MUSCLE DEVELOPMENT IN AMPHIOXUS - MORPHOLOGY, BIOCHEMISTRY, AND MOLECULAR-BIOLOGY

This review concerns the structure and biochemistry of muscle in amphi oxus. Most work has focused on the segmented swimming (axial) muscles. These muscles derive from the medial wall of the somites, which arise as evaginations from the gut wall. The myotomal muscle cells of amphi oxus, unlike those of vertebrates, never fuse, but remain mononucleate , contain only one myofibril, and span the entire length of the myotom e. The muscle cells are very thin and lack a T-tubule system. There ar e two, maybe three, types of fibers. Innervation is via muscle tails, which contact the basal lamina of the nerve cord. The notochord is als o composed of striated muscle cells, which similarly send muscle tails to the nerve cord. Less is known about the biochemistry of muscle. Th e notochord, like molluskan catch muscle, contains paramyosin. Among t he muscle-specific proteins sequenced are alkali myosin light chain, t roponin C and sarcoplasmic calcium-binding proteins, calcium-vector pr otein, and its target protein calcium vector-target protein. The only muscle regulatory factors identified are two MyoD proteins. Almost not hing is known about muscle enzymes in amphioxus.