The myosepta in Branchiostoma lanceolatum (Cephalochordata): 3D reconstruction and microanatomy

摘要

Myosepta have been subject to comparative and evolutionary studies in aquatic groups of the Craniata, because they are likely to play a role in transmission of muscular forces to axial structures during swimming. Based on gross morphological observations, the V-shaped myosepta of Branchiostoma lanceolatum appear to be simpler than craniate myosepta that lack the dorsal- and ventralmost anterior pointing arm. However, these small and delicate sheets of connective tissue have never been studied in terms of 3D morphology and collagen fibre architecture. We posed the following questions. What are the shape and collagen fibre architecture of the myosepta of Cephalochordata compared to those of Craniata? Do they exhibit the same structures as the corresponding parts of the W-shaped myosepta of Craniata? We adapted methods used for craniate myosepta (clearing, microdissections and polarized light microscopy, DIC microscopy) and additionally used computer-based 3D reconstruction to address these questions in B. lanceolatum. We found four features of complex myoseptal folding that are not present in any craniate group: (1) the medial attachment line is divided into an anterior and posterior line along their traverse on the neural tube, giving rise to a lumen between dorsal nerve cord and medial attachment line, (2 and 3) the myosepta exhibit two vertical anterior lamellae (AVL-1 and AVL-2) and (4) a posterior vertical lamella (PVL) originates from a small anterior depression in the epaxial part. The AVLs and PVL are situated in a paramedian plane near the axis and serve as attachment sites for muscle fibres. Muscle fibres exclusively run from myoseptum to myoseptum and in contrast to the vertebrate condition never attach to the chordal sheath. The myoseptal collagen fibre architecture is different from any of the conditions among Craniata: it is a system of crossing fibres (MLF-1, MLF-2) and longitudinal fibres (LF), that lacks distinct tendons. The MLFs and LFs are hypothesized to be involved in transmission of muscular forces during swimming. Given these findings it is likely that cephalochordate myomeres rather represent a specialized locomotory design than the notochordate ground pattern. Evolutionary transformations of the myoseptal system during early notochordate evolution are discussed in the light of current phylogenies including extinct taxa (for example conodonts, Yunnanozoon, Haikouella).