Figure 1

Scanning electron micrographs of an early mouse embryo, with three somite pairs, viewed from the posterior. (A) The node (white arrow) is clearly visible as a depression with a rounded posterior that continues forward opening out into the notochord. The cells surrounding the top of the node are often called the crown cells, and these have been implicated in responding to leftward nodal flow. Left (L) and right (R) sides are indicated. (B) A higher magnification of the node shown in panel (A). At this magnification, the presence of a single cilium on each of the cells within the pit of the node is visible. These cilia beat in a circular manner, but due to a posterior tilt, they drive a leftward flow of fluid in the node. It is this leftward nodal flow that normally establishes the direction of the left-right axis (credit: Dominic Norris).