Cytology

The endothelium is a simple squamous epithelium. The flat endothelial cell has a longitudinal axis orientated along the direction of flow. It sits on a basement membrane. Ultrastructural studies have revealed more detail.

The cell contains small microvilli on its surface. Within its cytoplasm are mitochondria, Golgi bodies, numerous pinocytotic vesicles, endoplasmic reticulum and contractile filaments. The contractile filaments are rarely witnessed in use. Exceptions include the contraction of venular endothelial cells during acute inflammation, and splenic capillaries, which seem highly active.

The vesicles have diameter of around 50-70 nanometres. They have been suggested as one route by which macromolecules might traverse the cell from the vascular lumen. Occasionally, distinct rod-shaped cytoplasmic structures are seen. Electron-dense, they are termed Weibel-Palade bodies and are believed to be the storehouse for factor VIII of the clotting cascade.

The intercellular junction accounts for less than 1% of the total surface area of the endothelial cell. Less than 20 nanometres wide, it may extend for up to 1000 nanometres between adjacent cells. At the closest points, the cells come into contact via tight junctions. Between the areas of tight junctions - junctional strands - there are interruptions in the barrier which permit the passage of water and small metabolites. The number and overlap of junctional strands determine the permeability of the endothelium. Hence, brain capillaries with almost total perimeter coverage of tight junctions have low permeability; the seal is termed a zonula occludens. The intercellular space, like the cell surface, is negatively- charged due to a network of sialoglycoproteins. The charge aids in endothelial filtering properties. The basement membrane counters the surface stresses of the cell; they are linked by glycoprotein laminin molecules.