Accumulation of PDGF+ cells and internalisation of the PDGF receptor at myotendinous junction following modified hindlimb muscle use in the rat

Morphological observations have shown previously that myotendinous junctions (MTJs) are sites where the associations between the cytoskeleton and the cell membrane are extensively remodelled during muscle growth and modified mechanical loading. The platelet derived growth factor (PDGF) molecule has been shown to induce cytoskeletal remodelling at focal contact sites of myoblasts in culture, the analogous structures of MTJs. The goals of the study were to determine whether PDGF is synthesised by mononuclear cells and whether PDGF receptors are internalised at the MTJs of the soleus muscle experiencing reloading. We also examined whether ED2⁺ macrophages that are nonphagocytic and activated inflammatory cells at MTJs during reloading secrete PDGF. Results obtained by immunohistochemistry showed that there was an increase in the number of cells expressing PDGF at remodelling MTJs and that the ED2⁺ macrophage population does not express PDGF at MTJs. According to morphological criteria, fibroblasts would be the logical candidates to secrete PDGF molecules near MTJs. Furthermore, the modification in muscle loading resulted in internalisation of PDGF receptors concentrated at the MTJ which accumulated predominantly around muscle nuclei. The enrichment of PDGF receptors and PDGF⁺ cells at MTJs and the internalisation of PDGF receptors during remodelling of MTJs suggest that PDGF may influence remodelling of MTJs following modified muscle use.