A new spin on handed asymmetry

摘要

In vertebrates, most internal organs develop asymmetrically with respect to the body's midline, with the liver on the right and the stomach on the left, for example. This phenomenon, known as left-right (L-R) asymmetry, raises an intriguing question — how is the early bilateral symmetry of the embryo first broken such that the L-R axis is always orientated the same way? A report by Nonaka et al. in Cell now provides a remarkable insight into this problem. They show that the mouse node, a cup-shaped cavity in the embryo's midline, uses anticlockwise rotation of cilia to create a directional flow of extraembryonic fluid. The authors propose that this 'nodal flow' concentrates critical L-R determinants to one side of the node, activating distinct downstream signalling pathways on the left and right sides of the embryo.