Fluorescent granular perithelial cells and granular pial cells in the brains of aged mice.

The majority of fluorescent granular perithelial cells (FGPs) in the brains of aged mice contain large honeycomb bodies which probably represent a specialised type of phagosome. The other characteristic feature of FGPs is the presence of large numbers of small coated vesicles in the cytoplasm. Granular pial cells are structurally identical to FGPs and in particular they contain large honeycomb bodies. The evidence indicates that FGPs and granular pial cells represent one cell type whose major function in the aged brain is to act as a scavenger of substances such as lipid but which does not phagocytose degenerating cells or large masses of cellular debris. FGPs and granular pial cells are different from intraventricular macrophages both structurally and in their response to ageing. The similarity between FGPs and granular pial cells in the ageing brain supports the view that FGPs are derived from pial cells.