Fig. 1. Expression analysis suggests that Shh from RGCs signals to cells in the retinal neuroblast, optic disc and optic stalk. (A,E) Sections through the optic vesicle of an E9 embryo hybridized with Shh (A) and Ptch (E) riboprobes demonstrate the upregulation of Ptch expression in the neuroepitheliun and cephalic mesenchyme adjacent to the Shh-expressing cells. (B,F) Frontal sections through the developing eye of an E11 embryo reveals a graded Ptch expression (F) in the ventral forebrain, with high levels in the anterior hypothalamic neuroepithelium (ahn) and low levels in the optic stalk (os), consistent with an established morphogen gradient of Shh (B) from the ventral midline. Ptch (arrows in F) expression in the diencephalic neuroepithelium is due to Shh from the zona limitans intrathalamica (not shown). The period from E12 to E14 is when most neuroepithelial cells of the optic stalk transform into astrocyte progenitor cells (Kuwabara, 1975), and this period is coincident with the rapid RGC differentiation and expression of Shh (C,D), as well as uniform Ptch (G,H) expression in the optic nerve. (I-P) The response of optic disc astrocyte precursor cells (odap) to RGC-derived Shh signaling. As RGCs differentiate in the central retina and express Shh (I), optic disc astrocyte precursor cells and retinal neuroblasts in close proximity to the Shh-expressing cells respond by upregulating the Hh target gene, Gli (J). It is likely that the Gli-expressing cells at the disc are the same cells that express Pax2 (K) and netrin 1 (Ntn1; L). Although RGCs continue to express Shh (M) into late embryogenesis and the underlying neuroblasts respond to this by expressing Gli (N), the Pax2-(O) and Pdgfra- (P) expressing retinal astrocyte precursor cells migrating into the retina (arrowheads in N,O,P), and those at the optic disc (arrows in N,O,P), downregulate their Hh responsiveness. ov, optic vesicle; tv, telencephalic vesicle; oc, optic cup; os, optic stalk; ahn, anterior hypothalamic neuroepithelium; odap, optic disc astocyte precursor cells.