Mesothelium is the name given to the membrane that lines most body cavities and surrounds the internal organs. Cells that shed from these membranes are commonly found in pleural, peritoneal, and pericardial fluids. (Pleural, pericardial, and peritoneal fluids are universally grouped as serous fluids because of similarities in the spaces or body cavities in which they are found as well as similarities in the formation of the fluids themselves.)
Mesothelial cells are large cells that may be found as single cells or in clusters and clumps. They tend to have a large round centrally placed nucleus with a generous amount of basophilic cytoplasm, which can appear frayed at the edges. They will have one or two small, well-defined, deeply staining nucleoli. While they may have small pinpoint vacuoles, they will not have the larger "foamy" vacuoles seen in macrophages or histiocytes.
Two mesothelial cells are indicated by the arrows in the image on the right. While they are different in size, they are definitely larger than the background normal and atypical lymphocytes. Notice the irregular frayed edges of the cytoplasmic membranes.