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Lecture 2: Epithelial Tissue - Coggle Diagram
Lecture 2:
Epithelial Tissue
Epithelial Tissue
covers body surface
lines hollow organs, cavities & ducts
forms the glands of the body
Key functions
selective barrier (filter/ limit, aid transport)
secretory (onto a free surface)
protective (especially from abrasions
Cells
Are arranged in continuous sheets as single or multiple layers
(held together by a variety of cell junctions):
Tight Junction
Adherens junction
gap junction
desmosome
hemidesmosome
Cytoskeleton
Comprised of a number of different proteins
Microfilaments (e.g Actin)
bundles beneath the cell membrane & cytoplasm
Function --> strength, alter cell shape & movement, links the cytoplasm to membrane, ties cells together, muscle contraction
Intermediate Filaments (e.g Keratin)
Function --> strength, moves materials through the cytoplasm
Types of epithelial tissue
Covering & Lining Epithelia
Inner covering
(blood vessels, ducts cavities, interior respiratory, digestive , urinary and reproductive systems
Outer covering
(skin & some internal organs)
Classifications
Arrangement
(layers)
simple
Single layer
secretion, absorption & filtration
stratified
2 or more layers
protective function
pseudostratified
appears to have multiple layers (but all cells are attached to the basement membrane).
Shapes
of cells
Squamous
flat & thin
Cuboidal
as tall as it is wide
Columnar
more tall than wide
transitional
stratified epithelium in which cells can change shape from cuboidal to flat (shape depending on organ shape
e.g urinary bladder
Glandular Epithelia
Endocrine gland
secrete directly into blood usually via traversing fluid
Example: pituitary, pineal, thyroid, parathyroid
distant effects
Exocrine gland
Secrete into ducts that empty onto the surface of a covering or lining epithelium
examples: sweat and salivary glands, oil glands, wax glands
local effects
Glands consist of a single cell or a group of cells that secrete substances into ducts onto surfaces or into blood
--> function = secretion
Classified according to where they secret there substances
Lateral Junctions
Desmosome Junctions
botton appearance
located in the lateral wall
had 'plaque' similar to adherens
gap is held open by transversing proteins
Cadherns span the gap
however link the cell surface to keratin (a cytoskeleton intermediate filament)
Keratin span from one desmosome to another on the other side of the cell
Function:
desmosomes help link the intermediate filaments between different cells
add connection between cells giving stability to epithelium
Most common in muscle cells
Gap Junctions
direct connection between cells
formed by the combination of 6 proteins in one cell and 6 proteins from another cell.
-These 6 proteins are called connexins
6 connexin protein molecules from a connexon or hemichanel
2 hemichanels make up the gap junction
gap junctions are very important in cardiac tissue
Adherens Junctions
belt like appearance
located more Basel in comparison to tight junctions
have a plaque layer of proteins on the inside of the cell to join the actin to cadherins
Cadherins --> span the gap
Catenins --> help link cadherins to actin (microfilament in the cell cytoplasm)
Function:
good for support and stopping the sharing of motion of the cells from moving on each other
prevents cell separation from tension forces (e.g contraction)
Tight Junctions
Located in the apical surface
"knitted appearance"
contains stands of trans membrane proteins holding the membrane together (more strands = tighter junction)
2 key proteins that bind into the membrane, joining the cytoskeletons of two adjacent cells:
claudins
occludins
This junction is considered electrochemically tight meaning ions can not pass easily between the cell (keeping the outside world out)
Most commonly found: stomach, intestines, bladder
Basel Junctions
Hemidesmosomes
button like appearance
-connects epithelia to basement membrane
links cellular basel intermediate filament (keratin) to basement membrane
Integrin --> linker protein binds too:
Laminin (in the basement membrane) & Keratin (intermediate filament in the cytoplasm)
Basement membrane
All epithelia overlay a basement membrane (acts like a platform)
comprised of two parts: 1.
Basel Lamina
2.
Reticular Lamina
Found between the epithelium & connective tissue
nutrients can pass through in order to feed the epithelium
nerves can pass through
but blood vessels are below the basement membrane as it is avascular
Functions:
supports the overlying epithelium
provides a surface along which epithelia cells migrate during growth and wound healing
acts as a physical barrier
participates in the filtration of substances in the kidney
Basel Lamina
secreted by epithelial cells.
contains collagen, laminin, other proteoglycans, glycoproteins, etc
Reticular Lamina
produced by the cells of the underlying connective tissue known as fibroblasts
contains fibrous proteins such as fibronectin, collagen etc