The bulk of these lesions is made of excess fat, collagen, and elastin. The elasticity of collagen improves fracture resistance. Cortisol stimulates degradation of (skin) collagen into amino acids. Hydroxyproline, a major component of the connective tissue collagen, is synthesised from proline. Ligands for integrins include fibronectin, vitronectin, collagen and laminin. The proliferation of fibroblasts and collagen deposits is also reduced. Interaction of ApoB with proteoglycans, collagen, and fibronectin is believed to cause atherosclerosis. Primarily, it composed of fibrous proteins such as collagen type I. The platelet cell membrane has receptors for collagen. Tendons are predominantly composed of dense collagen fiber bundles. Among the ligands of integrins are fibronectin, vitronectin, collagen, and laminin. They produce and maintain the cartilaginous matrix, which consists mainly of collagen and proteoglycans. It forms large, eosinophilic fibers known as collagen fibers. Some are simple structural molecules, like the fibers formed by the protein collagen. These synthesise collagen within the cell, and then secrete collagen fibrils.