Representative photomicrographs of intravascular tumor. Endothelial cells were detected and visualized using immunostaining with (A, C) anti-CD31 and (B, D) anti-CD34 antibodies. (A) An example of a naked tumor embolus, that is, no evident endothelial covering, within an efferent vein in a case of breast carcinoma. (B) In a case of esophageal carcinoma, a tumor embolus within a vein is entirely covered with endothelial cells. (C, D) Tumor emboli originating from a breast carcinoma are enveloped by endothelial cells. (E) The same case as D. The tumor possessed well-developed sinusoidal vasculature. Original magnifications: (A, E), ×200; (B-D), ×400.