Positive feedback
Positive feedback can create asymmetry from homogeneity. In cases where the external or stimuli that would cause asymmetry are very weak or disorganized, through positive feedback the system can spontaneously pattern itself. Once the feedback has begun, any small initial signaling is magnified and thus produces an effective patterning mechanism. This is normally what occurs in the case of lateral inhibition in which neighboring cells induce specification via inhibitory or inducing signals (see Notch signaling). This kind of positive feedback at the single cell level and tissue level is responsible for symmetry breaking, which is an all-or-none process whereas once the symmetry is broken, the cells involved become very different. Symmetry breaking leads to a bistable or multistable system where the cell or cells involved are determined for different cell fates. The determined cells continue on their particular fate even after the initial stimulatory/inhibitory signal is gone, giving the cells a memory of the signal.