Vertebrate left-right asymmetry: Old studies and new insights

During vertebrate embryonic development, the organs of the chest and abdome n, heart, lung and gastrointestinal tract, aquire characteristic asymmetric positions with respect to the left-right body axis. In the beginning of th e 20th century Hans Spemann and his co-workers described manipulations of a mphibian embryos which resulted in inversion of organ laterality in a predi ctable manner. Hedwig Wilhelmi concluded from these experiments that determ inants on the left side of the embryo specifiy laterality, and Meyer postul ated that a mediator should transfer this positional information to the for ming heart. In this review we discuss the classical experiments in the ligh t of recent advances in the molecular understanding of left-right developme nt, with a focus on the mediator role of the homeobox gene Pitx2.