The density and distribution of follicular dendritic cells in rat popliteal lymph nodes: a decrease in their density...

To determine whether follicular dendritic cells (FDCs) increase after stimulation, and also to confirm the widely accepted finding that the density of FDCs in the light zone is higher than that in the dark zone, we examined the density of FDCs in lymph follicles of rat popliteal lymph nodes. Rats aged five weeks were stimulated by injection of sheep erythrocytes, and then examined 10 days later. Unstimulated rats were also examined. After embedding in paraffin, the removed lymph nodes were immunostained with anti-S-100 protein antibody as a marker of rat FDCs. The density of FDCs was determined by measuring the area of the lymph follicle and counting the number of S-100 protein-positive cells within it. The density of FDCs in the lymph follicles of stimulated rats was found to be significantly lower than in 5-week-old and unstimulated rats. The density of FDCs in the light zone was similar to that in the dark zone when germinal center bordering cells (GCBCs), distributed at the border between the dark zone and the adjacent corona, were counted as FDCs. We conclude that the density of FDCs in lymph follicles decreases after stimulation, and suggest that FDCs have no or only very slight proliferative activity under normal conditions. Investigators may need to consider GCBCs in order to understand how FDCs differentiate.