Introduction to Extracellular Matrix (ECM) and Its Components
Introduction to Extracellular Matrix (ECM)
The ECM is a complex network of macromolecules that acts as “cement” between cells in biological tissues. It forms part of highly specialized structures like cartilage, tendons, and bones. Beyond its supportive function, the ECM regulates the behavior of cells in contact with it, influencing their differentiation, development, migration, and shape. The ECM is produced by specialized cells called connective tissue cells.
Supporting Tissues
Supporting tissues broadly refer to the ECM and the cells embedded within it.
ECM-Producing Cells
These cells are primarily derived from the mesoderm, with some originating from the ectoderm. They share common structural features and possess cell adhesion mechanisms that interact with ECM materials instead of other cells. They represent fixed cell populations in supporting tissues where the ECM is the predominant component.
1. Mesenchymal Cells
These cells are morphologically stellate with long, thin extensions that form a network. They are located in the embryonic mesenchyme and some remain in adult tissues, primarily near blood vessels (adventitial cells). Due to their multipotency, they are often referred to as stem cells. Functionally, they have a high mitotic index and the ability to differentiate into various cell types.
2. Fibroblasts
These are common tissue-specific cells responsible for synthesizing the ECM. When fully differentiated, they cease ECM production and are known as fibrocytes. Structurally, they exhibit a stellate morphology in culture and a fusiform shape in tissues. They have a large, central nucleus and PAS-positive cytoplasm. Functionally, they synthesize ECM, resist aggression, actively divide, participate in tissue repair, and can transform into histiocytes, fat cells, and smooth muscle cells.
- Myofibroblasts: These cells share characteristics with fibroblasts and smooth muscle cells and are located in the adrenal glands, testes, and seminiferous tubules.
- Pericytes: These cells are located in the walls of some venules and capillaries and are involved in the formation of the basal lamina.
3. Chordoblasts/Chondrocytes
These are cells of the chordoid tissue that produce a specific ECM. When they stop actively synthesizing ECM, they are called chondrocytes. They have an ectodermal origin and resemble epithelial cells with little intercellular substance between them.
4. Chondroblasts/Chondrocytes
These are cartilage cells that produce a specific, solid, and non-mineralized ECM. When they stop synthesizing ECM, they are called chondrocytes. Structurally, chondroblasts are round or oval with a central nucleus and loose chromatin. Chondrocytes have a variable appearance and reside in spaces within the ECM called lacunae. All cells within an isogenous group originate from the division of a single chondroblast.
5. Osteoblasts/Osteocytes
These are the ECM-secreting cells in bone tissue. The newly formed material, called osteoid, is rapidly mineralized by the deposition of calcium phosphate salts. Osteoblasts have an epithelial appearance and exhibit signs of high metabolic activity, located on bone-forming surfaces. Osteocytes are cells with fine cytoplasmic processes and long extensions, an oval nucleus, and dense cytoplasm. They play a crucial role in regulating calcium and phosphate balance.
6. ECM-Producing Cells in Dental Tissues
- Ameloblasts: Produce enamel (ectodermal origin)
- Odontoblasts: Produce dentin (mesodermal origin)
- Cementoblasts: Produce cementum (mesodermal origin)
Macromolecules of the Extracellular Matrix
A) Fibrillar Proteins
1. Collagen
Collagen has a unique amino acid composition, rich in glycine, proline, and hydroxyproline. Collagen molecules consist of tropocollagen subunits, which have a triple-helical structure.
Types:
- Type I: Found in the deep dermis, tendons, and bones, forming high-strength fibers.
- Type II: Found in cartilage, forming fibers.
- Type III: Found in fetal skin, forming collagen and reticular fibers.
- Type IV: Found in the basal lamina, forming mesh-like structures.
- Type V: Found in small amounts with diffuse distribution, forming thin fibers.
- Type VI: Forms thin fibrils.
- Type VII: Forms short, striated fibrils anchored in the basal lamina of the skin.
Synthesis: Collagen synthesis begins within ECM-producing cells, and the polymerization of protein subunits into fibrils and fibers occurs in the extracellular space.
Collagen Fibers: These fibers appear white and form undulating bundles. They have high tensile strength, are flexible, and slightly elastic. They are found in supporting tissues that withstand significant tensile stress.
Reticular Fibers: These are collagen microfibrils coated with carbohydrates and lipids, preventing aggregation into larger fibers. They tend to form networks and are found in the liver, bone marrow, and lymphoid organs.
2. Fibrillin
Fibrillin is a fibril-forming glycoprotein and the principal component of extracellular microfibrils. It is associated with elastin and helps organize elastic fibers. It is found in the mesangium of renal glomeruli and the spleen.
3. Elastin
Elastin is an insoluble fibrous protein with tropoelastin as its structural unit. It is rich in cysteine, proline, and glycine. Elastogenesis occurs after the establishment of cross-links between tropoelastin molecules, forming desmosine and isodesmosine.
Elastic Fibers: These fibers are formed by the interaction between elastin and fibrillin. Fibrillin microfibrils organize secreted elastin, which is deposited among them to form individual fibers. Their most prominent property is elasticity. They can appear as fibers, networks, parallel sheets, or bundles.
4. Fibronectin
Fibronectin is a fiber-forming glycoprotein consisting of two chains linked by disulfide bonds. It appears in the ECM as aggregates and plays a role in cell adhesion and migration during embryonic development. It can bind to various tissue components, including collagen and heparin. Cell membranes contain receptors for fibronectin, which belong to the integrin family. Fibronectin participates in the orientation of collagen fibrils and the arrangement of proteoglycans.