Openings between defective endothelial cells explain tumor vessel leakiness

Leakiness of blood vessels in tumors may contribute to disease progression and is key to certain forms of cancer therapy, but the structural basis of the leakiness is unclear. We sought to determine whether endothelial gaps o r transcellular holes, similar to those found in leaky vessels in inflammat ion, could explain the leakiness of tumor vessels. Blood vessels in MCa-IV mouse mammary carcinomas, which are known to be unusually leaky (functional pore size 1.2-2 mu m), were compared to vessels in three less leaky tumors and normal mammary glands. Vessels were identified by their binding of int ravascularly injected fluorescent cationic liposomes and Lycopersicon escul entum lectin and by CD31 (PECAM) immunoreactivity, The luminal surface of v essels in all four tumors had a defective endothelial monolayer as revealed by scanning electron microscopy, In MCa-IV tumors, 14% of the vessel surfa ce was lined by poorly connected, overlapping cells. The most superficial l ining cells, like endothelial cells, had CD31 immunoreactivity and fenestra e with diaphragms, but they had a branched phenotype with cytoplasmic proje ctions as long as 50 mu m. Some branched cells were separated by intercellu lar openings (mean diameter 1.7 mu m; range, 0.3-4.7 mu m). Transcellular h oles (mean diameter 0.6 mu m) were also present but mere only 8% as numerou s as intercellular openings. Some CD31-positive cells protruded into the ve ssel lumen; others sprouted into perivascular tumor tissue. Tumors in RIP-T ag2 mice had, in addition, tumor cell-lined lakes of extravasated erythrocy tes. We conclude that some tumor vessels have a defective cellular lining c omposed of disorganized, loosely connected, branched, overlapping or sprout ing endothelial cells. Openings between these cells contribute to tumor ves sel leakiness and may permit access of macromolecular therapeutic agents to tumor cells.