Embryos need to flex their growing muscles if developing cells are to give rise to joints, says a team led by Elazar Zelzer at the Weizmann Institute of Science in Rehovot, Israel.
They found that mutant mouse embryos with defective muscles fail to form various joints, including elbows, shoulders and hips (a normal embryo is pictured). Without muscle contraction, the cells that generate joint tissues do not activate a key regulatory pathway controlled by the protein β-catenin, and the progenitors switch fate to form cartilage instead.
One idea put forward by the authors is that the mechanical stress created by developing muscles might inform the cells of where they are and what cell type they should generate. The study could be relevant to rare human cases in which babies whose movement is restricted in utero develop abnormal joints.