Epithelia Flashcards
What is epithelia?
Boundary between controlled internal environment and uncontrolled external environment
Present in every organ
- Covers surfaces
- Lines cavities e.g. mouth
- Forms glands
What are the functions of epithelia?
Protection
Barrier
Diffusion
Absorption
Secretion
What are the common properties of epithelia?
Epithelial cells form dynamic barrier - able to import or expel substances
- Connection via tight junctions
- Distinct apical and basolateral domains (polarised)
- Lacks blood vessels (avascular)
- Lacks extracellular fibres
- Minimal EC space
What is polarity of epithelia?
Apical-Basal polarity
Apical membranes - in contact with external environment e.g. GI lumen
BLM - in contact with ECF compartment (BM)
There is specialisation on both surfaces
What is the basement membrane?
- Separates cells from underlying connective tissue
- Made from collagens, laminins, proteoglycans
- Provides structural support and organisation
- Anchors epithelial cells to underlying connective tissue
Why are junctional complexes important?
For cell adhesion and communication
What are tight junctions?
- Joins adjacent cells together
- Impede paracellular movement
What are claudins?
Protein strands that determine the tightness of tight junctions
Very tight = high barrier function e.g. renal TAC
or Leaky e.g. renal PT
The combination of claudin genes present determines permeability
What are adhering junctions?
- Form a belt around the cell under the TJ
- Linked with actin filaments and cadherins
- Disruption decreases the organisation - can lead to metastasis in epithelial tumours
What are gap junctions?
- Permit small molecule diffusion between cytoplasm of neighbouring cells
- Provides lateral communication
- Therefore cells electrically coupled due to ion movement
What are desmosomes?
- Form strong adhesion points between cells
- Have EC cadherin domains
- Have anchor proteins that link cadherin to intermediate filaments
- Myosin filament interactions for contraction
How are cells anchored to the BM?
Actin-linked cell-matrix junctions
Hemidesmosomes
Why is there continuous replacement of cells?
One side of epithelia in contact with hostile external environment
(Also high amounts of abrasion)
- Replacement by stem cells
Intestine self renewal = 5 days
Lung epithelium = 6 months
What are the two main types of epithelia?
Simple - single cell layer e.g. lung
Stratified - many cell layers e.g. skin
What is pseudo-stratified epithelia?
One tall cell layer - two nuclei layers
e.g. upper respiratory tract
What is simple squamous epithelia?
- Appears as thin scales
- Facilitates rapid molecule passage
- Secretion/absorption via AT
What is simple cuboidal epithelia?
- Secretion/absorption via AT
- Many have single, non-motile cilium on AM - deformation senses fluid flow
What are the types of columnar epithelia
Simple columnar - majority of GI tract
- Can have cilia or microvilli
- AT
Pseudostratified columnar
- Single layer
- All touching BM
What are the types of stratified epithelia?
Stratified squamous
- Most common type of stratified e.g. skin
- Apical cells appear squamous
- Basal cells appear cuboidal or columnar
Stratified cuboidal
- Glands
Which type of epithelia appears in areas of high abrasion?
Stratified squamous
What is stratified columnar epithelia?
Allows tissue stretch / contraction
e.g. conjunctiva, pharynx, anus male urethra
What is transitional epithelia?
Cells are rounded when relaxed
Facilitates shape change in distension without damage to epithelial lining
e.g. Bladder empty (rounded cells) bladder full (distended cells)
What are exocrine glands
- Multicellular glands
- Secrete directly onto epithelial cells
- Can have complex ducts
What are endocrine glands?
- Have single cell types or multiple cell types
- Secrete directly into blood
- Ductless
