Fibrotic scar is a common component of MS mature lesions; active lesions are enriched in collagen-producing cells, aggregates of fibronectin and collagen, mesenchymal perivascular aggregates of platelet-derived growth factor receptor (PDGFR)β-bearing cells, and pro-fibrotic factors such as biglycan and decorin.305,306 In contrast to TBI, the MS scar is mainly produced by resident fibroblasts and not by vascular infiltrating stromal cells.89 Development of MS is also associated with direct damage to astrocytes. This evidence concerns the gene PDGFRB and myeloid sarcoma.