Morphology and Function of Lateral Hypaxial Musculature in Salamanders1

Myoseptal architecture of sarcopterygian fishes and salamanders with special reference to Ambystoma mexicanum

During axial undulatory swimming in fishes and salamanders muscular forces are transmitted to the vertebral axis and to the tail. One of the major components of force transmission is the myoseptal system. The structure of this system is well known in actinopterygian fishes, but has never been addressed in sarcopterygian fishes or salamanders. In this study we describe the spatial arrangement and collagen fiber architecture of myosepta in Latimeria, two dipnoans, and three salamanders in order to gain insight into function and evolution of the myoseptal system in these groups. Salamander myosepta lack prominent cones, and consist of homogenously distributed collagen fibers of various orientations that never form distinct tendons. Fiber orientations are difficult to homologize with those of fish myosepta. The myosepta of Latimeria and dipnoans (Protopterus and Neoceratodus) illustrate that major changes in architecture occurred in the sarcopterygian clade (loss of horizontal septum), in the rhipidistian (dipnoans + tetrapods) clade (loss of epineural and epipleural tendon), and in tetrapods (loss of lateral tendons and myoseptal folding). When compared to fishes, the myosepta of wholly aquatic salamanders (Ambystoma mexicanum, Amphiuma tridactylum, Necturus maculosus) do not have the lateral tendons we suppose serve to transfer muscular forces posteriorly. We propose that alternative structures (most conspicuously present in Ambystoma) perform this function: posteriorly the relative amount of connective tissue increases considerably, and myosepta are disintegrated to horizontal lamellae of connective tissue. The structures thought to be involved in modulation of body stiffness in fishes during swimming are also absent in salamanders. Our data also have implications for the hypothesis that salamander hypaxial myosepta are designed to increase shortening amplification of the hypaxial muscle fibers. The posterior hypaxial myosepta of all three salamander species possess only mediolaterally directed collagen fibers, which would indeed amplify the shortening of the associated muscle.

Morphology and mechanics of myosepta in a swimming salamander (Siren lacertina)

In contrast to the complex, three-dimensional shape of myomeres in teleost fishes, the lateral hypaxial muscles of salamanders are nearly planar and their myosepta run in a roughly straight line from mid-lateral to mid-ventral. We used this relatively simple system as the basis for a mathematical model of segmented musculature. Model results highlight the importance of the mechanics of myosepta in determining the shortening characteristics of a muscle segment. We used sonomicrometry to measure the longitudinal deformation of myomeres and the dorsoventral deformation of myosepta in a swimming salamander (Siren lacertina). Sonomicrometry results show that the myosepta allow some dorsoventral lengthening, indicating an amplification of myomere shortening that is greater than that produced by muscle fiber angle alone (10% muscle fiber shortening produces 28.7% myomere shortening). Polarized light and DIC microscopy of isolated hypaxial myosepta revealed that the collagen fiber orientation in hypaxial myomeres is primarily mediolateral. The mediolateral collagen fiber orientation, combined with our finding that the hypaxial myosepta lengthen dorsoventrally during swimming, suggests that one possible function of hypaxial myosepta in S. lacertina is to increase the strain amplification of the muscle fibers by reducing the mediolateral bulging of the myomeres and redirecting the bulging toward the dorsoventral direction.