Fusion of human MSCs in regenerating mouse TA muscle. The TA muscle of the NOD/SCID β2m^–/^– mouse was damaged by cardiotoxin injection followed 24 hours later by injection of MSCs. The regenerating TA muscles were evaluated 7 days later. Immunoflorescence staining of regenerating TA muscle showed the presence of human nuclei in muscle fibers. (A) Red rhodamine-phalloidin staining of mouse TA muscle with blue DAPI nuclear staining. (B) Human nuclear staining (green) with antihuman nuclear antibody. (C) Composite of A and B. (D) H&E staining of a frozen section of regenerating TA muscle. (E) Fluorescence microscopy of the same section showing a GFP-expressing fiber. (F) Two GFP-positive fibers, each longer than 100 μm, are shown in a snapshot of a 3-dimensional projection of a stack of 44 green and red 2-channel images of a regenerating TA muscle. See Supplemental Video S1 for a rotating video of this section. Panels A-C imaged by Deltavision UApo 340; × 40/1.35; D-E, Nikon Plan Apo × 10/0.45; and F, Confocal Plan Apo × 20.0/0.70.