Fig. 7. A normal glial distribution is required for cell survival in the developing medulla. Cell death was compared in wild-type and so¹ animals in relation to the distribution of glia. (A-A'') A wild-type late third instar optic lobe is stained to reveal a normal organization of the medulla neuropile (med. n'pil, anti-HRP staining, green) and distribution of MNG and Xi glia (anti-Repo, red). Very few apoptotic cells are evident by staining with anti-activated caspase 3 antibody (blue in A,A', shown in grayscale in A''). (B-B'') In so¹ animals, an increased frequency of apoptosis was detected in the late third instar optic lobe (see the text for details). In the specimen shown, retinal axons have innervated the dorsal (dorsal is upwards) optic lobe, resulting in a relatively normally organized medulla neuropile. The ventral optic lobe lacks innervation and glia accumulate in the vicinity of the Wg domain (white arrow in B'). Few glia are found in the ventral neuropile region. An increased frequency of anti-activated caspase 3-positive cells (blue in B and B', grayscale in B'') is seen throughout the medulla cortex. More apoptotic cells are found in the ventral region (regions between white, yellow and blue arrowheads; see text for quantification), where glia are rare. (C-C'') An animal in which glial migration was blocked by expression of UAS-Ras1^N17 in somatic clones (marked by co-expression of UAS-CD8::GFP (green) (see the legend to Fig. 6, and Materials and methods for experimental details). A Ras1^N17-expressing clone in the dorsal Wg domain region has blocked glial migration into the dorsal optic lobe in this late third instar stage specimen. Glia (red) accumulate at the position of the yellow arrowhead shown in the boxed area in C. A locally increased frequency of activated Caspase 3-positive cells is visible (blue in C; grayscale in C', shown alone in grayscale between arrowheads in C''). Scale bar: 20 µm.