Figure 1.

Type I collagen production, degradation, and uptake. The fundamental structural unit of type I collagen (Col1) is a triple helix comprised of two procollagen type I a1 (pCol1a1) chains and a single a2 (pCol1a2) chain. Each procollagen chain is flanked by C-terminal propeptide (PICP) and N-terminal propeptide (PINP) which are cleaved off in the extracellular space before a Col1 monomer can be incorporated into a fibril. Antibodies against Col1 are typically polyclonal, derived using purified Col1 or a synthetic peptide corresponding to a sequence within Col1a1 or Col1a2 as an antigen. Therefore, these antibodies against Col1 (black) do not recognize PICP or PINP which distinguish procollagen produced by a fibrocyte (top) from Col1 taken up by a macrophage (bottom) leading to the misidentification of some macrophages as fibrocytes. Use of transgenic mice expressing a collagen-GFP fusion protein (green, bottom) may also result in fibrocyte misidentification as the collagen-GFP fusion protein may be taken up by macrophages and present in endosomes prior to further degradation.