Fig. 6. During the progression of the morphogenetic furrow, Hh signalling activates at least two different signals. One is Dpp which, at long range, primes cells to adopt a ‘pre-proneural’ state. These cells are ready to initiate neural differentiation, because they express Atonal and Daughterless, but are held in check because they also express the inhibitors Hairy and Emc, which keep the Atonal expression and activity at low levels. Delta/Notch activation provides a second Hh-dependent signal, which only works at short range (within the morphogenetic furrow), as Delta is a membrane-bound ligand. The activation of Notch causes the downregulation of Hairy and Emc, and consequently the upregulation of Atonal expression and activity. A different short range signal, unidentified but transduced by Raf, has also been proposed to upregulate Atonal (Greenwood and Struhl, 1999) (here shown in grey, see text).
