The microenvironment of collapsed myofibers is associated with alterations in ECM and trophic factors. (A) Histogram showing the relative proliferation of MPCs cultured in the presence of conditioned media from cultured intact or collapsed myofibers. Results were obtained from four different preparations of conditioned media that were exposed to four separate primary MPC cell lines. (B) Graph showing the qPCR quantification of IGF-1 relative mRNA expression. Results were obtained from four independent cultured intact and damaged myofiber preparations isolated from four different mice. (C) Histograms showing qPCR relative mRNA expression of the basal lamina genes coding for collagen VI, laminin, fibronectin, and collagen IV. The results compare cultured intact and collapsed myofibers from four different myofiber preparations to the contralateral muscle isolated from separate mice. *P < 0.05, **P < 0.01, ***P < 0.0001, and ****P < 0.00001 for the proliferation and qPCR assays. All results are expressed as means ± SEM. (D,E) Representative micrographs of fibronectin immunostaining in cultured intact and collapsed myofibers (D) and in the skeletal muscle of WT and mdx mice (E). (D,E) Fibronectin staining in cells at the periphery (arrowheads), at the basal lamina (arrows), and in the cytoplasm of the collapsed/damaged myofibers (asterisk) can be seen.