Type II forms thin fibrils, found primarily in cartilage. Triple helical domains of Type IV collagens are interrupted by several flexible non-helical regions. These types of diseases include Ehlers-Danlos syndromes, osteogenesis imperfecta, and some forms of osteoarthritis. There are a number of acquired diseases that affect the normal synthesis, processing or turnover of collagens. In the absence of ascorbic acid, collagen is insufficiently hydroxylated thereby preventing a-chains from forming a stable triple helix.
Tissues often require elasticity in addition to the high tensile strength provided by collagen.
Elasticity is imparted by a network of elastic fibers . Elastic fibers are composed of an amorphous material made up of elastin and microfibrils containing fibrillin. Elastin is a hydrophobic protein that is asembled into a covalently cross-linked network, which expands and contracts like a random coil. Fibrillin is a large hydrophilic glycoprotein that surrounds the amorphous core of elastin. Mutations in fibrillin result in Marfan s syndrome , a genetic disorder that affects elastic tissues.