Fibroblast derived soluble factors attract MDA-MB-231 breast cancer cells and enhance their invasiveness. A. Schematic representation of the 3D invasion assay shown in panel B. B. MDA-MB-231 cells were labeled with DiI (green fluorescence) and normal fibroblasts labeled with DiAsp (red fluorescence). Tumor cells were seeded in collagen gels in the presence or absence of fibroblasts as described in methods. Shown is a perpendicular section of a representative collagen gel examined with a fluorescent microscope. C. Quantification of tumor cells induced to invade by fibroblasts was performed as described under Figure 2.