Telocytes as “nursing” cells for satellite stem cells: morphological evidence from an experimental model of eccentric contraction-induced skeletal muscle injury

Telocytes (TCs) have recently emerged as unique stromal cells characterized by a small cell body and extremely long and thin moniliform cytoplasmic extensions, called telopodes. In the skeletal muscle interstitium, TCs have been proposed to play a “nursing” role in tissue regeneration mediated by resident muscle stem cells, namely satellite cells (SCs). However, at present there is no evidence of morpho-functional TC/SC interactions following skeletal muscle injury. Hence, the present study was undertaken to explore the TC presence and interaction with SCs in a mouse model of skeletal muscle (extensor digitorum longus) damage induced by forced eccentric contraction (EC) in isometric condition. After EC, skeletal muscles showed evidence of structural damage as assessed by light microscopy and transmission electron microscopy (TEM), along with significant changes in sarcolemnic functionality as determined by electrophysiological analyses. TCs were identified in both control and EC injured muscle interstitium by either CD34/CD31 double confocal immunofluorescence (i.e. CD34+CD31– TCs) or TEM. In EC muscles, an extended network of CD34+ telopodes was preferentially observed around activated SCs displaying Pax7 and MyoD nuclear positivity. TEM clearly demonstrated the presence of TCs invading the SC niche, as testified by the evidence of telopodes passing through a fragmented basal lamina and contacting the underlying activated SCs. TC/SC interaction was confirmed in vitro by culturing single muscle fibers isolated from EC muscles and the sprouting TCs and SCs. TCs from EC muscles showed an increased expression of VEGF-A, whose role in promoting myoblast proliferation/differentiation is well established. SCs isolated from the same samples exhibited increased expression of MyoD and a greater tendency to fuse into myotubes. Here, we provide the first demonstration of a morphological interaction between TCs and SCs following skeletal muscle damage. The possible role of VEGF-A in mediating juxtacrine-paracrine TC/SC interactions is worth investigating further.