Direct correlation of histochemical profile to the ultrastructure of single myofibers and their neuromuscular junctions from a mixed muscle.

Abstract

AbstractIt has been demonstrated that there are significant differences in the ultrastructural morphology of the three mammalian myofiber types and their neuromuscular junctions (NMJ). The description of these ultrastructural differences was based primarily on samples taken from muscles that were composed of predominantly one fiber type. The objectives of this study were 1) to determine if previously described ultrastructural characteristics could distinguish between the three fiber types and the NMJ in a mixed muscle containing similar percentages of each fiber type and 2) to determine if there were significant differences in the morphology of like fiber types when compared between two histochemical regions in a muscle with a compartmentalized fiber‐type distribution. The muscle used in this study was the flexor carpi radialis (FCR) of the cat. The objectives were achieved by a direct correlation of ultrastructure to histochemical profile on the same isolated myofibers. This study revealed a wide range of morpohological variation among fiber types in the FCR. Although many fibers from the FCR could be classified by using accepted ultrastructural criteria, most could not. Compartmental differences were present in several of the ultrastructural parameters measured. The slow‐twitch oxidative (SO) and fast‐twitch oxidative‐glycolytic (FOG) fibers in the oxidative compartment had a significantly larger myofiber core volume when compared to the fast‐twitch glycolytic (FG) fibers from both compartments. Also, the FG fibers in the glycolytic region had a significantly larger membrane volume (terminal cisternae, T tubules, and sarcoplasmic reticulum) when compared to the FOG fibers in the oxidative compartment and the SO fibers in both regions.This study has demonstrated that considerable variation exists in the most commonly accepted ultrastructural features used for distinguishing the three myofiber types when sampled from a mixed muscle that is not dominated by one fiber type. These results suggest the possibility of a greater functional range in the same type of histochemically identified motor units from mixed muscle than in those from muscles containing predominantly one fiber type.