ABSTRACT
Left-right asymmetry is an essential feature in bilateral animals. The mechanism underlying the left-right asymmetrical organ morphogenesis is a central question in developmental biology. Studies in vertebrates show that left-right asymmetry formation needs three essential steps: the initial left-right symmetry breaking, the left-right asymmetrical gene expression, and the left-right asymmetrical organ morphogenesis. Many vertebrates use cilia to produce directional fluid flow to break symmetry during embryonic development, asymmetric Nodal-Pitx2 signaling to pattern the left-right asymmetry, and Pitx2 and other genes to control the morphogenesis of asymmetrical organs. In invertebrates, there are left-right mechanisms independent of cilia and even others more different from that of vertebrates. In this review, we summarize the major steps and relevant molecular mechanisms of left-right asymmetric development in vertebrates and invertebrates, aiming to provide a reference for the understanding of the origin and evolution of the left-right developmental mechanism.
