Fig. 7. A model of the terminal branch guidance mechanism. Hh is expressed in the P compartment and forms a concentration gradient that diminishes toward the anterior and posterior directions. Terminal cells adhere to the basal surface of the epidermis and receive Hh, which promotes cell adhesiveness and thereby maintains terminal cell localization at the top of the gradient. Dpp, which is expressed at the dorsal edge of the epidermis, acts as a barrier against terminal branching to ensure monopolar growth.