Evidence for a reaction-diffusion system as a mechanism controlling mammalian hair growth

A scheme is outlined for the differentiation of epithelial cells in hair follicle bulbs which requires several spatial pattern properties characteristics of a reaction-diffusion system. Spatial patterns in the reacting and diffusing substances, called morphogens , develop spontaneously depending on the size and shape of the follicle bulb. The patterns correctly predict the cross-sectional shape of several classes of fibres known as awls , auchenes and zigzags found in mice. The spatial patterns also determine the distribution of ortho- and para-cortical cells within the fibre cross-section. In order to explain the observed variation in the morphology of zigzag fibres it is necessary to use the capacity of a reaction-diffusion system to produce spatial patterns with a preferred orientation. Moving the dermal papilla of the hair follicle bulb off-centre causes a homogeneous pattern of morphogens to become unstable and a bilateral pattern to develop with a preferred orientation. This phenomenon forms the basis of a control mechanism for the formation of zigzag fibres.