Signalling molecule that designates the front or back of a structure in early development. For example, a zone of polarizing activity was discovered the 1960s, which has been shown to at acts as organizer that mediates cell-to-cell interactions responsible for limb patterning in vertebrate embryos
See Cytokines, Growth cones, Lipids, Maturational gradients, Neurulation, Retinoic acid (RA