Morphometric characteristics of myonuclear distribution in the normal and denervated fast rat muscle fiber

Unlike the majority of mammalian tissues, which have mononuclear cells as a basic unit of the tissue composition, skeletal muscle fiber is a polynucle ar syncytium. Polynuclear organization offers an additional possibility for the regulation of protein expression: it can also be controlled at the lev el of myonuclear distribution and specialization. Distribution of myonuclei can be considered as the distribution of genes. Variability in gene concen tration may have important impact on the regional differentiation of the mu scle fiber, since it leads to different regional concentrations of various gene products including factors controlling their expression. The aim of th e present study was: 1) to provide morphometrical data on the myonuclear di stribution in the junctional and extrajunctional regions of the normal fast rat muscle fiber, and 2) to analyze, whether morphomertical parameters of nuclear distribution change after mechanical denervation of this muscle. Si ngle muscle fibers were isolated from the normal and denervated fast rat m. sternomastoideus. Their neuromuscular junctions were stained by thiocholin e histochemical procedure and myonuclei were fluorescently labeled by Hoech st 33342. Myonuclear distribution in each individual muscle fiber was morph ometrically analyzed on the image analyzer. Synaptic concentrations of myon uclei were found to exceed extrasynaptic concentrations by a factor of 17. The number of myonuclei accumulated at the endplates did not change after o ne or two weeks of denervation, neither did the morphometric parameters of these nuclei. A higher concentration of myonuclei due to muscle atrophy was observed in the extrasynaptic region and the longitudinal axis of these nu clei also became significantly shorter. Unchanged morphometric parameters o f the junctional myonuclei after denervation are indicative of either irrev ersibility of the nerve-induced formation of nuclear clusters in this regio n or persistence of the factors responsible for their formation and mainten ance. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.