Résumé :
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The two components of the extracellular matrix in skeletal muscle, i.e the perimysium and endomysium, are generally considered to differ in their ability to control muscle flexibility. For instance, the endomysium permits the transmission of forces by joining the cytoskeleton to adjacent myofibers. The perimysium, however, is considered as a simple packing tissue, and little is known about its organization and subsequent role in force transmission. We have analysed the features of the perimysium collagen network in the bovine Flexor carpi radialis muscle using various techniques including scanning and transmission electron microscopy and immunohistochemistry. We have observed that the perimysium forms a network of collagen fibres at three successive levelds : (i) a regular loose isocele lattice, (ii) collagen plexi at the angles, that finally bind adjacent myofibers at level of (iii) a particular domain on each myofibre that we call Perimysial Junctional Plate (PJP) (100 um dimension). The associated myofibre cytoplasm content always appears to subsarcolemmal accumulations of mitochondria we found to be colocalized with myonuclei clusters, which is confirmed by statistical analysis. We call this particular cytoplasmic arrangment, Associated Mtonuclear Domain. Observation of perimysium adhesion to adjacent myofibres at the PJP reveals that the perimysium of the extracellular matric could carry forces from the cytoskeleton to the perimysium lattice, and play a role in force transmission and subsequenty mechanosensing role for the regulation of myofibre activity. Furthermore, the close association of myonuclei and mitochondria with PJP also suggests their possible implication in the adaptation of the energy metabolism of the muscle cellnto contractile activity. These findings could also be of particular importance for the understanding of muscle physiopathology and mitochondria disorders.
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