Fig. 8. A model for the onset of lamina column formation, which depends on Single-minded induced by Hh delivered from R axons. (A) Schematic illustrations of the sim mutant phenotype. Wild-type (upper) and sim mutant (lower) lamina. sim mutant neurons (pale blue circles) fail to be incorporated into the assembling domain and accumulate in the pre-assembling domain. Lateral perspective of the wild-type and sim mutant lamina are also illustrated (right). (B) A model for the incorporation of the lamina neurons into the process of lamina column formation. Newly arriving axons project to the lamina layer along existing R axons (I). G1-phase lamina precursor cells receive Hh from R axons, which triggers Dac and Sim expression and differentiation to lamina neurons (II). Sim enables lamina neurons to associate with one of the paired R axon bundles (III). The R axon bundle is enveloped by lamina neurons and is incorporated into the lamina column. Lamina neurons become mature by receiving Spitz, an Egf ligand, which is also provided by R axons at a later stage. (C) Hh delivered from R axons induces dac, egfr and sim independently.