Increase in rat soleus myotendinous interface after a 14-d spaceflight.

Myotendinous junctions (MTJs) transmit contractile force from skeletal muscles to tendons. The effects of a 14-d spaceflight on MTJ were studied in the soleus muscle of male adult Sprague Dawley rats by transmission electron microscopy and histomorphometric techniques. We showed that the length of the junctional membrane relative to the muscle fiber diameter increased by 58% after 14 d of spaceflight. This increase accompanies morphological changes at MTJs. The flight MTJs appeared more shredded. The ends of the muscle fibers exhibited T tubule dilatation, swollen mitochondria, Z-disk streaming, loss of myofilaments, a thinning down of subplasmalemmal densitites, multivesicular bodies and signs of junctional membrane and basal lamina remodelling. The ultrastructural observations suggest that the increase in myotendinous interface could result from the extracellular matrix spreading into remodelling muscle fiber, whereas the constraints related to unloading were reduced by spaceflight conditions.