Symmetry breaking of the embryo
Nonaka S, Shiratori H, Saijoh Y, Hamada H
Determination of left−right patterning of the mouse embryo by artificial nodal flow
Nature 418:96-99, 2002

The body plan of organisms can be defined by three major axes that are patterned during embryogenesis: the anteroposterior (A-P), dorsoventral (D-V), and left-right (L-R) axes. Different organisms employ different strategies for axis specification, and even within a single organism the three axes are determined by distict mechanisms. Here we focus on the mechanism of L-R axis formation during mouse embryogenesis.

In the mouse embryo, Left-Right specification takes place after the A-P and D-V axes are determined. Once A-P and D-V axes form, we can also define the midline that separates left and right halves. Because the internal structure of the embryo contain left-right asymmetry, however, a distinct mechanism must exist that breaks the symmety to specify which is left or right. This has long been a fascinating problem in the field of developmental biology and medical science.

In this paper, the authors address a specific question: what is the relationship between L-R specification and the "nodal flow"? The "node" is a special structure located at the tip of the mouse embryo. The node initially acts as an organizer for gastrulation, but it also seems to have an important role in L-R symmetry breaking. Each cell of the node has a single motile cilium, which rotate to make a leftward fluid flow (nodal flow). Although this directional flow had been implicated in L-R symmetry breaking, direct evidence for the role of this flow in L-R specfication had been lacking. This is the issue that the authors decided to tackle.

The most interesting point in this paper is the strategy. I still remember how I felt when I read this paper; it was really "a change of thinking". The authors adopted an unusual methodology that developmental biologists had never tried before --- even though the method itself is rather simple and straightforward. I would like you to enjoy this paper by learning how the authors addressed an important question regarding symmetry-breaking.

If you are not so familiar with the mouse embryo and its node, you can find information in the following review:
Lee JD and Anderson KV (2008). Morphogenesis of the Node and Notochord: The Cellular Basis for the Establishment and Maintenance of Left−Right Asymmetry in the Mouse.
Developmental Dynamycs 237: 3464-76.