muscle interstitial fibroblasts are the main source of collagen vi synthesis in skeletal muscle implications for congenital muscular dystrophy types ullrich and bethlem mutations in the extracellular matrix molecule collagen vi underlie the congenital muscular dystrophy types ullrich and bethlem establishing the origin of collagen vi in muscle is important for understanding the pathophysiology of these diseases and for developing future treatment approaches involving cellspecific delivery because the cells that produce collagen vi cannot be identified by histologic analysis we examined the production of collagen vi in pure cultures of primary myogenic cells and muscle interstitial fibroblasts from limb muscle of neonatal mice immunofluorescence staining and western blot analysis revealed secretion and matrix deposition of collagen vi by interstitial fibroblasts but not by myogenic cells in vitro using northern blot and realtime reversetranscriptasepolymerase chain reaction analysis for the collagen vi genes col6a1 col6a2 col6a3 transcript levels for the 3 mrnas were high in interstitial fibroblasts whereas in primary myogenic cells they were indistinguishable from background furthermore retention of mutant collagen vi in muscle from 3 patients with collagen vi mutation was identified in interstitial fibroblastic cells but not in their myofibers these results suggest that interstitial fibroblasts but not myogenic cells contribute significantly to the deposition of collagen vi in the extracellular matrix in skeletal muscle and imply major roles of this cell type and the extracellular matrix in the pathogenesis of these diseases