Increased metastasis associated with dense stromal collagen. (a) Increased number of lung metastases per lung at 15 weeks in mice that formed tumors in collagen-dense mammary glands (PyVT/Col1a1) when compared with mice that formed tumors in control glands (PyVT/wt; mean ± standard error of the mean (SEM), n = 4 of each genotype). (b) Three-dimensional tumor cell invasion assay showing that tumor explants from collagen-dense tumors (PyVT/Col1a1) resulted in more invasion into three-dimensional collagen gels and colony formation after 10 days than explants from PyVT/wt tumors (mean ± SEM; n = 4 PyVT/wt and n = 14 PyVT/Col1a1 tumor explants from four sibling mice). (c) Tumor cells extracted from collagen-dense tumors (PyVT/Col1a1) showed increased migration when compared to tumor cells from control tumors (PyVT/wt) as measured by transwell migration assays with serum as the chemoattractant (mean ± SEM; n ≥ 3 independent experiments). *Indicates a statistically significant difference (p < 0.05) following analysis with t-tests.