Fig. (1) Hypothetical Model of Cellular Mechanisms of SPARC. SPARC binds procollagen as it is secreted from the cell (or procollagen is secreted bound by SPARC) and prevents interaction of procollagen with cellular receptors, such as DDR2 and integrin α₂β₁ (or others). Therefore, procollagen is appropriately processed and incorporated into collagen fibrils. In the absence of SPARC, procollagen accumulates at the cell surface and is inefficiently incorporated into the collagenous ECM, resulting in less total collagen and fewer thick collagen fibers.