Epithelia and cell junctions Flashcards
What are epithelia?
Avascular tissues composed of cells
What are epithelia usually organised into?
Sheets or tubules
What are the sheets/tubules that epithelia are organised into connected to?
An underlying extracellular matrix basement membrane
What are the five main types of epithelia?
Simple, stratified, columnar, cuboidal and squamous
What is an example of simple cuboidal epithelia?
Kidney tubules
What is an example of simple columnar epithelium?
Small intestine
What is an example of squamou epithelium?
Lung alveolus
What is an example of stratified epithelium?
Oesphagus
What are the functions of epithelia in the small intestine?
Permeability barrier and absorption
What are some other functions of epithelia?
Filtration, secretion, diffusion of gases or fluids and mechanical protection
What are epithelia held together by?
Cell junctions
What is a cell junction?
A specialised site on a cell at which it is attached to another cell or the extracellular matrix
What are the four functional groups of cell junctions?
Anchoring, Occluding, Channel-forming and Signal-relaying
What do anchoring junctions do?
Link cells together or to the extracellular matrix
What do occluding junctions do?
Seal the gaps between cells
What do channel-forming junctions do?
Create passageways linking the cytoplasm of adjacent cells
What so signal-relaying junctions do?
Allow signals to be communicated from cell to cell
Role of adherens junctions?
Link cadherins to the actin filaments of the cytoskeleton
Role of desmosomes?
They link cadherins to intermediate filaments of the cytoskeleton
What is the role of focal adhesions?
They interact with actin filaments within the cell, and anchor the cell to the basal lamina
What do cadherins mediate?
cell-cell attachment
How do cadherins mediate cell-cell attachment?
A cytoskeletal filament is attached to a linker protein, which is in turn attached to a cadherin molecule. Cadherin molecules then attach to each other in the extracellular space
How do epithelial sheets form a tube or vesicle?
They bend
How do occluding junctions help maintain cell polarity?
They seal cells in a way that prevents small molecules from passing through the barrier–> maintains concentrations of molecules on either side of the barrier
What do channel junctions allow ions and small molecules to do?
Pass directly from cell to cell
Example of channel forming junctions in an organ?
Gap junctions allow the passage of ions through them–> permits changes in membrane potential pass from cell to cell–> rhythmic contraction of the heartbeat
What are channel forming junctions in plants called?
Plasmodesmata
What do signal relaying junctions allow?
Communication of signals between cells
Tight junction?
Seals neighbouring cells together in an epithelial sheet to prevent leakage of molecules between them
Adherens junction?
Joins an actin bundle in one cell to a similar bundle in a neighbouring cell
Desmosome?
Joins the intermediate filaments in one cell to those in a neighbour
Gap junctions?
Forms channels that allow small water-soluble molecules, including ions, to pass from cell to cell
Hemidesmosome?
Anchors intermediate filaments in a cell to the basal lamina
