Changes in cross-sectional area and capillary supply of the muscle fiber population in equine gluteus medius muscle as a function of sampling depth

Abstract

SUMMARY The right and left gluteus medius muscles of 4 mature Andalusian stallions were examined by repeated needle biopsy over a specific area and depth, as well as at different depths, to determine whether the cross-sectional area and capillary supply of the various fiber types are homogeneous throughout the muscle. The muscle biopsy specimens were histochemically analyzed for fiber types (myofibrillar adenosine triphosphatase) and capillaries (amylase-periodic acid-Schiff method). Differences between contralateral sites were not identifiable for any of the analyzed variables. Differences between sampling depths were larger than those between sample sites. The percentage of type-1 fibers increased, and that of type-2B fibers decreased as a function of increased sampling depth, but the relative frequency of type- 2A fibers did not change with depth. The mean crosssectional area of type-1 and type-2A fibers increased significantly, and that of type-2B fibers decreased between the most superficial and the deepest sampling sites. The capillary density and the mean number of capillaries per fiber increased as a function of sampling depth, but significant changes were not recorded between sampling depths for overall mean fiber area. The mean number of capillaries in contact with fibers of each histochemical type increased significantly with increasing sampling depth only for type- 1 fibers. However, the number of capillaries in contact with fibers changed significantly, relative to the fiber area, as a function of sampling depth; for type- 28 fibers this variable increased, whereas for type-2A and type-1 fibers, it decreased. These results indicate that fiber size and capillary supply of the various fiber types in the gluteus medius muscle of horses vary as a function of depth. These patterns of variation most likely indicate the functional difference between various portions of the muscle, and support the general opinion that the functional demands placed on the fiber population are an important factor in the development of muscle fiber properties.