Paraxial <b><i>Nodal</i></b> Expression Reveals a Novel Conserved Structure of the Left-Right Organizer in Four Mammalian Species

Nodal activity in the left lateral plate mesoderm is a conserved sign of irreversible left-right asymmetry at early somite stages of the vertebrate embryo. An earlier, paraxial nodal domain accompanies the emergence and initial extension of the notochord and is either left-sided, as in the chick and pig, or symmetrical, as in the mouse and rabbit; intriguingly, this interspecific dichotomy is mirrored by divergent morphological features of the posterior notochord (also known as the left-right organizer), which is ventrally exposed to the yolk sac cavity and carries motile cilia in the latter 2 species only. By introducing the cattle embryo as a new model organism for early left-right patterning, we present data to establish 2 groups of mammals characterized by both the morphology of the left-right organizer and the dynamics of paraxial nodal expression: presence and absence of a ventrally open surface of the early (plate-like) posterior notochord correlates with a symmetrical (in mice and rabbits) versus an asymmetrical (in pigs and cattle) paraxial nodal expression domain next to the notochordal plate. High-resolution histological analysis reveals that the latter domain defines in all 4 mammals a novel 'parachordal' axial mesoderm compartment, the topography of which changes according to the specific regression of the similarly novel subchordal mesoderm during the initial phases of notochord development. In conclusion, the mammalian axial mesoderm compartment (1) shares critical conserved features despite the marked differences in early notochord morphology and early left-right patterning and (2) provides a dynamic topographical framework for nodal activity as part of the mammalian left-right organizer.