Histologic changes in smooth muscle-to-collagen ratios in the corpus cavernosum. Panels A to C show representative light micrographs of Masson trichrome-stained frozen sections of penile mid-shaft specimens in the sham, vehicle, and bone marrow–derived MSC groups, respectively, to assess the composition of smooth muscle (red) and collagen (green). Panel D shows quantification of smooth muscle-to-collagen ratios in penile tissue in the sham (n = 6), vehicle (n = 6), and bone marrow–derived MSC (n = 6) groups. Note that the ratio of smooth muscle to collagen in the MSC group is significantly higher than that in the vehicle group. *P < .05. Scale bar = 500 μm. MSC = mesenchymal stem cell.