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EPITHELIA AND CELL JUNCTIONS - Coggle Diagram
EPITHELIA AND CELL JUNCTIONS
WHAT ARE EPITHELIA?
avascular tissues composed of cells
usually organised into
sheets or tubules
EPITHELIAL SHEETS CAN BEND TO FORM A TUBE OR VESICLE
SHEET OF EPITHELIAL CELLS LINKED ACROSS WHOLE SHEET
(by an adhesion belt with associated actin filaments)
INVAGINATION OF EPITHELIAL SHEET CAUSED BY AN ORGANIZED TIGHTENING ALONG ADHESION BELTS IN SELECTED REGIONS OF CELL SHEET
EPITHELIAL TUBE PINCHES OFF FROM OVERLYING SHEET OF CELLS
EPITHELIAL TUBE -
HUGELY IMPORTANT IN DEVELOPMENT
how CNS and EYE is formed
Epithelial movements are crucial during embryonic development
attached to an underlying ECM
basement membrane
cover both the
internal
and
external
surfaces of the body
some are modified to form
glandular structures (exocrine glands)
TYPES OF EPITHELIA
simple
stratified (multiple layers) eg. skin
oesophagus
many layers originating from basal generative zone
columnar
small intestine
cuboidal
kidney tubules
squamous
lung alveolus
Simple or statified organisations can be made of columnar, cuboidal or squamous cells
FUNCTIONS OF EPITHELIA
mechanical protection (eg. skin) - damage
permeability barrier (eg. small intestine - prevents uncontrolled movement
absorption (eg. small intestine)
filtration (eg. epithelium of renal corpuscle) - pressure filtration
secretion (eg. exocrine sweat glands) - specialised component of skin
diffusion of gases or fluids (eg. lung alveoli)
sensory (eg. retina) - light sensitive, sound by hair in ears
Can have complex biochemical functions. Epithelia cover the external surface of the body and line all its internal cavities
polarised
Apical
basolateral region
basal
Basal lamina is a thin tough sheet of extracellular matrix made of type IV collagen and laminin proteins which provide adhesive sites
APICAL BASAL POLARITY ARE SPECIALISED SURFACES
HOW IS PHYSICAL INTEGRITY MAINTAINED
epithelia are held together by
cell junction
anchoring junctions
- linking cells together or to the extracellular matrix
anchoring/strengthening junctions link cells together or to the extracellular matrix
adherens junction
actin filaments
cadherin
protein plaques holding together cells
mediate cell-cell attachment
undergo homophilic binding
cadherin molecule + linking protein - linking protein attached to cytoskeletal filament
alpha-catenin
beta-catenin
link Cadherins to the
actin filaments of the cytoskeleton
- can be contractile
desmosome
intermediate filaments
cadherin (desmoglein)
plakoglobin
desmoplakin
link cadherins to intermediate filaments of the cytoskeleton
cadherin family proteins
cytoplasmic plaque made of intracellular anchor proteins
intercellular space
keratin filaments anchored to cytoplasmic plaque
interacting plasma membranes
hemidesmosome
intermediate filaments
integrin/collagen
dystonin
anchor epithelial cells to the basal lamina
focal adhesion
actin filaments
integrin
play a central role in mediating cell-matrix contacts
keratin filaments
intern molecules
basal plasma membrane of epithelial cell
basal lamina
focal ahesion kinase
occluding junctions
- seal the gaps between cells (epithelium acts as a total barrier)
TIGHT JUNCTION
claudin occludin
specialised for sealing proteins
Arranged in strands along lines of junctions to create the seal
basal lamina
intestinal epithelial cell
zona occludin scaffold protein
barrier function
: regulate passage of ions, water and macromolecules - seal neighbouring cells together
fence function
: maintains cell polarity (apical basal)
occluding/tight junctions help maintain cell polarity
mesh of sealing strands of occludin and claudin proteins seals sides from each other
ECM can project up but can't get through
Also function as sites for assembly for complexes of intracellular proteins that govern the apical-basal polarity of the cell interior
channel-forming junctions
create passageways linking the cytoplasm of adjacent cells
allow ions and small molecules to pass directly from cell to cell with no association from cell to cell
eg.
cardiac muscle
- gap junctions allow the passage of ions this permits changes in membrane potential to pass from cell to cell = rhythmic contraction of the heartbeat
membrane proteins from adjacent cells line up to form a channel
allow passage of ions
connexion composed of six subunits
PLASMODESMATA
Plant tissues lack all cell junctions discussed, held together by cell walls
Water filled channels (?)
signal-relaying junctions
- allow signals to be communicated from cell to cell (nervous system)
eg. axon and dendrite
ORGANISATION OF CELL JUNCTIONS ON EPITHELIA
tight junction
defines apical from basolateral
seals neighbouring cells together in an epithelial sheet to prevent leakage of molecules between them
adherens junction
towards apical but in lateral
joins an actin bundle in one cell to a similar bundle in a neighbouring cell
desmosome
scattered centrally
joins the intermediate filaments in one cell to those in a neighbour
gap junction
relatively basal
forms channels that allow small water-soluble molecules, including ions, to pass from cell to cell
hemidesmosome
always basal
anchors intermediate filaments in a cell to the basal lamina
a specialised site on a cell at which it is attached to another cell or the extracellular matrix
DISEASES
DEFECTIVE DESMOSOMES
pemphigus vulgaris
severe blistering
dehydration and infection
death
autoimmune destruction of desmosomal protein
WHAT CAUSES SCURVY?
Vitamin C deficiency = defective collagen synthesis
Vitamin C is a cofactor for propyl hydroxylase
HYDROXYPROLINE
stabilises triple stranded collagen
Fibrodysplasia ossificans progressiva
muscle and connective tissue like tendons are gradually replaced by bone (ossified)
LOSS OF FUNCTION OF TIGHT JUNCTIONS IS ASSOCIATED WITH MANY HUMAN DISEASES
BARRIER FUNCTION
CROHN'S DISEASE
inflammation of the bowels
permeability disorder
FENCE FUNCTION
CANCER
loss of cell polarity and cell contacts (epithelial-mesenchymal transition - EMT)
increase in motility and eventual metastasis
ABNORMAL GAP JUNCTIONS
CATARACTS (connexion 50)
VOHWINKEL SYNDROME (connexion 26)
SIGNAL RELAYING JUNCTIONS
Myasthenia Gravis
autoimmune destruction of neuromuscular junction
first indication of disease is a droopy eye
severe muscle weakness