Pia arachnoid mesothelial (PAM) cells are often found in spinal fluid because they line the arachnoid space between the skull and the cerebrum. Ependymal cells which line the ventricles may also be present. Since ependymal and mesothelial cells are normal findings, the term mesothelial is sometimes used as a general term for both. It is essential to differentiate these tissue cells from blast cells or tumor cells which have diagnostic significance. An example of a pia arachnoid mesothelial cell, as indicated by the arrow, is present in this slide. Differentiating characteristics of this type of cell include an off-center, single, round nucleus and irregular cytoplasm that exhibits pseudopods.