Electron Microscope Studies on the Vessel Wall and the Leucocyte Emigration

Emigration of leucocytes from the inflamed blood vessels as well as the normal fine structure of vessel walls in the mouse skin has been observed by electron microscopy.The emigration occurs exclusively through the wall of small veins. The cytoplasm involved in junctions between contiguous endothelial cells of the veins appear to be highly elastic. These endothelial cells are surrounded by varying layers of peri-endothelial cells. No smooth muscle cells of the tunica media nor perivascular nerve fibers exist in the veins.During acute inflammation, many leucocytes stick to the endothelial surface and insert their pseudopods mostly into the intercellular junctions between endothelial cells. Then they promptly pass through the junctions and are held between endothelial and peri-endothelial layers for relatively long time before their eventual escape through spaces among the periendothelial cells into the surrounding connective tissue. No large intercellular gap is formed in the endothelial lining during the process of emigration. After the passage of leucocytes through the endothelium no residual sign of it is recognizable.