The three-dimensional structure of interdigitating cells.

The in vivo three-dimensional architecture of lymph node and spleen interdigitating cells (IDCs) has been studied in mice by means of a computer reconstruction program and polystyrene models. Tissue fragments treated with an osmium-zinc iodide fixative solution, which gives a brilliant and quite specific impregnation of IDCs, were embedded in Epon and cut into 2 microns thick serial sections. The data was transferred into a "MOP-Videoplan 3D-cell reconstruction" software program and both tridimensional plottings and morphometric measurements were obtained. Then polystyrene models were assembled. Our findings show a cell population of high spatial complexity. IDCs are large cells whose numerous cytoplasmic processes form an extensive three-dimensional network which envelopes lymphoid cells. The most characteristic feature of IDC' architecture is the large, flattened, sheet-like processes which expand for several microns from the cell body, embracing numerous lymphocytes and lymphoblasts. In certain cases surface invaginations create channel-like structures which cross through the IDC peripheral cytoplasm. The impressive extension of the IDC membranous network provides a powerful anatomic strategy which facilitates the interaction between IDCs and T-lymphocytes and creates a unique microenvironment for T-cell activation and proliferation. This study gives new morphological evidence for the strict physical interaction between IDCs and T-lymphocytes and supports the concept that IDC-T lymphocyte aggregation represents a very special microanatomical and functional unit.