A morphogen is a substance governing the pattern of tissue development, and the positions of the various specialized cell types within a tissue. It spreads from a localized source and forms a concentration gradient across a developing tissue.
In developmental biology a morphogen is rigorously used to mean a signaling molecule that acts directly on cells (not through serial induction) to produce specific cellular responses dependent on morphogen concentration.
Well-known morphogens include: Decapentaplegic / Transforming growth factor beta, Hedgehog / Sonic Hedgehog, Wingless / Wnt, Epidermal growth factor, Fibroblast growth factor, and Retinoic acid.
Morphogens are defined conceptually, not chemically, so simple compounds such as retinoic acid (the active metabolite of retinol or vitamin A) may also act as morphogens.