New roles of lipid receptors in cardiac and vascular development. Receptors for S1P are critical for the migration of cardiac muscle precursors and smooth muscle cells during heart tube formation and blood vessel formation, respectively. See the text for details. White boxes, Edg-1 (vessel), mil (heart); purple arrows, S1P; blue cells, endothelial cells (vessel), midline cells (heart); orange cells, smooth muscle cells (vessel), cardiac muscle precursors; green arrows, chemoattractant(s) or permissive microenvironment.