Mechanisms of tissue patterning. (A) Turing reaction–diffusion systems, in which cells produce a system of two morphogens with different rates of diffusion, one an activator and the other an inhibitor of a tissue pattern. The activator stimulates both its own production as well as that of its inhibitor. Because the activator diffuses slowly, it concentrates and acts locally, whereas the quickly diffusing inhibitor can only act over a limited distance before it is degraded. The two acting together yield a standing wave pattern of activation, which can give rise to periodic patterns in nature. A simulated reaction–diffusion system is illustrated on the lower right, illustrating the nature of the activator and inhibitor’s diffusion through a tissue and two types of patterns that can emerge. (B) An immunofluorescence image of P-cadherin (P-cad) in embryonic day 16.5 mouse epidermis, revealing the regular pattern and AP angling of hair follicles. The positions of hair follicles are believed to be determined by a reaction–diffusion system comprised of Wnt and Dkk4. Bar, 10 µm. (C) Positional information as an alternative patterning mechanism, in which cells interpret their position within a concentration gradient (left). Different threshold levels of a morphogen can specify multiple cell fates. An example of such a system is the patterning of Drosophila body segments (right), in which pair-rule genes are expressed in alternating stripes, giving each segment a unique identity. The pattern of pair-rule gene expression, in turn, specified by expression of gap genes that interact with maternal gradients of bicoid and caudal expression. Simulations in A were produced using code from Leppänen (2015).