Telling left from right: left-right asymmetric controls in sea urchins.

Left-right asymmetry of internal organs is a common feature in bilateria. It is generally considered that three steps are required to establish the left-right axis. These include the initial symmetry-breaking step that leads to asymmetric gene expression and ultimately results in side-specific organogenesis along the left-right axis. During the development of indirect-developing sea urchins, the adult rudiments consistently form on the left side of the feeding larvae. Recent studies have revealed that several signaling molecules are required to regulate the initial right-sided nodal expression in the archenteron. Right-sided Nodal signaling then restricts BMP signaling to the left side. These two TGFβ signals are important for asymmetric gene expression, cell behavior, and morphogenesis along the left-right axis. Although considerable progress has been made regarding sea urchin left-right asymmetry, many questions remain unanswered. This review attempts to synthesize what is known about the left-right asymmetric controls in sea urchins. Additionally, questions raised from previous findings and possible mechanisms concerning symmetry breaking are also discussed.