Histochemical evidence for microglia-like macrophages in the rat trigeminal ganglion

Of the 4 major cell types in CNS parenchyma, microglia appear to serve the unique functional role of tissue macrophages. The distribution of equivalent cells in the PNS is unclear. Recently, the B4 isolectin of Griffonia simplicifolia was shown to bind selectively to microglia as well as to other macrophages under specific conditions. In the present study, this lectin was used to assess the existence of macrophages in the rat trigeminal ganglion. Vibratome sections of fixed ganglia were incubated with horseradish peroxidase (HRP)-conjugated isolectin, an HRP reaction subsequently performed, and sections processed for histology and viewed by light microscopy. Staining activity was found to be localised to a population of cells throughout the ganglion. These cells possessed small oval somata and several thin crenated processes, an appearance typical of ramified microglia. Stained cells also exhibited a regular, evenly spaced tissue distribution similar to CNS microglia. Finally, similar cells were also labelled by thiamine pyrophosphatase histochemistry, a cellular marker for CNS microglia/macrophages. It was concluded that there are microglia-like macrophages in the trigeminal ganglion and that these cells may function in immune reactions.