RCA-I lectin histochemistry after trypsinisation enables the identification of microglial cells in thin paraffin sections of the mouse brain.

A biotinylated lectin from Ricinus communis (RCA-I) and avidin-biotin-horseradish peroxidase complex (ABC) were used to identify microglial cells in 3-microns-thick sections of formalin-fixed paraffin-embedded brains of adult mice required for quantitative cell kinetic studies. In 3-microns-thick sections of the mouse brain the staining intensity of RCA-I-positive cells compared to background staining was too low for evaluation, quite in contrast to rat brain. However, perikarya and cytoplasmic processes of microglial cells were clearly stained in 10- and 20-microns-thick sections. The low contrast characteristic of thin mouse brain sections could be enhanced by pre-incubating the sections with trypsin before application of the lectin. We assume that different densities of RCA-I binding sites among microglial cells of rats and mice, respectively, are responsible for the different staining intensities observed. Our protocol of lectin staining did not influence subsequent autoradiography for studies of cell proliferation after [3H]thymidine application.