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General Features of Epithelia
Epithelial tissues are one of the main types of animal tissues and are the first barrier to
infection
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Covering epithelia cover or line body surfaces, cavities or tubes such as skin
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Specialised cell junctions join adjacent epithelial cells
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Epithelial cells are polarised
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The golgi body in these cells is polarised, facing the direction of the flow
of blood and in front of the nucleus
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The apical surface is the free side of the epithelial cell and often features microvilli or cilia,
usually facing a lumen
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Collagen and elastic fibres form the basement
membrane attached to the basal surface
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They also attach to cells and have growth factors
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Epithelia are classified according to their shape, the number of cell layers, the presence of
cell-surface specialisations, and function
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Squamous epithelia have a central nucleus and a greater width than height- they are flat
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Cuboidal epithelia have a similar height and weight and are isodiametric with a central
nucleus
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They
often have a basal nucleus
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All of the cells are in contact with the basement
membrane
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Stratified epithelia have multiple layers
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Cell-surface specialisations include the presence of microvilli or cilia, or keratin (a fibrous
protein for protection)
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They are found in the blood vessels and in the lungs
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They also have lots of mitochondria for the production of large amounts of ATP as
an energy source for the active transport of non-gaseous materials (such as ions and
nutrients)
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Ciliated epithelia are non-transporting tissues
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The
cilia in the fallopian tube are responsible for moving the egg
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Secretory epithelia produce substances excreted into extracellular space, and can be
isolated or associated with other secretory epithelia in glands
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They produce watery substances such as tears and sweat, and mucous
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Both are made of epithelial
cells
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Pseudostratified epithelia appear to be stratified, but are in fact only one layer of cells- so
are, in fact, simple
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Skin is made up of epithelia, but the top layer is dead
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Skin epithelia is also keratinised for strength
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The basal cells are cuboidal and divide constantly
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More staining will occur if the cell is taking is less sugar
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Connective tissue contains cells as well as gaps between them, composed mainly of
collagen
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Contact inhibition occurs when epithelia grow and divide to fill all of the available free
space and then cease growth
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Podocytes are visceral epithelial cells found in the kidneys
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The basement membrane of the epithelial cells found near blood vessels
is negatively charged; this prevents blood leaks
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Cilia are
an extension of the cell’s cytoskeleton and are composed of a core of nine microtubules
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Cilia are
not exclusive to epithelia
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This is in contrast to microvilli, which are composed of actin filaments and cannot move (but
the cytoplasm that they are attached to can)
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They are considerably smaller than cilia and can not be easily detected with a light
microscope; an electron microscope must be used
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In microvilli, F-actin filaments run throughout the length of the microvillus
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Villin and plastins
are F-actin filament binding proteins that anchor the F-actin to the plasma membrane
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They have basolateral folds to increase their surface area
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Stereocilia are found in outer hair cells in the ear
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These specialisations contain actin, similar to microvilli, rather than
microtubules
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They do not
move, but can bend from fluid moving
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Cell-to-cell junctions are found on the lateral surface of epithelial cells
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They are very thin
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 Desmosomes or Macula adherens connect cells tightly via the cytoskeleton to
strong keratin filaments
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 Gap junctions allow cell-to-cell communication using small substances
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The
proteins in gap junctions are called connexins and allow the regulation of pore
opening
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They anchor the epithelium to the
basement membrane, thus providing structural support
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Plectin is another substance that has the
same effect, alongside muscle loss
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Together, these
substances make up H+E staining
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In an H+E staining, white sections signify glands
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The basement membrane has a structural purpose
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This scaffolding function becomes
apparent if the basement membrane is destroyed in a wound such as a burn, which takes
considerably longer to heal than if it was not destroyed
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Exocrine glands secrete their product via a duct (simple glands have a single duct,
compound glands have a branched duct)
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The following are types of exocrine glands:
 Merocrine glands function by fusing the vesicles containing their product with the
plasma membrane so that the product is released
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 Apocrine glands release their product in membrane-bound vesicles surrounded by
cytoplasm and plasma membrane
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Endocrine glands also use exocytosis, after which the product diffuses directly into the
bloodstream
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 Protein-secreting cells have a large nucleus, abundant rough endoplasmic reticula
and electron-dense vesicles
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 Steroid-secreting cells have a well-developed smooth endoplasmic reticulum,
abundant mitochondria for ATP synthesis and have lipid-containing vacuoles
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