 The cytoneme-mediated model suggests that Dpp is directly transported to target cells via actin-based filopodia called cytonemes that extend from the apical surface of Dpp-responding cells to the Dpp-producing source cells.These cytonemes have been observed, but the dependence of the Dpp gradient on cytonemes has not been definitively proven in imaginal wing discs.However, Dpp is known to be required for and sufficient to extend and maintain cytonemes.