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. Author manuscript; available in PMC: 2010 Jul 6.
Published in final edited form as: Int Rev Cytol. 2007;259:173–224. doi: 10.1016/S0074-7696(06)59005-9

FIG. 3. Midline hedgehog signaling is required to establish cell -intrinsic timing of retinal neurogenesis.

FIG. 3

(A) At the time of neurulation (10–15 hpf), the Hh signal from the prechordal plate (red) (Masai et al., 2000; Stenkamp and Frey, 2003a) regulates pax2 expression in optic stalk and separates the eye fields ( Macdonald et al., 1995). Evidence suggests that Hh signaling also establishes a proximal–distal gradient of an unknown intrinsic factor (Kay et al., 2005) that predicts the fan gradient (Hu and Easter, 1999; Li et al., 2000) of neurogenesis, as revealed by the spatiotemporal pattern of ath5 expression ( Kay et al., 2005; Stenkamp and Frey, 2003a). (B) If Hh signaling is blocked by cyclopamine treatment at the time of neurulation ( Kay et al., 2005; Stenkamp and Frey, 2003a), pax2 expression may be disrupted (Stenkamp and Frey, 2003a), and neurogenesis is either blocked or proceeds with inappropriate timing ( Kay et al., 2005; Stenkamp and Frey, 2003a). N, nasal; V, ventral.