Electron Micrographic Observations on the Emigration of Leucocytes

Electron micrographs have been made of small venules in the rat's mesentery after exposure to mild mechanical trauma. Stages in the migration of blood cells are illustrated. Polymorphonuclear leucocytes seem sometimes to penetrate the endothelium at or near cell junctions, but they may also be able to penetrate the endothelial cytoplasm at other points. After the passage of the leucocytes through the endothelium no sign of a passage way can be seen. The emigrating leucocytes may be held up for a time by the basement membrane of the endothelium or by the periendothelial tissues but these are eventually penetrated so that the leucocytes become free in the surrounding tissue. Eosinophils and probably monocytes were seen passing through the vessel walls apparently in a manner similar to that of the polymorphs. Red blood cells were seen in process of going through the endothelium but the pictures do not show whether they pass through it in holes made by leucocytes.