GEMBALLA, Sven: Myoseptal tendons in vertebrates: Spatial arrangement, functional and evolutionary implications

A comparative study of myosepta was carried out considering representatives of the major gnathostome taxa (chondrichthyans, actinopterygians, sarcopterygians, urodelans). Tendinous structures embedded in myosepta were detected using a modified clearing and staining technique, dissection of complete septa and light- DIC- and SEM- microscopy. Throughout all investigated gnathostomes except urodelans a uniform pattern of tendinous structures was found in myosepta and thus represents the groundpattern of gnathostomes. The epaxial sloping part of myosepta exhibits a Y- shaped arrangement of two tendons referred to as epineural tendon and lateral band. In the posterior part of the trunk this arrangement is found in the hypaxial parts as well; epaxial and hypaxial arrangement are mirrorimaged by the horizontal septum. The architecture of the epaxial and hypaxial flanking parts is also mirrorimaged by the horizontal septum. These parts show tendinous structures that are oriented longitudinally and are referred to as myorhabdoid tendons. The data indicate that the pattern described is a unifying evolutionary feature enabling for undulatory locomotion in gnathostomes. From these findings the role of myoseptal tendons in transmission of muscular forces during undulation is evaluated. Arch- like units consisting of epineural tendons, lateral bands and white muscle fibres may cause bending if at the same time white muscle fibres near the axis would be active and thus by radial expansion generate internal pressure. Comparative data show that only little of this arrangement is found in urodelans. The same is true for the myoseptal architecture of agnathans and Branchiostoma. Thus major evolutionary steps in the locomotory system must have taken place on the turn to gnathostomes and tetrapods.