Histomorphology of the MINO model mammary precancer with early invasive carcinoma. (a) Low-power hematoxylin and eosin histomorphology of the MINO precancer filling the precleared fat pad. Intact gland is seen for comparison in the bottom left corner. The lymph node is seen (LN) at the left, and no invasion of the node by precancer tissue is seen. (b) The edge of the precancer is seen at higher power and is similar to a normal developing mammary gland terminal end bud with mitoses (arrowhead) and remodeling via apoptosis to clear the luminal space. In the area of precancer in the center of the growth there is an organized relationship between the transplanted MINO tissue and the host stroma. (c) The MINO cells differentiate to form acinar and ductal structures with high intralesional cell heterogeneity. (d) In this heterogeneous differentiated zone of precancer tissue, an area of transformation to invasive carcinoma is characterized by increased mitoses (arrowheads) and much less residual host stromal tissue. MINO, mammary intraepithelial neoplasia outgrowth.