Examination of intrafascicular muscle fiber terminations: implications for tension delivery in series-fibered muscles

Mammalian skeletal muscles with long fascicle lengths are predominantly composed of short muscle fibers that terminate midbelly with no direct connection to the muscle origin or insertion. The manner in which these short fibers terminate and transmit tension through the muscle to their tendons is poorly understood. We made an extensive morphological study of a series-fibered muscle, the guinea pig sternomastoid, in order to define the full range of structural specializations for tension transmission from short fibers within this muscle. Terminations were examined in single fibers, teased small bundles of fibers, and in sections at both the light and electron microscopic level. In many cases, sites of fiber termination were defined by reactivity for the enzyme acetylcholinesterase, which also marks myotendinous junctions. Additionally, transport of the lipophilic fluorescent dye, DiI, or injection of Lucifer Yellow were used to visualize undisturbed fiber terminations in whole muscles using confocal and fluorescence microscopy. At the light microscopic level, we find that intrafascicularly terminating fibers end about equally often in either a long progressive taper, or in a series of small or larger blunt steps. Combinations of these two morphologies are also seen. However, when analyzed at higher resolution with confocal or electron microscopy, the apparently smooth progressive tapers appear also to be predominantly composed of a series of fine stepped terminations. Stepwise terminations in most cases join face-to-face with complementary endings of neighboring muscle fibers, some via an extended collagenous bridge and others at close interdigitating myomyonal junctions. These muscle-to-muscle junctions show many of the features of myotendinous junctions, including dense subsarcolemmal plaques in regions of myofibrillar termination and we suggest that they serve to pass tension from fiber to fiber along the longitudinal axis of the muscle. In addition, we observe regions of apparent side-to-side adhesion between neighboring fibers at sites where there is no apparent fiber tapering or structural specialization typical of myofibril termination. These sites show acetylcholinesterase reactivity, and large numbers of collagen fibers passing laterally from fiber to fiber. These latter connections seem most likely to be involved in lateral transmission of tension, either from fiber to fiber, or from fiber to endomysium. Overall, our results suggest that tension from intrafascicularly terminating fibers is likely to be passed along the muscle to the tendon using both in-series and in-parallel arrangements. The results are discussed in light of current theories of tension delivery within the series-fibered muscles typical of large, nonprimate mammals.