Figure 1. Establishment of left-right asymmetry in echinoderms.

Left-right asymmetry in echinoderms is characterized by the asymmetric positioning of the imaginal rudiment on the left side of the bilateral pluteus larva. The adult emerges from this imaginal rudiment through metamorphosis. Formation of the rudiment is intimately linked to development of two mesodermal derivatives, the coelomic pouches, that form from an unpaired coelomic sac that budds off from the tip of the archenteron. The coelomic pouches are bilateral structures, but only the coelomic pouch located on the left side of the larva proliferates and differentiates to form the rudiment. Precursors of the coelomic pouches have a double origin. Part of these precursors derives from the small micromeres that form by asymmetric division of the large micromeres at 5th cleavage. These cells are thought to contribute to the germ line. Another population of coelomic pouch precursors derives from the non-skeletogenic mesoderm that is induced during blastula stages by Delta signals emanating from the skeletogenic mesenchymal cell precursors.