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(A) How FGF10 and Sonic hedgehog are thought to induce the growth and branching of the buds of the bronchial tree. Many other signal molecules, such as BMP4, are also expressed in this system, and the suggested branching mechanism is only one of several possibilities. (B) A cast of the adult human bronchial tree, prepared by injecting resin into the airways; resins of different colors have been injected into different branches of the tree. (B, from R. Warwick and P.L. Williams, Gray's Anatomy, 35th edn. Edinburgh: Longman, 1973.)