The left-right polarity puzzle: determining embryonic handedness

Abstract

Whenever symmetry is broken in nature to yield only one of two equally probable outcomes, there is an intriguing problem to be solved.

Figure 1. Mirror-Image Symmetry of the Enantiomeric Molecules D- and L-Tartaric Acid

Pasteur discovered that a solution of tartaric acid from grape juice (now known to contain only the D form) rotated plane-polarized light, whereas chemically synthesized tartaric acid did not. Pasteur solved this puzzle by showing that the chemically synthesized compound was a mixture of the two forms, which when separated could rotate light in opposite directions.

Figure 2. Ventral View of Monocilia on the Mouse Node in Early Gastrulation

The diagram shows how clockwise-rotating cilia on the nodal cells can move a fluid suspension of small vesicles containing signaling molecules (nodal vesicular particles [NVPs]; red spheres) toward the left, creating a right-to-left asymmetric gradient across the midline. Key to the cilia's function is the posterior tilt of their rotational axes, as explained in the text. Connection arrows show the trajectory of the tip of one cilium as it rotates.