Ultra-thin transmission electron microscope sections showing morphological changes in blood vessels during the adenoma-carcinoma sequence in ApcMin/+ mice. A, A vessel in a normal region. The luminal surface of the endothelium generally has a smooth contour, but the thin margins of adjacent cells may overlap slightly and the distance to the lumen is short. Asterisks, endothelial cells; arrows, marginal fold at the cellular edge. Scale bars: 2 μm. B, Adenoma. Marked structural changes in the microvasculature were observed in aberrant villi of benign tumors that lacked endothelial cell junctions. Asterisks, endothelial cells; arrowheads, abnormal microvilli. Scale bars: 2 μm. C, Adenocarcinoma. The morphology of the vascular luminal surface became irregular; the endothelial cells accumulated lipid droplets and the irregularity of tumor vessels in terms of size variation became more prominent. Asterisks, endothelial cells; arrowheads, abnormal microvilli; double arrowheads, lipid droplet. Scale bars: 4 μm (left), 1 μm (right). D, Ultra-thin sections showing morphological changes in the basement membrane. Note the multi-layered basement membranes in adenoma and adenocarcinoma. Asterisks, endothelial cells; double arrows, a layer of the basement membrane. Scale bars: 0.5 μm.