Fig. 6. A model for Bar regulation during retinal neurogenesis. Ato expression (gray region) in the morphogenetic furrow (MF) is activated by Hh produced by photoreceptor cells (orange region) and initiates the generation of photoreceptor neurons. Bar homeodomain proteins, which are essential for transcriptional repression of ato (Lim and Choi, 2003), are expressed in basal undifferentiated cells behind the furrow (green region) by several mechanisms. Positive and inhibitory relationships labeled by arrows may be indirect. (i) Prior to photoreceptor differentiation at the time of furrow initiation, Bar expression in the basal undifferentiated cells is induced initially by a secreted signaling factor, Hh, from the posterior margin (yellow region). (ii) During furrow migration, Bar expression near the furrow is induced by Ato from the furrow. EGFR signaling may partially mediate nonautonomous effects of Ato on Bar expression. (iii) Hh produced in photoreceptor cells induces Dpp expression and may also contribute to Bar expression during furrow migration. Finally (iv), Bar is autoregulated to maintain its expression.