Fig. 5. JAK/STAT pathway activation leads to cellular overproliferation. (A,B) Late third instar Drosophila larvae: (A) wild type or (B) carrying a gain-of-function hop allele. Constitutively activated JAK/STAT signalling leads to the proliferation of lamellocytes, which aggregate to form prominent and, ultimately lethal, melanised tumours (dark cells). (C) Wild-type and (D) upd-overexpressing Drosophila eye imaginal discs, at the same magnification, are stained to show the structure of the disc (red) and mitotic cells (green). The morphogenetic furrow (MF) is marked, with the first and second mitotic waves visible ahead and behind it. The increased density of mitotic cells in the first mitotic wave is visible in the dorsal region of the upd-overexpressing disc. (E,F) Scanning electron micrographs of the adult eyes derived from (E) wild-type and (F) upd-overexpressing Drosophila showing the dorsal overgrowth of the adult eye that results from pathway mis-activation. (E) Reproduced, with permission, from Bach et al. (Bach et al., 2003).