The filopodia only become visible when single cells are labeled. Their inhibitory action is mediated by Delta attached to the filopodia, which contacts membrane-bound Notch on the surrounding, inhibited cells. The extent of visible filopodia roughly matched the range of inhibition. And overexpression of Delta increased the range of the filopodia, so Delta actually promotes formation of its own means of transport.
Inhibition of filopodial outgrowth did not prevent local Delta–Notch signaling, but did shut down longer range inhibition. The result was an increased density of neural cells and more of the associated structures such as microchaetes. Thus, epithelial cells can communicate at long distances without resorting to either diffusible mediators or cell relay mechanisms. ▪