Effect of chronic undernutrition on the composition and organization of fibers in fascicles of the rat extensor digitorum longus muscle

Skeletal muscles are conformed by three different types of fibers, which are classified according to their contractile and metabolic capacity in: slow, intermediate and fast, or glycolytic and oxidative, respectively. The proportion ofeach fiber type in muscles are modified by several intrinsic and-or extrinsic factors such as the degree of muscle development or by diverse adverse environmentalconditions, such as malnutrition or injury. One of the most studied muscles is the extensor digitorum longus muscle (EDL m.) which is composed of four fasciclesand it is involved in ankle dorsiflexion and flexion of fingers (2 to 5 in paws). In this study we analyzed the effect that chronic undernutrition exerts on the composition and organization of each fiber types in fascicles (F2, F3, F4, F5) of the EDL m. For this purpose, two groups of Wistar rats (control and undernourished) with 35 days of age were used and each fascicle in the EDL m. was dissected free and prepared for histological analysis. Sections of 10 ?m thickwere obtained from each fascicle and stained with the histochemical ATPase (pH 9.4), NADH-TR and COX techniques. Photomicrographs of stained sections weretaken (10X) and the fibers were identified according to their staining intensity as follows: transparent - slow; dark - intermediate and gray - fast, or clear - glycolyticor dark - oxidative, respectively. Subsequently the number and relative proportion of each fiber type were determined and the geometrical coordinates (XY) of each fiber were used to construct binary images of the position of them. Methods of fractal correlation and density distribution function were used to establish the degree of organization and the distribution patterns of fibers in muscles. Theresults obtained indicate that fascicles contain different percentages of fiber types, both in control and in undernourished muscles. Fractal analysis of data indicatesthat there is a non-random distribution of fibers in muscles (D> 1), suggesting that the fibers are distributed in clusters of different size, separated from each other.Both the distribution and the fractal organization of fiber types are modified as a result of chronic undernutrition. Our results may suggest that chronic undernutrition alters the internal structural organization of fascicles in the EDL m.