Abstract:. The longitudinal muscle bands at the rear of the abdominal cavity were severed with scissors. The sea cucumbers were then reared in sea water containing antibiotics (100 IU/mL penicillin and 100 μg/mL streptomycin), and the structural changes of the regenerated longitudinal muscle bands were documented using morphological and histological methods. Morphological analysis revealed a gap of 0.5–1 cm around the broken muscle bands caused by contraction of the longitudinal muscle after surgery. Ivory and flocculent tissue (muscle precursors) were observed at the trauma site 15 d after the operation. The muscle precursor thickened 30–45 d after injury and the ends of the severed muscle were reconnected. The muscle precursor gradually transformed into muscle bands that were half the thickness of the normal longitudinal muscle 60–90 d after surgery. The regenerated longitudinal muscle bands grew further, and were only slightly smaller in diameter 110–130 d after surgery. The longitudinal muscle bands were completely regenerated 150 d after surgery, and there was no significant difference from undamaged muscle fibers. Histological analysis revealed that the muscle precursor was composed of connective tissue and individual muscle fibers 15 d after surgery. At 30–45 d, the number of new muscle cells increased significantly, and there was evidence of “bridged’ connections between thickening muscle precursors and the body wall. At 60–90 d, the thickening muscle precursor was largely replaced by muscle fibers, and could be classified as longitudinal muscle bands. Concurrently, the number of “bridged’ connections increased. At 110–130 d, the number of new muscle fibers increased significantly and the number of “bridged” connections decreased. At 150 d, the longitudinal muscle bands had regenerated completely and the “bridged” connections had disappeared. Our observations suggest that the longitudinal muscle bands have a strong ability for regeneration and the new muscle cells are derived from the connective tissue cells in the body wall and the coelomic epithelium.
