cells
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Endocrine Glands

• Endocrine glands are the hormone producing
  structures of the body.
• Some, like the thyroid are large and obvious.
•  
• Others, for instance the Islets of Langerhans
  found in the pancreas, are small islands of
  endocrine cells embedded within the larger
  exocrine portion of this organ.
• Endocrine cells release their
  
  secretory products into the
  
  interstitial spaces around the cells.  
• The hormones then diffuse into
  nearby
  capillaries and are
  carried to all
  parts of the body.  
• Only when the hormones encounter
  a
  target tissue do they exert an effect.

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Endocrine Glands
Only cells in the target tissue or target organ
have the correct membrane receptors for this
hormone
Secretory cell in endocrine gland
Hormone diffuses into the blood
Cells without the appropriate membrane receptors
remain unaffected by this hormone
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Key terms

• Exocrine gland a gland that secretes externally
  through a duct
• Endocrine gland a gland that secretes hormones
  internally directly into the bloodstream
• Hormone the secretion of an endocrine gland that
  is transmitted by the blood to the tissue(s) on
  which it has a specific effect
• Target tissue cells of an organ that are
  affected by specific hormones

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Hormones

• There are 2 major types of hormones
• Lipid-based Hormones
• Protein-based Hormones

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Lipid-based hormones

• The commonest type of lipid-based hormones are
  the steroids
• Examples include aldosterone and the sex hormones
  such as oestrogen and testosterone

testosterone
Aldosterone hormone produced in the adrenal
gland, regulating salt and water balance in the
kidney
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Lipid-based hormones

• Lipid based hormones are hydrophobic which means
  they are lipid soluble.
• They can diffuse through the cell membrane
  without the need for a receptor on the membrane.
• These hormones bind to receptor proteins in the
  cytoplasm or nucleus.
• This steroid-protein complex then binds to DNA
  and either starts or inhibits transcription of a
  particular gene.
• This means it coordinates which genes should be
  transcribed as mRNA, to make a protein.

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How steroid hormones work
1. Steroid hormone diffuses across the plasma
membrane
2. Steroid hormone binds with protein receptor
3. Steroid-protein complex diffuses into the
nucleus
4. Steroid-protein complex binds to a specific
gene
5. Gene is transcribed to produce a section of
mRNA
6. mRNA is translated by ribosomes in the
cytoplasm to produce the required protein
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Protein-based hormones

• These include
• Polypeptides e.g. insulin ADH
• Glycoproteins e.g. FSH LH
• Catecholamines e.g. adrenaline noradrenaline

adrenaline
ADH anti diuretic hormone FSH follicle
stimulation hormone LH luteinizing hormone
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Protein-based hormones

• Protein based hormones are hydrophilic and are
  not lipid soluble. That means they cannot diffuse
  across a membrane.
• The protein hormone acts as a first messenger by
  binding to receptors in the plasma membrane.
• This causes an enzyme to be activated.
• This enzyme causes the formation of cyclic AMP
  from ATP.
• Cyclic AMP acts as a second messenger to activate
  a particular reaction pathway in the cytoplasm.

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How protein-based hormones work outline of
mechanism
first messenger
second messenger
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What is cyclic AMP?
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How protein-based hormones work detailed
mechanism
1. Protein hormone binds to a protein in the
plasma membrane of a target cell
2. G-Protein activated
3. Cyclase enzyme activated. This catalyses the
conversion of ATP to cyclic AMP (cAMP)
4. cAMP activates an enzyme in the cytoplasm.
This enzyme will catalyse the required reaction
in the target cell.
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How adrenaline affects target tissues
adenylyl cyclase
FIRST MESSENGER
adrenaline
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ATP
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inactive enzyme
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SECOND MESSENGER
inactive enzyme
cAMP
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G protein
active enzyme
inactive glycogen phosphorylase
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active enzyme
active glycogen phosphorylase
receptor protein
glucose phosphate
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glycogen
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How adrenaline affects target tissues
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The pancreas

• Both endocrine and exocrine functions can be
  found in the same organ.
• The pancreas simultaneously produces exocrine
  secretions like digestive enzymes and endocrine
  secretions like insulin and glucagon that are
  released into the blood to exert their effects
  throughout the body.

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