Transmission electron microscope observation of mouse lymph nodes demonstrated that reticular fibers in the paracortex were invested not only by reticular cells, but occasionally also by migratory cells such as interdigitating cells, macrophages, and lymphocytes. Quantitative analysis of electron micrographs covering wide areas revealed that about 90% of the surface area of the reticular fiber was enclosed in the sheath of reticular cells in both nude and hetero mice, whereas the rest of the surface area was associated with migratory cells. In nude mice, whose lymph nodes contain more numerous interdigitating cells than hetero animals, about 9% of the surface area was occupied by interdigitating cells including Langerhans cells; in hetero mice only about 3% was associated with the interdigitating cells. Actively phagocytizing macrophages occupied about 3% of the surface area in both nude and hetero mice. Contact between lymphocytes and reticular fibers was observed in hetero mice, whereas this relation could not be demonstrated in nude mice whose lymph nodes contain very few lymphocytes. These results suggest that the association between reticular cells and reticular fibers in the paracortex of lymph node is flexible, allowing for the interposing of migratory cells.

The basic framework of the lymph node is constructed of reticular cells and reticular fibers. The reticular cells are connected to each other by desmosome-like junctions and extend along the extracellular skeleton of the reticular fibers. The mesh of the framework is occupied by lymphocytes and non-lymphocytec migratory cells such as macrophages and interdigitating cells. Early studies by electron microscopy demonstrated that the reticular fibers were regularly surrounded by a sheath of reticular cells, and thus separ-