Follicular dendritic reticular cells in the germinal center of the rat lymph node as studied by immuno-electron microscopy.

Follicular dendritic cells (FDCs) in the lymph follicles extend many dendritic cytoplasmic processes and are embedded among abundant lymphoid cells of the germinal centers, making it difficult to visualize their profiles and clearly identify their precursors. In rats pre-immunized with horseradish peroxidase (HRP), three cell types revealed a positive HRP reaction in the germinal center of the lymph node. One type was the typical FDC with remarkable cytoplasmic processes showing complicated invaginations. The second type comprised cells which were associated with reticular fibers enclosed by their cytoplasmic processes. These cells possessed fewer cytoplasmic processes than the typical FDCs, but their cytoplasmic invaginations were remarkable. Cells of the third type were attached to the outside of capillaries. Some of them surrounded reticular fibers with their cytoplasmic processes, while others extended complicated cytoplasmic processes like those of the typical FDCs. These observations support a new view that FDCs belong to the reticular cell population. Reticular cells, which generally are considered to be a mere supportive structure in lymph nodes thus are presumed to have the potential to transform into FDCs, thereby playing a role in the immune response.