Progression of spontaneous mammary tumors in wild type and NG2 null mice. A. Whole mounts of #4 mammary glands from MMTV-PyMT female mice (WT) and NG2 null MMTV-PyMT females mice (NG2 KO) at six, eight, ten, and 12 weeks of age were stained with carmine alum to visualize the developing mammary epithelium and associated MINs. Arrowheads indicate MINs. LN: lymph node. B. MIN progression in wild type and NG2 null MMTV-PyMT mice was quantified by using image analysis to determine the area occupied by MINs at each time point. Large masses in the nipple area were excluded from this analysis. Eight glands were examined at each time point for each genotype. C. At 14, 17, and 20 weeks of age, total tumor burdens were determined in wild type and NG2 null MMTV-PyMT female mice by dissecting and weighing all tumors. Weights are in grams. At 14 weeks, n = 8 mice of each genotype. At 17 weeks, n = 7 mice of each genotype. At 20 weeks, n = 8 mice of each genotype. Dashed lines: average tumor weights. D. Semi-log plots of the data from panel C show that after an initial fast start by tumors in wild type MMTV-PyMT mice, subsequent tumor growth is similar in wild type and NG2 null hosts. * P = 0.03; ** P = 0.005. E-F. Sections of 17-week tumors from wild type (WT, E) and NG2 null (KO, F) MMTV-PyMT female mice were H & E stained to visualize tissue morphology. Tumors in both wild type and NG2 null hosts are multifocal, heterogeneous for cellularity and tissue morphology, and very cystic in nature. G-J. Sections of 10-week (G, H) and 12-week old (I, J) mammary gland from wild type (WT, G, I) and NG2 null (KO, H, J) MMTV-PyMT female mice were H & E stained to visualize MIN development. At both ages, MINs occupy larger areas in wild type than in NG2 null mammary glands. Scale bars = 100 μm in E, F and 200 μm in G-J. MIN, mammary intraepiethelial neoplasis; MMTV-PyMT, mammary tumor virus-driven polyoma middle T; NG2, nerve-glial antigen 2.