Post-autotomy regeneration of respiratory trees in the holothurian Apostichopus japonicus (Holothuroidea, Aspidochirotida).

Specialised respiratory organs, viz. the respiratory trees attached to the dorsal part of the cloaca, are present in most holothurians. These organs evolved within the class Holothuroidea and are absent in other echinoderms. Some holothurian species can regenerate their respiratory trees but others lack this ability. Respiratory trees therefore provide a model for investigating the origin and evolution of repair mechanisms in animals. We conducted a detailed morphological study of the regeneration of respiratory trees after their evisceration in the holothurian Apostichopus japonicus. Regeneration of the respiratory trees occurred rapidly and, on the 15th day after evisceration, their length reached 15-20 mm. Repair involved cells of the coelomic and luminal epithelia of the cloaca. Peritoneocytes and myoepithelial cells behaved differently during regeneration: the peritoneocytes kept their intercellular junctions and migrated as a united layer, whereas groups of myoepithelial cells disaggregated and migrated as individual cells. Although myoepithelial cells did not divide during regeneration, the peritoneocytes proliferated actively. The contractile system of the respiratory trees was assumed to develop during regeneration by the migration of myoepithelial cells from the coelomic epithelium of the cloaca. The luminal epithelium of the respiratory trees formed as a result of dedifferentiation, migration and transformation of cells of the cloaca lining. The mode of regeneration of holothurian respiratory trees is discussed.