Development and evolution of the muscle system in the Echinodermata

摘要

It is generally accepted that echinoderm muscles have an epithelial origin. The available hypothe-ses on the origin and evolution of the muscles (Rieger, Lombardi, 1987; Stauber, 1993; Candia Carnevali et al., 1995) are based on the studies of normal anatomy of contractile systems and therefore hardly take into account the actual myogenic events. Our recent data on the development and regeneration allow deeper understanding of the evolution of the echinoderm muscles. The simplest metazoan contractile system is an epithelium composed of myoepihtleial cells (MCs). The first step toward the development of complex contractile systems involves grouping of myoepithelial cells into clusters and displacement of the latter into the basal region of the epithe-lium resulting into a pseudostratified myoepithelial organization. During the next stage, clusters of the MCs gradually penetrate into the underlying connective tissue, so that the myoepithelium splits into an apical layer of peritoneal cells and a deeper mass of myoepithelial cells immersed into the connective tissue. However, these two layers remain connected by a continuous basal lamina. At the final stage, the MCs detaches completely from the epithelium to give ruse to muscle bundles.