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Epithelia
Gwen V. Childs, Ph.D.
Graduate Microanatomy
Common features of epithelia
• Cells are connected to one another providing a lining for a surface or a hollow organ or tube
• Sit on a layer of fine filaments, called a "basal lamina".
• Form a boundary between the external environment and the remainder of the organ.
• Control movement of substances into and out of that organ.
How are epithelia classified?
Depends on number of layers
One layer
Simple
Two or more layers
Stratified
Pseudostratified
Multilayered nuclei
Classification depends on shape
Flattened, scale-like
Squamous
Cells are cubes Cells are columns
Cuboidal Columnar
lumen
lumenlumen
Transitional Epithelium (Bladder)
Pseudostratified Columnar epithelium
Importance of cell shapeFlattened, squamous
cells
Reduced surface for passive transport
across cells
Can be stacked toprovide protective
layers
Taller, cuboidal or columnar cells
often expressstructural and
functional polarity
Apical surface functions
Basal/lateral surface functions
Absorption
secretion
Water and iontransport
Enzymeactions
receptivitysecretion
junctionsIon transport
communication
sensory
Importance of layering
Protect against friction and injury
Multilayered epithelia:
Lower layers regenerate upper layers
Barrier to water, diseasesome toxins, etc
Single layered epithelia:
Important in regulatedtransport of cells/molecules
Communication/gateway
Key cell structures at each surface
Microvilli, Cilia,Stereocilia
Specialized junctions
(Cellular interdigitations)Basal lamina + receptors
Increase surface area of cells 15-30X; Covered by glycocalyx; enzymes important in absorption are associated with this region
Core Actin filaments
Actin filamentsare held in bundlesby actin-bindingproteins: fimbrin, villin, or fascin
Actin filamentsare held at the tipof microvilli by amorphousanchoring proteins;capping preventsdepolymerization
Actin is boundto myosin along lateral walls
Terminal WebBase of
microvillus
Microvillus actin binds to more actin running horizontally.
Underneath the actin filaments are cytoskeletal filaments,also running horizontally
Terminal web actin is cross-linked byspectrin.
CiliaNine microtubuledoublets surrounda central pair
Doublets connectedby Dynein arms thatwalk along adjacent microtubule
Nexin spokes radiateinward, connectingthe doublets to the central pair
Specialized Microvilli
• Stereocilia are long microvilli
• Found in absorptive/secretory cells in epididymus (non-motile)
• Also found on special sensory cells in ear (cochlea)– Bending is a stimulus for sound or
position sensation.
Cilia/flagellar functions
• Move egg down fallopian tube
• Move Sperm
• Move mucous in respiratory tract.
• Olfactory epithelium– Bear receptors that bind chemicals– This receptor binding is the stimulus for
smell
SEM view of cilia and microvilli
Can you classify this tissue?
What kind(s) of junction(s) are in stratified squamous epithelium?
Anchoring junctions at thelateral surface
• Bind cells together via special ligands and their cytoskeletal systems – ligands are cadherins
• Junction differentiated by type of microfilament– Actin: adherent junctions– Intermediate filaments: desmosomes
Anchoring junctions : Desmosomes
• Like a spot weld or button.– Formed by a dense intracellular plaque on each
side into which keratin filaments are looped. (desmoplakins; plakoglobin; 75 kD polypeptide; desmoglein)
– Cadherin molecules (“calcium dependent adhesion molecules”) extend across the extracellular space to bind the cells together. (Desmocollins-I, II & desmogleins)
Desmosome
Two dense plaques:one on each cellmembrane
Cadherins connect the two plaques via homophilic binding
Cadherin bindingrequires Calcium
Intermediate filamentsloop into plaque
Adherent junctions
Two dense regionsone on each cellmembrane (alphaactinin)
Cadherins connect the two plaques via homophilic binding
Cadherin bindingrequires Calcium
Actin connects with dense regions on membrane
Pemphigus: skin disease
• Patients make antibodies against one of their desmosomal cadherans.
• Antibodies bind to desmosomal sites (only in skin and oral mucosa)
• disrupt connection between skin cells.
• Body fluid leaks into epithelial cells and causes blistering.
• Can be fatal if left untreated
Penphigus vulgaris
Penphigus Vulgaris
• Skin lesion histology:– Notice that the
separation occurs in that layer where the desmosomes are abundant.
Can you classify this epithelium?
Junctional complex
• Tight (occluding) junctions– Zonula occludens
• Zonula adherans (belt-like zone)• Desmosome
• Found frequently in columnar-type epithelia• Prevents entry between cells and helps to
maintain cell polarity
Microvilli
Occluding junction
Gap Junction
Specializations at the basal surface
• Basal/Lateral interdigitations
• Hemidesmosomes– Interactions between intermediate
filaments and basal lamina
• Focal contacts– Interactions between actin filaments
and basal lamina
Basal/Lateral Interdigitations
• Cells sit on multiple processes (like an octopus).
• These processes interdigitate with one another.
• Greatly increase surface area. • Process filled with mitochondria• On membrane are Na/K pumps (ATPase’s)
(important for active transport of sodium and water conservation)
Anchoring junctions: Hemidesmosomes
• Like a half desmosome at the base of the cells • links to basal lamina
– A plaque anchors intermediate filaments inside the cell--ends are buried in plaque (contains desmoplakin-like protein).
– anchoring proteins called “integrins”, bind from the keratin molecules inside to laminins in the basal lamina. (Integrins are receptors for specific extracellular matrix proteins). Bind like “velcro”
Anchoring junctions at thebasal surface
• Bind cells to extracellular matrix– ligands are integrins
• Junction differentiated by type of microfilament– Actin: focal contacts– Intermediate filament: hemidesmosome
Dense plaque (desmoplakin)
Cytoskeletal filaments(intermediate)anchored to plaque
Integrin molecules: Transmembrane receptorsfor extracellular matrix proteins (fibronectins,laminins, etc) found in basal lamina
fibronectin
Plasma membrane
Basallamina
Thickenedarea on membrane
Actin filamentsanchored to plaque
Integrin molecules: Transmembrane receptorsfor extracellular matrix proteins (laminins) found in basal lamina
laminin
Plasma membrane
Basallamina
What are integrins?• Extracellular matrix receptors in the cell membrane:
– Affinity relatively low (Ka= 106--109 liters/mole): Why would that help the cell?
– Depends on extracellular divalent cations (Ca++ or Mg++)
– binding activates signalling cascades
• Composed of two glycoprotein subunits: alpha and beta. Combination and types of subunits may dictate selectivity of binding.
How do integrins function, in general?• Regulated adhesion via integrins
controls route and movement of cells in the body.
• Could you apply this to wound healing?