Mst KO MDSCs failed to generate myotubes in vitro , but in vivo stimulate tissue repair comparable to the WT MDSCs. Aged (10-month-old) mdx mice were used to maximize myofiber loss and lipofibrotic degeneration in the gastrocnemius. (A) Muscles were cryoinjured, implanted with 0.5 × 106 DAPI-labeled WT MDSCs, and allowed to undergo repair for 10 days. Frozen muscle sections were stained for MHC-II with Texas red streptavidin, and merging of blue and red fluorescence was obtained (200×). MDSC nuclei centrally located within myofibers are indicated with yellow arrows. (B) Similar picture, but for Mst KO MDSCs. (C) Gastrocnemius injury in the aged mdx mice was performed in the two apexes of the muscle with notexin, and muscles were injected 4 days later with saline or with 1.0 × 106 WT MDSCs or (D) Mst KO MDSCs in saline (n = 5/group). Repair was allowed to proceed for 3 weeks. Hematoxylin/eosin staining was performed in frozen sections, and a representative picture for each case shows myofibers from the gastrocnemius implanted with MDSCs, with arrows pointing to abundant central nuclei (200×). (E) Quantitative image analysis of these tissue sections (WT), in comparison to tissue sections from Mst KO MDSC-implanted mice (KO) and saline-injected controls, based on 12 fields per section, three sections per animal. ***P < 0.001. Mst KO, myostatin knockout; MDSC, muscle-derived stem cell; WT, wild type; mdx, X chromosome-linked muscular dystrophy; DAPI, 4', 6-diaminido-phenylindole; MHC, myosin heavy chain; KO, knockout.