**2.4. Fibronectins**

lular interstitial space and they have a wide variety of functions that reflect their unique

Collagens are the most abundant proteins in ECM and the whole human body [5]. Collagens have many functions, which depend on the type and tissue they are in. Depending on the tissue, collagen fibers can provide tensile strength, can regulate cell adhesion, can support chemotaxis and migration and can direct tissue development [6]. Generally in normal phys‐ iologic states, different types of collagen fibers form a heterogeneous mixture but usually there

Type I: skin, tendon, vascular ligature, organs and bone (main component of the organic part

A majority of collagen molecules are in the form of triple strands, which form supramolecular complexes like fibrils and networks depending on the type of collagen. Network collagens are incorporated into the basal membrane and fibrous collagens form a skeleton for the collagen

Elastins are the main ECM element, which gives tissues elasticity and allows a tissue to stretch and return to its original state if needed. Their tight association with collagen fibrils crucially limits their stretchability. Fibroblasts and smooth muscle cells secrete elastin in the form of its precursor, tropoelastin. Secreted tropoelastin molecules assemble into elastin fibers. Elastin fibers are covered by glycoprotein microfibrils. The most common glycoprotein covering elastin fibers is fibrillin. The presence of fibrillins is also essential for the integrity of elastin [1].

Laminins are glycoproteins that form heterodimers containing 1 α, 1 β and 1 γ chain. They are synthesized in podocytes and endothelial cells. Laminin trimerization occurs inside the cell in the endoplasmic reticulum. Once trimerization completed they are secreted into the extracel‐

buffering, hydration, binding and force–resistance properties.

224 Composition and Function of the Extracellular Matrix in the Human Body

**2. Fiber-forming ECM elements**

is a dominant type of collagen in every given tissue.

Type II: main collagenous component of cartilage

Type IV: forms basal lamina and basement membrane

Type III: main component of reticular fibers

Type V: placenta, cell surfaces and hair

fibril bundles in the interstitium [1].

The five most common collagen types are the following:

**2.1. Collagen**

of bone)

**2.2. Elastins**

**2.3. Laminins**

Fibronectins are glycoproteins that establish connection between cells and collagen fibers in the ECM. It is secreted as a dimer form, which are joined by two disulfide bonds. Fibronectin fibers have several binding sites through which they form a connection with other fibronectin dimers, collagen fibers, heparin and cell-surface integrin receptors [1].

Fibronectins have vital importance for mediating cell attachment and function and they have an important role in the organization process of the interstitial ECM. During tissue develop‐ ment, fibronectins are important for cell migration, and also studies showed that they have roles on cardiovascular disease and tumor metastasis [6].
