Exam 3 Study Guide - ECM/Junctions Flashcards
THE EXTRACELLULAR MATRIX
- A complex network of macromolecules in the extracellular space
- Secreted by the cells which inhabit it
- Gives structural and organizational support to tissues
- Makes up a substantial part of tissue volume
Collagen
30% of total body protein, 90% of it is Type I
• Triple-helical structure, strengthened and stabilized by crosslinks
• Fibrillar and nonfibrillar collagens
• Different types and combinations of collagen molecules predominate in different tissues
• Collagen is frequently remodeled, and synthesis is affected by physiologic conditions such as diet or mechanical stress
• Osteogenesis/dentogenesis imperfecta is the result of a genetic mutation that causes the glycine in Type I collagen to be replaced by another amino acid
Elastin
provides flexibility and resilience to the ECM
• Predominant ECM molecule in blood vessels
• Found in tissues that require elasticity
• Stabilized by two-way crosslinks
• Not synthesized after adolescence
Fibronectin
facilitates communication between intracellular and extracellular
compartments
• Binds to other ECM molecules
• Binds to integrin molecules on cells
• Important for cell adhesion, growth, migration, and wound healing
• Soluble and insoluble forms – interaction with oral pathogens
Laminin
found primarily in basal lamina
• Gives structural support to the ECM
• Major component of basement membranes in epithelial and muscle tissues
• Binds to cells, often through the integrin family of proteins
• Diseases of abnormal laminin
Proteoglycans
the gel-formers of the ECM
• Structure of molecules, diversity
• Have many negative charges, which attract cations and water. The result is hydration of the ECM, which provides rigidity and durability.
• Proteoglycans are <10% of ECM protein, but because of their hydrophilicity, GAG chains fill most of the extracellular space
• Aggrecan
• Can also bind growth factors, regulate enzyme activity, and act as co-receptors
• Lysosomal storage diseases
CELL JUNCTIONS:
- Specialized regions where cells are attached to each other and the ECM
- Epithelial tissues are particularly rich in cellular junctions
Tight Junctions
Cell-cell junctions found in all epithelia.
• Severely limit intercellular permeability and maintain cell membrane polarity (different protein components of apical and basolateral membranes)
• Formed by sealing strands anchored in the membrane
• Required for active transport of solutes across the epithelial barrier
• All tight junctions are impermeable to macromolecules, but permeability to small molecules varies greatly, depending on the number of sealing strands and the different transmembrane proteins (occludin and claudin families) that comprise them.
Anchoring junctions
Bind cells to each other and to the ECM. They are found in all tissues, especially those subject to mechanical stress.
Adherens Junctions
- Cell-cell junction found primarily in epithelia
- Form a continuous adhesion belt just below the tight junction
- Transmembrane proteins are cadherins
- Connect bundles of actin filaments – contractile
- Necessary for tight junction formation and epithelial tube formation in morphogenesis
Desmosomes
- Cell-cell junctions that “rivet” cells together
- Found primarily in epithelia and muscle cells
- Contribute to tissue strength and durability
- Transmembrane proteins are cadherins
Hemidesmosomes
- Resemble half-desmosomes, but bind cells to the ECM
- Transmembrane adhesion proteins are the integrins
- Attach the junctional epithelium to the tooth enamel
Focal Adhesions
- Spot-type junctions that link the cell cytoskeleton to the ECM
- Can contain as many as 100 different proteins
- Dynamic: they can form, change, or disappear in response to intracellular or extracellular signals
- Transmit external mechanical signals via integrin receptors
- Important in cell migration, motility, differentiation, proliferation, and orthodontic tooth movement
Gap Junctions
communicating junctions found in nearly all cells except skeletal muscle and blood cells
• Allow small molecules and electrical charges to pass directly from cell to cell
• Channels, called connexons, are hexamers of connexin proteins
• Connexon permeability varies with the identity and combinations of connexins present
• Gap junction permeability can be regulated by both intra- and extra-cellular signals
• Allow the cells of a given tissue to function together quickly and in a coordinated fashion
