Tissue cells that are never seen in peripheral blood but are often seen in spinal fluid samples are presented in the table below:

Cells	Causes
macrophages	RBC's in CSF viral meningitis tubercular meningitis
ependymal	normal - due to shedding of cells that line the ventricles
pia arachnoid mesothelial cells (PAM)	normal - due to shedding of cells lining the arachnoid space

These cells are important because they must be differentiated from tumor cells and blast cells.