across the eye disc
diagram © 1999: Jessica Treisman, Skirball Institute
See specification of the eye disc primordium and establishment of dorsal/ventral asymmetry.
Progression of the morphogenetic furrow (blue vertical line) from posterior to anterior (here, right to left) across the eye disc is driven by
hedgehog (hh) expressed in the developing photoreceptors (red dots).
hh activates the
expression of decapentaplegic (dpp) and the proneural gene atonal (ato) in
the furrow. ato expression is refined to the future R8 photoreceptor and is
required for the development of this cell.
Photoreceptors further away from the morphogenetic furrow (on the right) are further along in their development. Other photoreceptors and accessory cells are recruited to each ommatidial cluster (composed of developing photoreceptors) by waves of expression of the ligand spitz (spi) for the EGF receptor.