Endothelial cells and mesothelial cells differ in location. Endothelium lines the blood vessels and other tube-shaped vessels, while mesothelium covers the outer surface of organs, according to the Histology Learning Center from Boston University.
Both of these simple squamous epithelial cells are responsible for lining free surfaces. Endothelium cells and mesothelium cells also differ in cytoplasm concentration. Endothelial cells usually have little-to-no cytoplasm present between nuclei. Mesothelial cells tend to have much more cytoplasm between structures.
Endothelium has direct contact with blood and lymph. Endothelial cells have a central nucleus and lie on a fibrous basement membrane. The structure of endothelium involves multiple overlapping cells to prevent leakage from the vessel. Endothelium can be further divided into continuous, fenestrated and discontinuous endothelium. Endothelium located in the lymph vessels act against infection by draining interstitial fluid out of cells and tissues.
Mesothelium forms a protective covering around many vital organs. Mesothelium is associated with internal body cavities, not blood and lymph. Like endothelium, mesothelium has a central nucleus and lies on a basement membrane. Mesothelial cells are subdivided into different types based on location and organ association. Both the endothelium and mesothelium are a single-cell-layer thick and are derived from the mesodermal layer of the embryo.