Localization of T-cell factor 4 positive fibroblasts and CD206-positive macrophages during skeletal muscle regeneration in mice

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Skeletal muscle regeneration is initiated by the activation of the transcription factor paired box 7 (Pax7), which is expressed in the satellite cells. The nuclear transcription factor T-cell factor 4 (Tcf4) is expressed in the fibroblasts and is involved in muscle tissue repair, while M2-like macrophages play an important role in skeletal muscle regeneration. However, the localization of M2-like macrophages and the expression of Tcf4 over a period of time during skeletal muscle regeneration remain unknown. Therefore, the murine masseter muscle was immunofluorescence investigated for the surface protein CD206 of M2-like macrophages and Tcf4 of fibroblasts during skeletal muscle regeneration to understand the changes in the CD206 and Tcf4 expression over time. We observed that CD206 entered the cytoplasm of some regenerating muscle fibers 5–7 days after the experimental muscle damage, that is, in the early stage of maturation of the regenerating muscle fibers with central nuclei. In addition, Tcf4 was expressed in the nuclei of the fibroblasts around the regenerating muscle fibers and in the central nuclei of the regenerating muscle fibers. Furthermore, the expression of laminin adjacent to Tcf4-positive cells was observed to partially disappear, and the shape of this missing part was observed to be identical to that of the nuclei of Tcf4-positive cells adjacent to the laminin. Clathrin was also expressed in these sites, demonstrating endocytosis. Thus, these results suggest that in the early stage of maturation of the regenerating muscle fibers, M2-like macrophages and Tcf4-positive fibroblasts enter the cytoplasm of the regenerating muscle fibers, thereby regulating the expression of various maturation factors.