Epithelia & cell junctions Flashcards
What are epithelia?
Avascular tissues composed of cells, usually organised into sheets/tubule, attached to an underlying ECM basement membrane
What are epithelia modified to form?
Glandular structures (exocrine glands)
How many types of epithelia are there?
What are they?
5
Layers:
> simple
>stratified
Cell shape:
>columnar
>cuboidal
>squamous
Where are simple cuboidal epithelia found?
Around kidney tubules
Where are simple columnar epithelia found?
Small intestine
Where are squamous epithelia found?
Lung alveolus
Where are stratified epithelia found?
Oesophagus
What are the 7 functions of epithelia?
> mechanical protection > permeability barrier > absorption > filtration > secretion > diffusion > sensory
How are epithelia polarised?
Apical-basolateral
How are the apical & basal membranes different?
Composition of lipids & membrane proteins
How is the cytoskeleton organised in epithelia?
So that secreting vesicles are produced & driven towards apical surface ready to be secreted
How is physical integrity of the epithelium maintained?
Epithelia held together by cell junctions
= specialised site attached to another cell or extracellular matrix
What are the 4 different types of cell junctions?
> anchoring junctions
occurring junctions
channel-forming junctions
signal-relaying junctions
What is the role of anchoring junctions?
Linking cells together or to the ECM
What is the role of occluding junctions?
Sealing gaps between cells
What is the role of channel-forming junctions?
Creating passageways (for ions & small molecules) linking the cytoplasm of adjacent cells
What is the role of signal-relaying junctions?
Allowing signals to be communicated from cell to cell
What are the 2 types of ‘strengthening’ junctions?
(between cells)
What are the 2 types of anchoring junctions?
(between cell & basal lamina)
> Adherens junction
Desmosome
> Focal adhesion
Hemidesmosome
What are the 3 components of an adherens junction?
> Actin filaments
> Cadherin
> alpha & beta catenin
What are the 3 components of a desmosome?
> intermediate filaments
> cadherin (desmoglein)
> plakoglobin & desmoplakin
What are the 3 components of a focal adhesion?
> actin filaments
> integrin
> focal adhesion kinase
What are the 3 components of a hemidesmosome?
> intermediate filaments
> integrin/collagen
> dystonin
What do cadherins do?
Mediate cell-cell attachment
Which type of junction links cadherin to the actin filaments of the cytoskeleton?
Adherens junction
How can epithelial sheets form a tube/vesicle?
Invagination of sheet due to an organised tightening along adhesion belts in selected regions of sheet
–> epithelial tub pinches off from sheet of cells
Which type of junction links cadherins to intermediate filaments of cytoskeleton?
Desmosomes
What is the role of integrin?
Which junction is it found in?
Mediates cell-matrix contacts
Hemidesmosomes
- anchor epithelial cells to basal lamina
Give an example of a disease caused by defective desmosomes
What are the symptoms?
Pemphigus vulgaris
- autoimmune destruction of desmosomal protein
> severe blistering > dehydration & infection - gaps open up between cells & fill w/ fluid --> blisters burst = infection > death
What are the 2 functions of occluding junctions?
Barrier: regulate passage of ions, water & macromolecules
Fence: maintains cell polarity
What are the 2 components of occluding junctions?
> Claudin & occludin
> Zona occludin scaffold protein
Which disease is loss of barrier function in occluding junctions associated with?
Crohn’s disease
- inflammation of bowels
- permeability disorder
Which disease is loss of fence function in occluding junctions associated with?
Cancer
- loss of cell polarity
- loss of cell contacts
- increase in motility
- eventual metastasis
What are the components of channel-forming junctions?
Connexin formed of 6 subunits
Explain the role of channel-forming junctions in cardiac muscle
Allow passage of ions
–> permits changes in membrane potential to pass from cell to cell
= rhythmic contraction of heartbeat
Which inherited disorders can abnormal gap junctions cause?
Cataracts
Vohwinkel Syndrome
What causes cataracts?
Dominant mutation in CX50
= no gap junctions, but hemichannels form
–> disrupted connections
What causes Vohwinkel Syndrome?
Upregulated CX26 in epidermis
–> hyper proliferation
(over production of epidermal cells)
What are channel-forming junctions in plants called?
Which organelle projects across them?
Plasmodesmata
- cytoplasm physically connected across junction
Smooth endoplasmic reticulum
What are the components of a signal-relaying junction in a neuromuscular junction between 2 neurones?
> cadherin
> neuroligin & neurexin
What happens when signal-relaying junctions are lost?
What are the symptoms?
Myasthenia Gravis
= autoimmune destruction of neuromuscular junction
> droopy eye
severe muscle weakness
