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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.
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