Scanning electron microscopy of the subarachnoid space in the dog: evidence for a non-hematogenous origin of subarachnoid macrophages.

Injection of viable BCG into the subarachnoid space of immunized and non-immunized dogs produced a 10-fold increase in the populations of pial free cells. In immunized animals injected three days previously with BCG, stereoscopic SEM revealed that many pial cells had rounded up and were protruding into the subarachnoid space. With continued rounding these cells took on amoeboid characteristics, with shapes that suggested a capacity for cell movement. Internally, these pial cells possessed an increased volume of perinuclear cytoplasm and organelles. Reactive pial cells could be distinguished from macrophages of presumed hematogenous origin on the basis of their surface morphology. These findings suggested that pial cells had the ability to alter their normal structural and behavioral characteristics and to become macrophage-like under these conditions of secondary challenge by BCG.