hAP + cells express chondrogenic and osteogenic markers. Serial sections were stained for hAP+ cells (A, B) and type II collagen in fractures at week 2 (A') or type I collagen in the peri-cortical region of open fracture samples at week 3 (B'). Collagen staining was seen in both the cells and the matrix. hAP+ cells were found to stain with collagen markers (arrowheads) indicating that formally myogenic progenitors had contributed to mature cartilage/bone tissue. Scale bar = 50 μm.