Venule, light micrograph. Longitudinal section showing venules filled with erythrocytes (red blood cells) and occasional leukocytes (white blood cells) with nuclei. Venules have thin walls comprised of endothelial cells with slender elongated nuclei. External to the endothelial wall are pericytes which are fibroblasts and mesenchymal cells. At upper left several small calibre capillaries are seen in longitudinal and cross-section just wide enough to accommodate erythrocytes seen within the vessel lumen. Paraffin section, Masson's trichrome stain. Magnification: x320 when width printed at 10cm.