Extracellular Matrix and Cell-Cell Junctions

Basal Lamina/ Basement Membrane

Layer 40 nm to 120 nm thick composed of laminin and type IV collagen that is tethered to the ECM by lamina and type VII collagen fibrils. Serves as a scaffold for regrowth following injury.

Laminin

Extracellular component that composes the basal lamina.

Fibronectin

A dimer that is held together by disulfide bonds at one end that bind to integrins on the cell surface. Each arm has functionally distinct domains linked with flexible polypeptide chains.

Elastin

A hydrophobic protein that has proline and glycine. Protein is made in ER and secreted in the ECM as tropoelastin and then cross-linked.

Collagen

Most abundant protein in mammals with 20 types. Provides tensile strength to tissues.

Pro-alpha chain

Component that composes Collagen that are produced by ribosomes. Undergoes hydroxylation and glycosylation and cross-linked with 2 other chains by hydroxyprolines and hydroxylysines. Secreted out of the cell as fibrils (0.5 um) or fibers (300 um)

Glycosaminoglycans (GAGs)

Macromolecule of the extracellular matrix. Resist pressure and provides tensile strength and makes hydrogel.

Proteoglycan

Glycosaminoglycans that are linked to proteins. Gel formed acts as a sieve and influences what makes it to the apical and basal surface. Plays role in cell signaling and binds to enzymes or proteins.

Hyaluronan

GAG not linked to core protein and found in all tissues and fluids of adults. It is produced at the cell surface by an enzyme complex in the cell membrane. Also serves as space filler during embryonic development.

Chondroitin, Heparan, and Keratin Sulfate

GAGs made in the ER and polysaccharides are added by the Golgi Apparatus. Up to 95% carbohydrate by weight and function is determined by core protein and GAG chains.

Fibroblasts

Cells that secrete the majority of the ECM molecules/proteins.

Cell Junction

Either Cell-Cell or Cell-Matrix contact. Three types: occluding, anchoring, and communicating junction.

Occluding Junctions

Prevents molecules from moving from apical to intracellular space. Includes the tight junction.

Anchoring Junction

Provides mechanical attachments. Includes Adherens junction, desmosomes, focal adhesions, and hemidesmosomes

Communicating Junction

Junction that allows for chemical and electrical signals to move between cells. Includes the Gap Junction.

Tight Junction

Type of Occluding Junction that prevents liquids from escaping between cells. Composed of the proteins claudins, occludins, and ZO proteins that are attached to actin.

Gap Junction

Type of communicating junction that allows for transport of ions, water, and other substances between cells. Composed of 6 connexin proteins that form connexons.

Adherens Junction

Type of anchoring junction that attaches to cell to cell. Composed of the proteins Cadherins and vinculin and alpha-actinin that are are attached to actin.

Desmosomes

Type of anchoring junction that attaches to cell to cell. Composed of the proteins cadherins and desmoplakin or plakoglobin that are either attached to the intermediate filaments keratin (in skin) or desmin (in the heart)

Focal Adhesions

Type of anchoring junction that hold cells to the extracellular matrix. Composed of the proteins vinculin, talin, focal adhesion kinase (FAK), and integrins that are attached to actin and laminin or fibronectin.

Hemidesmosomes

Type of anchoring junction that hold cells to the extracellular matrix. Composed of the proteins plectin and (beta and alpha) integrins that are attached to the intermediate filaments keratin and laminin.

Extracellular Matrix

The connective structure beneath and in between cells that they build. Unique to each tissue type.