The endothelium

The cells are nearly always large and rich in granular cytoplasm. They contain distinct round or oval, vesicular nuclei, of which there is usually only one in each cell.
The intercellular substance is very small in amount and is clear and homogeneous. The arrangement of the cells and their size and shape all vary greatly, giving rise to a number of varieties of epithelium, which are classified according to the shape and arrangement of the cells. In pavement-epithelium the cells are thin and arranged in a single layer, not unlike endothelium. In cubical epithelium the cells are thicker and also usually arranged in but a single layer. In columnar epithelium the cells are prismatic in form and rest with their bases upon the surface of the tissues beneath.
They are usually separated at their bases by pyramidal cells, so that the layer of epithelium cannot be said to consist strictly of but one layer of cells, and in some situations there are several distinct layers. In stratified epithelium the cells are superimposed upon each other to form a layer of cells, the thickness of which is several times the diameter of a single cell. The cells of the variety of epithelium called ciliated epithelium differ from those of the other varieties in possessing delicate, hair-like processes which project from the free surface of the tissue.
Epithelium resembles endothelium in being composed almost exclusively of cells separated by a minimal amount of intercellular substance. Like endothelium, it is nearly always found covering other tissues and having one free surface. The two tissues differ greatly in the character of their cells, with one notable exception.
This exception is found in the epithelial lining of the pulmonary alveoli, where the pavement-epithelium contains cells that closely resemble those of endothelium. These cells are, however, directly exposed to the inspired air, while endothelium is only found in situations where it is protected from all contact with the exterior.