Mesothelial cell

Many organs in the body are not loose: they sit inside a slippery wrapping. That wrapping is a serous membrane, and what lines it on the inside is the mesothelium, the layer of cells called mesothelial cells. On the outside it is held in shape by connective tissue, the body's filler and support material. Think of a cloth bag: the mesothelium is the inner lining and the connective tissue is the cloth that gives it its shape.

What matters about those cells is what they release. They produce serous fluid, a watery fluid that moistens the membrane and lets the surfaces glide instead of scraping, the way two dry fingers drag when you rub them together but slide once you add a drop of water between them. In the chest that same fluid has a name of its own: pleural fluid, which comes from the mesothelial cells of both layers of the pleura, the wrapping around the lung, and lubricates their surfaces.