Ultrastructural and morphometrical studies on the reticular framework and reticular fibers in the reticuloendothelial system of rats.

The reticular framework and reticular fibers in the thymus, cervical lymph node, spleen and bone marrow of Wistar rats were studied by transmission electron microscopy and morphometrical method. The reticular framework was usually observed in these organs as a common structure that consisted mainly of the slender cytoplasmic processes of the fibroblastic reticular cells interconnected with tight junction. Ultrastructurally, the reticular fibers were a mixture of collagen fibrils and amorphous materials, and were almost completely ensheathed by the cytoplasm of fibroblastic reticular cells. Such characteristic structure of the reticular fibers was found not only in the thymus, lymph node and spleen, but also in the bone marrow where it has not been clearly demonstrated previously. Morphometrical analysis revealed that the content of collagen fibrils in the reticular fiber in the lymphoid tissues (the thymus, lymph node and splenic white pulp) was much greater than that in the hematopoietic tissues (the bone marrow and splenic red pulp). Based on these evidences, it was reasonably considered that the reticular framework and reticular fiber ensheathed by the cytoplasm of the fibroblastic reticular cells were the most representative common structure in the reticuloendothelial system (RES) and played some important roles in the functions of RES.