Fig. 2. The optic stalk expands before R axon innervation. (A-D) Optic stalks at the early larval stages before R axon projection. (A,B) Optic stalks carrying omb^P1 at the late second instar (A,A') and the early third instar (B,B'). lacZ expression is visualized with anti-ß-galactosidase (ß-gal) antibody (white). The BM is visualized with anti-Perlecan. (A) At the late second instar before R axon innervation, glial cells (white) form a tubular structure around Bolwig's nerves (magenta, anti-HRP). (B) At the early third instar, the optic stalk expands and glial cells increase in number. One ommatidial column is detected in this specimen, but R axon innervation is not observed (arrow). (C,D) A late second instar (C) or an early third instar (D) larval optic stalk carrying NP4702-Gal4 and UAS-mCD8GFP, viewed from horizontal perspective. (E,E') Cells in mitosis are visualized with anti-phospho-Histone H3 (magenta) in an early third instar optic stalk. Glial cells (blue, anti-Repo) in the optic stalk undergo mitosis (arrow). Repo-positive glial cells are shown alone in E'. (F,F') An early third instar larval optic stalk carrying NP4702-Gal4, UAS-mCD8GFP and ptc-lacZ (visualized with anti-ß-gal). The optic stalk consists solely of SG cells (green), while no WG cells (white) are seen in the optic stalk. (G-G'') Clonally labeled glial cells surrounding the optic lobe (green) never entered into the optic stalk (white, visualized with -Tubulin). (A-G'', except for C and D) Anterior is to the up, dorsal right. (C,D) Lateral is down, dorsal right. os, optic stalk.