GAL4c855areveals the proliferation centers of the developing optic lobe. (a) A late third instar larval central nervous system (CNS): ventral nerve cord (VNC), central brain (CB) and optic lobes (OL). Subsequent images show frontal confocal sections, as shown in the inset diagram (OPC in green). Anterior and posterior refer to the neuraxis of the larval CNS. (b) A frontal section through a brain lobe at mid third instar: the OPC (green), the inner proliferation centre (IPC, yellow) and the medulla cortex (me). Discs large (Dlg; grey) outlines all cell cortices in the larval brain and highlights the morphology of the two optic lobe proliferation centres. (c) GAL4c855abegins to drive expression of UAS-pon-gfp (green; Dlg in red) at first instar. At late first/early second instar (24 hours ALH; after hatching), the OPC and the IPC can be distinguished as two closely associated epithelia. The cells belonging to the proliferation centers (green) are clearly distinguishable by their columnar shape, in contrast to the round, isolated central brain cells. (d) At the end of second/early third instar (48 hours ALH) the epithelia of the OPC and IPC separate from each other and smaller progeny cells are located between the two epithelia. (e) As development progresses during second to mid third instar (72 hours ALH) the OPC cells at the medial edge of the epithelium loose their columnar shape (to the left of the arrowheads). (f) At late third instar (96 hours ALH) the OPC epithelium decreases in size while the number of round neuroblast-like cells increases at the medial edges (to the left of the arrowheads). All images are single confocal sections, with anterior on top and lateral to the right. Scale bar is 50 μm (a) and 20 μm (b-f).