The fine structure of subarachnoid macrophages in the rat

Macrophages associated with the subarachnoid space have fine structural characteristics which distinguish them from macrophages found elsewhere. The specific location of a macrophage in this area influences its morphology. Macrophages found in the subarachnoid space characteristically have a highly vacuolated cytoplasm. When cells of the pia mater separate the macro‐phage from the subarachnoid space, the extreme vacuolization is not present and a more compact cytoplasm prevails. Frequently macrophages found in the median longitudinal fissure contain compound reticulated inclusions. Extracellular bodies resembling compound reticulated inclusions have been observed. The presence of lobulated nuclei, lysosome‐like inclusions, phagosomes, and other characteristics of these cells is reported. The possible origin and function of subarachnoid space macrophages is discussed.