These are reactive mesothelia and not malignant cells. Some reactive mesothelia may have striking degrees of nuclear atypia, but note how smooth their nuclear membranes are and how open the chromatin pattern is. Nucleoli are one to several and always small. Cytoplasm is always present, sometimes with peripheral microvesiculation or small "windows" between adjacent cells. No matter how cellular the sheets or clusters are, nuclei seldomly overlap one another. To rezognize reactive mesothelia is very important in fluid cytology interpretation.