Timing of conversion of marrow cells to skeletal muscle. Digital photomicrographs of frozen sections of muscle. C57BL/6 mice that received a transplant of tibialis anterior GFP marrow were injured with cardiotoxin. (A-B) Frozen sections of the muscle 3 and 6 days after injury demonstrate a massive infiltration of hematopoietic cells in the area of injury (arrows). (C). At 2 weeks after injury, the first GFP-positive myocytes appear, and, at 4 weeks after injury (D), many GFP-positive fibers can be identified (arrows). Pictures were taken with a FITC filter. Magnification, × 20.