Figure 11. Comparison between vertebrates and echinoderms: the laterality information emanating from a mesendodermal left-right organizer propagates to distant tissues through Nodal/Univin signaling.

In vertebrates, the asymmetric expression of nodal is initiated in a discrete region on the midline that plays the role of a left-right organizer (the node in mammals, the Küpffer vesicle in zebrafish or the gastrocoel roof in amphibians). H⁺/K⁺-ATPases, Delta/Notch and FGF signaling as well as cilia driven leftward flow have been implicated in the symmetry breaking process. Nodal expression subsequently propagates to the lateral regions of the embryos by mechanisms that involve long-range signaling by Nodal and GDF1 as well as communication with endodermal cells through gap junctions. In the sea urchin embryo, H⁺/K⁺-ATPase, Delta/Notch and FGF signaling are involved in the initiation of asymmetrical expression of nodal in a discrete region of the archenteron that appears to play the role of a left-right organizer. In the sea urchin, the laterality information is relayed from the left-right organizer to more distant regions through long range Nodal-Univin signaling.