Fig. 6. Ectopic expression of Hedgehog interferes with midline cell differentiation. (A) At stage 11, about six midline cells per segment express the proneural gene lethal of scute (L'sc, black). (B) Ectopic expression of Hedgehog in the neuroectoderm and developing CNS (sca-GAL4) induces Lethal of scute expression in all midline cells. (C) At stage 13, the first axons cross through the anterior commissure (arrow; Futsch, brown). Engrailed-positive midline cells (En, dark purple) cluster at the developing posterior commissure. (D) Ectopic expression of Hedgehog activates Engrailed expression in all midline cells and prevents the crossing of axons through the anterior commissure. (E) Axons (BP102, brown) in the mature CNS form a ladder with the anterior (arrowhead) and posterior commissure as rungs. (F) Ectopic expression of Hedgehog deletes the anterior commissure (arrowhead). (G) At the end of embryogenesis, midline glia cells (star; Slit, dark purple) tightly enwrap the commissures. (H) In embryos expressing Hedgehog ectopically, midline glia cells do not enwrap the remaining commissure. Often midline glia cells become apoptotic (arrow). (I) In each segment Odd (brown) is expressed in two midline-derived MP1 neurons and two dMP2 neurons. (J) Ectopic expression of Hedgehog eliminates Odd expression from most MP1 neurons (brown, arrow) but does not affect expression in the dMP2 neurons. (K) At the end of embryogenesis, midline cells (GFP, green) show an intricate axonal pattern and a subset of cells continues to express Engrailed (En, red). (L) The ectopic expression of Wingless in all midline cells does not interfere with the differentiation of the cells or the expression of Engrailed. (M) Ectopic expression of Hedgehog in all midline cells activates the ectopic expression of Engrailed in the midline. (N) Ectopic Hedgehog causes a severe reduction in the number of midline cells. Ventral views, anterior to the left; brackets outline the midline.