Morphological left-right asymmetry of Hensen's node precedes the asymmetric expression of Shh and Fgf8 in the chick embryo

Hensen's node and the rostral part of the primitive streak of chick embryos at HH-stage 4-7 were investigated using scanning electron microscopy, a series of semithin sections, and whole-mount in situ hybridization. An asymmetric expression of Shh and Fgf8 was first found at HH-stage 5. The asymmetric expression of both laterality genes is preceded by an asymmetric morphology of the avian organizer. The right lip of the streak and the node is much more prominent than the left one and contains a cylindrical cell condensation that is connected with the head process. Since the densely packed cells in Hensen's node and in the cranial part of the primitive streak connect the epiblast with the endoderm, a cilia-generated "nodal flow" between epiblast and endoderm in the avian embryo seems to be unlikely.