9+0 and 9+2 cilia are randomly dispersed in the mouse node

The initial determination of left-right asymmetry is an essential process in embryonic development. In mouse embryo, cilia in the node play an important role generating the nodal flow that subsequently triggers left-right determination in the embryo. Although nodal cilia have historically been thought to have a 9 + 0 axonemal configuration, the existence of 9 + 2 cilia has been reported so far. Because the distribution of those two types of cilia within the node has not yet been reported, we assessed the arrangement of 9 + 0 and 9 + 2 cilia in the node. In this study, we concluded that most of the nodal cilia were 9 + 0 in structure and there were much fewer 9 + 2 cilia than 9 + 0 cilia. Furthermore, the two types of cilia were randomly distributed in the node with no regularity. In addition, we studied the embryonic origin of the crown cells surrounding the node to better understand their identity.