Platelet-derived growth factor receptor-β-positive telocytes in skeletal muscle interstitium

Telocytes (TCs) represent a new cell type recently described in mammalian skeletal muscle interstitium as well as in other organs. These have a specific morphology and phenotype, both in situ and in vitro. Telocytes are cells with long and slender cell prolongations, in contact with other interstitial cells, nerve fibres, blood capillaries and resident stem cells in niches. Our aim was to investigate the potential contribution of TCs to micro-vascular networks by immunofluorescent labelling of specific angiogenic growth factors and receptors. We found that in human skeletal muscle TCs were constantly located around intermediate and small blood vessels and endomysial capillaries. Epi-fluorescence and laser confocal microscopy showed that TCs express c-kit, platelet-derived growth factor receptor (PDGFR)-beta and VEGF, both in situ and in vitro. Telocytes were constantly located in the perivascular or pericapillary space, as confirmed by double staining of c-kit/CD31, PDGFR-beta/CD31 and PDGFR-beta/a-smooth muscle actin, respectively. Electron microscopy (EM) differentiated between pericytes and other cell types. Laminin labelling showed that TCs are not enclosed or surrounded by a basal lamina in contrast to mural cells. In conclusion, a) PDGFR-beta could be used as a marker for TCs and b) TCs are presumably a transitional population in the complex process of mural cell recruitment during angiogenesis and vascular remodelling.