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The left-right body axis, along with the dorso-ventral and antero-posterior axis, is certainly very important, but at the same time the most neglected body axis of bilaterally symmetrical animals. The asymmetrical distribution of visceral organs along this body axis is vital for many animals. In vertebrates, this asymmetry becomes established at early embryonic stages. An essential role in this process is played by the organizer, which is responsible for the correct establishment of the left-right axis. Among vertebrates, organizers of left-right asymmetry are found in different parts of the embryonic body, and with few exceptions, they are composed of cells with motile cilia, which create an unidirectional leftward flow of extracellular fluids in the organizer. The flow is probably sensed by the cells that surround the left side of the organizer. These cells respond to the aforementioned mechanical stimulus by triggering the Nodal signaling pathway. This signaling cascade results in the left-sided expression of the Pitx2 gene, which specifies the left side of the embryonic body. It appears, that the role of Nodal signaling pathway in determining left-right asymmetry is not only present in all vertebrates, but also is probably ancestral to bilaterally symmetrical animals. Another mechanism ensuring...

Left-right asymmetry determines the orientation of visceral organs during gastrulation in the mouse embryo. The asymmetry originates in ventral node which is located on the anterior end of primitive streak. Cells of the ventral node possess motile nodal cilia. These cilia rotate and generate leftward flow of extra-embryonic fluid. The leftward flow initiates asymmetric expression of Nodal signalling pathways in nodal cells. Abnormal nodal flow and the interruption of asymmetric gene expression cause reversal arrangement of visceral organs, called situs inversus. The mechanosensing and chemosensing models are the main theories addressing the question of how the leftward nodal flow is sensed by nodal cilia. The mechanosensing model, also referred to as 'two cilia model', is based on two types of cilia, motile and immotile. Motile cilia generate nodal flow while immotile cilia act as mechanosensors with polycystin cation channels. According to the chemosensing model, morphogens are secreted and form gradient that induces asymmetric gene expression in the node. It is still unclear which model is the right one, but it is possible that the final model is combination of both.