A major challenge in developmental biology is to understand how cells move and interact to form complex multicellular structures. Jennifer Zallen describes the discovery of a system of spatial information that organizes cell movements in the developing fruit fly embryo to elongate the body axis from head to tail. Interestingly, the global positional code is provided by an ancient family of Toll-related receptors that are widely used for pathogen recognition in the innate immune system. This research elucidates general principles that link molecular signals to the physical forces and collective cell behaviors that establish tissue structure, and may help to reveal how errors in these processes contribute to birth defects and cancer in humans.

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