L-cells/LE-II cells coculture. Induction of endogenous PAI-1 expression in fibroblasts contacting endothelial cells. (A and C) (brightfield and darkfield, respectively) show in situ hybridization of L-fibroblast cells (L)/ LE-II endothelial cells (E) cocultured with a PAI-1–specific probe. (B and D) (brightfield and darkfield, respectively) show in situ hybridization of a L-fibroblast cell culture displaying a gradient of cell density.