High mammary collagen density promotes tumor formation. (a) Histology of mammary glands from 10-week-old wild-type and heterozygous Col1a1tmJae mice showing increased stromal collagen and hypercellularity associated with the Col1a1tmJae mouse model. Scale bar 25 μm. (b) Significantly increased tumor numbers per mouse in collagen-dense (Col1a1) mammary glands. (c) Whole mount preparations of the fourth inguinal mammary glands from PyVT/wt and PyVT/Col1a1 mice at 10 weeks of age. Quantitative analysis of the area of hyperplasia from three pairs of glands calculated from a common threshold value set with density slicing in ImageJ software revealed a greater than 1.5-fold increase in hyperplasia associated with increased stromal collagen (t-test: p = 0.03). In addition, at age-matched time points, tumors in mice with dense stroma not only displayed more hyperplastic area but also tumor regions that grew out away from the gland (arrows in (c) and (d)). (e) Low (i), (ii) and high (iii), (iv) magnification images of H&E stained histology sections from 10-week-old mice showing increased collagen in PyVT/Col1a1 tumors ((ii) and (iv)) and a more invasive phenotype when compared with PyVT/wt (i) and (iii) tumors. Scale bars 50 μm in (i) and (ii) and 25 μm in (iii) and (iv).