Lecture 16- Tissue organization Flashcards
Cellular Tissues
- Composed mostly of cells
- Cell-cell and cell-ECM interactions (adherens junctions) in conjunction with interactions with the cytoskeleton within the cell provide the strength/integrity of the tissue
Connective Tissues
- Composed primarily of ECM with a few cells
- Cells synthesize and bind to the ECM that surrounds them
- Physical properties of tissue are provided by organization of the ECM
Junctions of cellular tissues
1) Tight junction
2) Adherens junction
3) Desmosome junction
4) Gap junction
5) Hemidesmosome junction
6) Focal adhesions (Cell-ECM junction)
Adhering junctions (anchoring)
(Adherens, desmosomes, hemidesmosomes, focal adhesions)
Maintain the tissue when subjected to mechanical stress
Three basic units of adhering junctions
1) Transmembrane glycoprotein component–interacts either with proteins on adjacent cells or with proteins of the ECM
2) A complex of proteins on the cytoplasmic face of the junction forming a plaque–mediates the association between the membrane protein and the third component of the junction
3) Cytoskeleton network–may be actin filaments or intermediate filaments (without this the glycoproteins could just be pulled out of the membrane!!)
Adherens junctions
- Cell-cell junction
- Cadherins are the major transmembrane protein component (glycoprotein)
- Associate with other Cadherins via homophilic interactions
- Actin filaments around the perimeter
Role of Adherens in development
- Positioning of adherens junctions toward the apical surface of cells allows for oriented contractions of the actin filaments to initiate an invagination of an epithelial sheet which can pinch off the form a tube
- Formation of neural tube during embryogenesis
Focal adhesions
- Cell-matrix anchoring junction
- Integrins are the transmembrane glycoproteins (alpha, beta heterodimeric proteins)
- Actin cytoskeleton
- Heterophilic type junction
- Interact with surrounding extracellular matrix
- Important in cancer cell migration, metastasis, immune surveillance, and tissue repair
Desmosomes
- Intermediate filament based adhering junction
- Cell-cell anchoring junction
- Cadherins are the gylcoprotein (homophilic interaction)
- Plaque proteins interact with intermediate filament cytoskeleton
Hemi-desmosomes
- Intermediate filament based adhering junction
- Cell-matrix anchoring junction
- Integrin a6B4 is the major glycoprotein
- It’s the only integrin to associate with intermediate filaments
- Link epithelial cells to the underlying basal lamina
- Heterophilic interaction
Pemphigus
Autoimmune disease to skin desmosomal cadherin causing destabilization of cell-cell interactions
Can cause severe blistering
Epidermolysis bulosa simplex
Defect in intermediate filament assembly causing loss of integrity of desmosomes and hemidesmosomes
Can cause severe blistering
Tight junctions
Two functions:
- Join neighbors to form permeability barrier across epithelial sheets
- Maintain the cells polarity
- Continuous strands of a transmembrane protein called claudin and occludin that interact with the same protein on the membrane of the adjacent cell (homophilic interaction)
- Cytoskeleton not required for integrity of tight junctions
Gap Junctions
-Facilitate communication between neighboring cells
-Apparent in tissues comprised of electrically excitable cells
-Can help set up concentration gradients
-Protein component is connexins
-6 connexins make a connexon
-Junction is composed of many individual connexons associating
-Permeability regulated by pH and Ca levels
(high Ca, low pH= closed)
Connective tissues
- Synthesized by a small number of cells
- Strength provided by ECM
- In many connective tissues cells called fibroblasts generate ECM
