Endothelial cells are thin wafer-like cells joined at their borders and form the inner lining of the entire blood vascular system. In capillaries, the outer tunics of smooth muscle cells are absent and only the endothelium is present. Capillaries are often invested in pericapillary contractile cells called pericytes. The layer of endothelial cells is a type of epithelium and is bounded at its outer surface by a basement membrane. The lumenal (blood compartment) and ablumenal (tissue compartment) surface of endothelial cells contain numerous flask-shaped invaginations called caveolae (little caves). In thin sections, there also appears to be free floating membranous vesicles in the cell's cytoplasm. The smallest capillaries are a single endothelial cell warpped around to join upon itself with a junction. In most instances this junction is of the zonlua occludens type through which no movement of solutes occurs.