Astrocytes surround the MB during developmental axon pruning.

<p>(A) Scheme of developmental pruning of MB γ neurons. During larval development and up to puparium formation (0 h APF), γ neurons extend a single process that sends out dendrites near the cell body and continues as an axon peduncle that bifurcates to form a dorsal and a medial branch. At the onset of puparium, a glial derived TGF-β signal induces the expression of the ecdysone receptor-B1 (EcR-B1) within γ neurons. Subsequently, an ecdysone pulse activates EcR-B1, resulting in a largely unknown transcriptional cascade. At 6 h APF both axonal branches, as well as the dendrites begin to undergo fragmentation, while the pedunclar axon remains intact and the neurons retain their cell body. The fragmented axons are then cleared by glia that surround these axons. Subsequently, γ neurons regrow axons that project into a new, adult specific and medially projecting lobe. (B–E) Confocal Z-projections of brains expressing alrm-GAL4 driving mCD8::GFP at 0 h APF (B), 6 h APF (C), 18 h APF (D) or 24 h APF (E). (B) At the onset of metamorphosis (0 h APF) astrocytic membranes are evenly dispersed in the region of the MB lobes (higher magnification in A₂). (C) By 6 h APF, at the onset of pruning, the astrocytes have changed their morphology and have begun to infiltrate the degenerating lobes. (D) At 18 h astrocyte membranes surround axon fragments (arrowhead in D₂). By 18 h to (E) 24 h APF there is a significant decrease in alrm-GAL4 positive membranes. Newly extended axon branches of α/β neurons, are also stained with anti-FasII antibody at 24 h APF (E). Magenta represents anti-FasII staining. Green is alrm-GAL4 driven mCD8::GFP. The scale bars are 20 μm. Genotypes: (B–E) y,w;alrm-GAL4;alrm-GAL4,UAS-mCD8::GFP/+.