Specialised tissues 1 Flashcards
Epithelial cell shape classifications
Squamous (flattened)
Cuboidal (roughly cube-shaped)
Columnar (pillar-shaped)
Epithelial cell layering classifications
Simple - single layer
Stratified - multi-layered
Epithelial cell classifications
Simple squamous Simple cuboidal Simple columnar Stratified squamous Pseudostratified
Explain the pseudostratified classification
- Falsely stratified
- Looks multi-layered but surface cells have contact with basal lamina
Two forms of cell-cell junctions
Zonulae (belts)
Maculae (spots)
Types of junction
Tight junctions Adhesion belts Desmosomes Gap junctions Synapses
Features of tight junctions
- Occluding junction
- Seals paracellular pathways
- Prevents molecules moving through space between cells
- Junction type closest to apex of cell
- Allows cells to establish and maintain apical-basolateral polarity by preventing mixing of proteins/lipids between different plasma membrane compartments
Features of adhesion belts
- Adherens junction
- Usually formed just basal to apical tight junction
- Transmembrane adhesion molecule is cadherin
- Cadherins associate with the actin cytoskeleton
- Controls assembly of other junctions
- “Master junction”
Cadherin properties
Part of a family of Ca2+ ion-dependent cell adhesion molecules
Features of desmosomes
- Spot junction
- Found at multiple points between adjacent cell membranes
- Linked to intermediate filament cytoskeleton
- Provides mechanical continuity between cells
Features of gap junctions
- Spot junction
- Clusters of pores
- Each pore formed of 6 identical subunits in membrane
- Pores are continuous with pores in adjacent cell membrane
- Allow passage of ions and small molecules between cells
- Opening and closing pores controls intercellular communication
Role of polarity in epithelial cells
Polarity in these cells allows functions to be unidirectional.
Difference in polarity between apical and basolateral domain allows direction to be established
Functions of epithelial cells
- Secretion (endocrine/exocrine)
- Fluid and solute transport
- Absorption
- Protection
Process of exocrine secretion
Secretory granules in apical domain move to and fuse with apical membrane to release contents into lumen or duct
- Golgi apparatus in middle of cell
- Extensive rough ER in basal cytoplasm
Process of endocrine secretion
Secretory granules in basolateral domain move to and fuse with apical membrane to release contents into bloodstream
- Golgi apparatus in middle of cell
- Extensive rough ER in the apical cytoplasm
Adaptations of epithelial cells for fluid and solute transport
- Passive transport of water and ions through ion and water channels in apical membrane
- Basal membrane has active transport transporters
- Many mitochondria in basolateral domain to supply energy to active transporters
- Transport direction apical to basal towards blood vessel
Adaptations of epithelia cells for absorption
- Microvilli increase SA
- Villi on small intestine surface increase SA
- Villi covered with intestinal epithelial cells
- Nutrient-transporting carriers found in microvillous brush-border membranes
Polarisation of fluid and transporting epithelia
- Ion pumps and channels have apical-basolaterla polarisation in plasma membrane
- Polarisation of transporter allows directional flow of fluids and ions
- Flow differs depending on function of cell (blood vessel to extracellular or other way round)
Polarisation of secretory epithelial cells
- Secretion polarised to ensure that secreted products are delivered to correct tissue compartment
- Unpolarised cells would have vesicles fusing and secreting their contents from nearest membrane regardless of function
Small intestinal epithelial cell turnover rate
Turnover every 3-5 days
Colon epithelial cell turnover rate
Turnover every 5-7 days
Epidermis (epithelium of the skin) turnover rate
48 days but dependent on region
Division process of epithelial cells in small intestine
- Cells lost from villus tip
- New cells produced by crypt stem cells
Differences between villus and Crypt of Leiberkhun
Villus - fold of cells of epithelia that extends out into lumen of small intestine
Crypt of Leiberkhun - Opposite of villus, epithelia invaginates away from lumen of small intestine lined with stem cells that produce new epithelial cells
