Morphogen

A morphogen is a signaling molecule that forms a concentration gradient across a field of cells and directs their developmental fates according to the concentration they experience. It is the physical implementation of positional information: cells at different positions in the gradient receive different signals and differentiate accordingly. The classic example is Bicoid in the fruit fly embryo, whose anterior-to-posterior gradient patterns the head and thorax. Morphogen gradients are not static maps but dynamical systems that emerge from the interplay of diffusion, degradation, and receptor-mediated uptake — and their robustness to perturbation is an active area of research in evolutionary developmental biology.