Venule, light micrograph. Longitudinal section through a venule with a smaller joining tributary vessel. Venules have a thin wall of inner endothelium external to which are fibroblasts and mesenchymal cells collectively referred to as pericytes. An adjacent arteriole of smaller calibre shows a thicker wall comprised chiefly of smooth muscle. Epoxy resin section, Toluidine blue stain. Magnification: x350 when width printed at 10cm.