Cardiac looping--an uneasy deal with laterality.

Cardiac looping is a key morphogenetic event in vertebrate heart development. In a complex progression, the linear heart tube adopts a sweeping rightward curvature, establishing an architecture for the multichambered organ it soon becomes. Looping is an asymmetric event, its rightward direction being determined by an embryonic left-right axial pathway. The dextral loop has been a vehicle for development of heart complexity during evolution, to the extent that in higher vertebrates and humans, heart structure and function is extremely susceptible to perturbations of laterality, which may be common. The first clues to genetic control of laterality and looping are now emerging. Key issues to resolve include the origin of laterality information in the embryo, the exact nature of the effector molecules that link laterality to looping and the intrinsic mechanisms that drive the looping process itself.