Structural Basis for Soft Tissue Viscoelasticity
Extracellular Matrix (ECM)
Soft tissue consists of two important components: cells and extracellular matrix (ECM). It is well known that cells are the fundamental functional unit of living organisms. But only by integrating cells with ECM, tissue can maintain its structural integrity and realize its unique function. By definition, ECM is any material part of a tissue that is not part of any cell. ECM is a complex mixture of structural and functional proteins, glycoproteins and proteoglycans arranging in a unique, tissue specific 3D ultrastructure.
Major Building Blocks of ECM
Among the various ECM substances, collagen, elastin, proteoglycans, bound and unbound water are the most important building blocks. Each of the aforementioned building blocks has its unique ultrastructural and mechanical properties. By various combinations, nature is able to generate different tissues with totally different functionality. For example, in tendon/ligament, uniaxially aligned collagen network is the dominant component that bestows tendon extraordinary material strength; in cartilage, large proteoglycan aggregates (highly negatively charged) is able to attract and trap a lot of water, which provides cartilage with lubrication and shock absorbing capability.
Ultrastructure, Mechanics, and Function of Major ECM
Collagens are, in most animals, the most abundant protein in the ECM. In fact, collagen is the most abundant protein in the human body and accounts for 90% of bone matrix protein content. Collagens are present in the ECM as fibrillar proteins and give structural support to resident cells. Collagen is exocytosed in precursor form (procollagen), which is then cleaved by procollagen proteases to allow extracellular assembly. Disorders such as Ehlers Danlos Syndrome, osteogenesis imperfecta and epidermolysis bullosa are linked with genetic defects in collagen-encoding genes. The collagen can be divided into several families according to the types of structure they form:
Fibrillar (Type I,II,III,V,XI) Facit (Type IX,XII,XIV) Short chain (Type VIII,X) Basement membrane (Type IV) Other (Type VI,VII, XIII)
Elastins, in contrast to collagens, give elasticity to tissues, allowing them to stretch when needed and then return to their original state. This is useful in blood vessels, the lungs, in skin, and the ligamentum nuchae, and these tissues contain high amounts of elastins. Elastins are synthesized by fibroblasts and smooth muscle cells. Elastins are highly insoluble, and tropoelastins are secreted inside a chaperone molecule, which releases the precursor molecule upon contact with a fiber of mature elastin. Tropoelastins are then deaminated to become incorporated into the elastin strand. Disorders such as cutis laxa and Williams syndrome are associated with deficient or absent elastin fibers in the ECM.
Proteoglycans are proteins that are covalently bonded at multiple sites along the protein chain to a class of polysaccharides, known as glycosaminoglycans. Glycosaminoglycans constitute approximately 95% of the mass of proteoglycans by weight, which results in proteoglycans bearing a resemblance more to polysaccharides than to proteins. The physiological properties of proteoglycans are a function of the particular glycosaminoglycans present. Examples of common glycosaminoglycans are chondroitin 6-sulfate, keratan sulfate, heparin, dermatan sulfate, and hyaluronate. As a result of the ionic character of glycosaminoglycans, proteoglycans carry at least one negatively charged carboxylate or sulfate functional group under physiological conditions. Examples of proteoglycans include Versican, Brevican, Neurocan, and Aggrecan.
The major functions/purpose of proteoglycans depends on the glycosaminoglycan component of the molecule. This component allows connective tissues of the Extracellular Matrix (ECM) to be able to withstand compressional forces through hydration and swelling pressure to the tissue. Aggrecan best portrays this particular function.