Dying cells make a joint

Aleg that doesn't bend is not much use. To make space for joints in the fruit fly limb, a thin band of cells are carved out by apoptosis, according to a new study by Cristina Manjón, Ernesto Sánchez-Herrero, and Magali Suzanne (Autonomous University of Madrid, Spain). The team shows that a sharp switch in morphogen signaling determines which cells get eliminated.

The morphogen in question is decapentaplegic (Dpp). In developing flies, it defines where and in which direction the legs should grow. Manjon et al. now find that, later on, Dpp initiates the formation of joints in the distal segments of the fly leg.

Across the would-be joints, Dpp signaling activity abruptly switches from high to low. At this transition point, a narrow band of cells undergoes apoptosis. Mutants that lack the signaling boundary due to...