Interdependent signaling loops across segment boundaries in Drosophila. Part of the maintenance of parasegment, and later segment, boundaries involves cell-cell signaling. Wnt-class signaling with the Wg protein, the wingless (wg) gene product, induces expression of the engrailed (en) gene in recipient cells. The En protein triggers expression of hedgehog, part of a non–Wnt-class signaling pathway. The Hedgehog protein (Hh) promotes expression of Wg in the original cells and completes the closed signaling loop.