Slit, but not astrocytes, leads to asymmetric migration from SVZ explants. A, SVZ explants were cultured in the proximity of aggregates of HEK cells (right), x-Slit-expressing HEK cells (middle), or astrocytes (left) in Matrigel. The dashed lines indicate the location of the aggregates relative to the explants. Migration appeared asymmetric only in the presence of x-Slit-expressing cells. Scale bar, 300 μm.B, Quantification of migration from SVZ explants cocultured in the vicinity of aggregates for 11 hr in Matrigel. Although approximately equal numbers of cells migrated from the proximal and distal sides of SVZ explants cultured with HEK and astrocyte aggregates, most of the cells were found migrating from the distal halves of explants cultured with x-Slit-expressing cells.