Extracellular Matrix and Cell Junctions

tissues

• Cooperative assembly of cells and matrix woven together to form a distinctive multicellular unit with a specific function (nervous, muscular, epithelial, connective tissues)

extracellular matrix

• material that cells secrete around themselves

• Structural support, mechanical strength and medium for cell communication

cell junctions

• link cells together in the flexible epithelial tissues of animals

  • Junctions transmit forces either from cytoskeleton of one cell to the next or from the cytoskeleton of a cell to the extracellular matrix

Connective tissues

extracellular matrix is abundant and carries the mechanical load

composition of connective tissue ECM

• water

• macromolecules: GAGs, proteoglycans, proteins

Cellular adhesion

the ability of a single cell to stick to another cell or an extracellular matrix (ECM)

cell adhesion molecules

cadherins, Ig superfamily, selectins, integrins

cadherins

CAM, homophilic

functions: Cell-cell adhesion, tissue integrity, development

Ig superfamily

CAM, homophilic + heterophilic (integrins)

functions: Immune response, cell-cell recognition, neural connections

selectins

CAM, heterophilic (carbs)

functions: Leukocyte rolling, inflammation, neutrophil extravasation

integrins

CAM, heterophilic (ECM)

functions: Cell-matrix adhesion, migration, signal transduction

collagens

Triple-stranded, fibrous protein that is a major component of the extracellular matrix and connective tissues

procollagen

• collagen molecules in a precursor form w/additional peptide extensions at each end that obstruct premature assembly into collagen fibrils 

integrins

• transmembrane receptor proteins present on cell surfaces that enable cells to make and break attachments to the extracellular matrix, helping them to crawl through a tissue

fibronectin

• Extracellular matrix protein that helps cells attach to the matrix by acting as a “linker” that binds to a cell-surface integrin molecule on one end and to a matrix component, such as collagen, on the other

Glycosaminoglycans (GAGs)

• polysaccharide chain that can form a gel that acts as a “space filler” in the extracellular matrix of connective tissues; helps animal tissues resist compression

proteoglycans

• molecule consisting of one or more glycosaminoglycan chains attached to a core protein; these aggregates can form gels that regulate the passage of molecules through the extracellular medium and guide cell migration (diverse in size, shape + chemistry)

Tight junction

• seals neighboring cells together in an epithelial sheet to prevent leakage of molecules between them

• cell-cell adhesion, prevent leakage 

Adherens junction

•  joins an actin bundle in one cell to a similar bundle in a neighboring cell

• provides strength of epithelium, vesicles + cell shape change, movement 

Desmosome

•  joins the intermediate filaments in one cell to those in a neighbor

• keratins (IFs), cell-cell, 

Gap junction

• allows the passage of small water-soluble ions and molecules

• direct communication (channel across for molecules) 

Hemidesmosome

• anchors intermediate filaments in a cell to the basal lamina  

• cell-matrix (basal lamina) → IF (keratins), anchor to ECM

Epithelia (epithelium)

Sheet of cells covering an external surface or lining an internal body cavity

basal lamina

• Thin mat of extracellular matrix, secreted by epithelial cells, upon which these cells sit

apical surface

• Describes the top or the tip of a cell, structure, or organ

  • free and exposed to air or bodily fluid

basal surface

• Situated near the base; opposite of apical

attached to the basal lamina

laminin

• protein that provides adhesive sites for integrin molecules in the basal plasma membranes of epithelial cells cells 

Focal adhesion

• domains of the MP where actin filaments interact w/ECM molecules via integrins 

Type I collagen

• makes up 90% of the body’s collagen → form fibers

collagen I synthesis

Intracellular steps

1. Transcription in the nucleus

2. Translation ar the ribosomes on the rough ER

3. Hydroxylation of proline and lysine (requires vitamin C) 

4. Triple helix formation (procollagen) 

5. Packing in the golgi apparatus

6. Secretion through the cell membrane (secretory vesicle)

Extracellular steps 

7. Cleave by procollagen peptidases (removes ends)

8. Assembly intro fibrils w/staggered arrangement

9. Cross-linking by lysyl oxidase enzyme