Simple squamous epithelium

Seen from the surface the cells fit together like floor tiles with wavy borders; in section they are so thin (often < 0.5 µm away from the nucleus) that only the flattened, bulging nucleus is obvious — the "fried-egg" appearance. The minimal thickness gives the shortest diffusion path, which is why this epithelium forms exchange surfaces.

Two important types have special names: endothelium lines the heart, blood vessels and lymphatics, and mesothelium forms the epithelium of serous membranes (pleura, pericardium, peritoneum). Others include the type I alveolar cells (pneumocytes) of the lungs, the parietal layer of the glomerular (Bowman) capsule, the thin limbs of the nephron loop, the posterior corneal "endothelium" and parts of the inner ear. Although endothelium and mesothelium are true epithelia functionally, they derive from mesoderm.