Extracellular Matrix: Structure, Composition, and Function
Extracellular Matrix: Introduction
The extracellular matrix (ECM) is a complex network of macromolecules that act as a “cement” between cells, forming highly specialized structures such as cartilage, tendons, and bones.
The ECM supports cells and regulates their behavior, influencing differentiation, development, migration, and shape.
The ECM is produced by specialized cells called mech-producing cells, which are derived from the mesoderm and ectoderm.
Mech-Producing Cells
1. Mesenchymal Cells
Mesenchymal cells are stellate-shaped cells with long, thin extensions that form a meshwork. They are located in the embryonic mesenchyme and remain in some adult tissues, such as near blood vessels.
2. Fibroblasts
Fibroblasts are common tissue-specific cells responsible for synthesizing the ECM. They are stellate-shaped in culture and fusiform in tissues.
Fibroblasts can transform into histiocytes, fat cells, or smooth muscle cells.
3. Cordoblastos/Citos
Cordoblastos are cells that produce the specific ECM of chordoid tissue. When they stop synthesizing ECM, they are called chondrocytes.
4. Chondroblasts/Cytes
Chondroblasts are cells that produce the specific ECM of cartilage. When they stop synthesizing ECM, they are called chondrocytes.
5. Osteoblastos/Citos
Osteoblastos are cells that secrete ECM in bone tissue. The newly formed material, called osteoid matrix, is quickly transformed into bone by the deposition of calcium phosphate salts.
6. Cells Producing ECM in Dental Tissues
- Ameoblasts: enamel
- Odontoblasts: dentin
- Cementoblasts: cement
Macromolecules of the Extracellular Matrix
A) Fibrillar Proteins
1. Collagen
Collagen is a unique protein with a high content of glycine, proline, and hydroxyproline. It consists of tropocollagen subunits that form triple-helical structures.
There are different types of collagen, each with a specific composition and location:
- Type 1: found in tendons, bones, and deep dermis
- Type 2: found in cartilage
- Type III: found in fetal skin
- Type IV: found in basal lamina
- Type V: found in small numbers in various tissues
- Type VI: found in thin fibrils anchored in the basal lamina of the skin
2. Fibrillin
Fibrillin is a glycoprotein that forms extracellular microfibrils. It is associated with elastin and is found in the mesangium of renal glomeruli and spleen.
3. Elastin
Elastin is an insoluble fibrous protein that forms elastic fibers. It is composed of tropoelastin molecules that cross-link to form desmosine and isodesmosine.
4. Fibronectin
Fibronectin is a fiber-forming glycoprotein that appears in the ECM as aggregates. It plays a role in cell adhesion and migration.