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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.
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