BMP in a hairy situation

Amorphogen must be both off and on for hairs to grow, according to Kobielak et al. on page 609. Their studies of bald mice reveal that bone morphogenetic protein (BMP) controls the switch from dividing to differentiating hair cells by working at the right time and place.

Hair cells arise from a pool of epithelial precursors that first proliferate and then differentiate to form the hair channel and the hair itself. Proliferation signals originate from a set of mesenchymal cells called the dermal papillae (DP) at the base of the hair follicle. Further up the follicle, differentiation is a response to Wnt signaling that works through stabilized β-catenin and Lef1 to turn on genes such as hair keratins. Now, Kobielak et al. find that the later differentiation depends on both the...