White pulp compartments in the spleen of rats and mice. A light and electron microscopic study of lymphoid and non-lymphoid celltypes in T- and B-areas.

The spleen of rats and mice was studied with the light and electron microscope. Special attention was paid to the delineation and composition of the white pulp compartments: periarteriolar lymphatic sheath (PALS), follicles and marginal zone. These three compartments each have their specific lymphoid and non-lymphoid cells. Reticulum cells and reticulin fibres, although occurring in all three compartments, from a characteristic pattern in each compartment. In the PALS two areas can be distinguished: a central area, largely devoid of reticulum cells, and a peripheral area where reticulum cells are arranged in cylindrical shells. The central PALS FORMS THE THYMUS DEPENDENT AREA OF THE SPLEEN, THE PERIPHERAL PALS contains both T- and B-lymphocytes. T-B-interactions requiring cell contact could take place in the latter area. Lymph vessels originate from the shells of reticulum cells around the smaller arterioles; these vessels follow the central arteriole to the hilus of the spleen. Circumstantial evidence suggests that the lymph vessels form a recirculation pathway for T-cells and possibly also for B-cells. In two areas of the splenic white pulp characteristic non-lymphoid cells are present. The central PALS contains interdigitating cells (IDC), which show a close contact with surrounding T-lymphocytes. The light zone of the follicle centre exhibits dendritic cells (DC). B-cells are found between the ramifications of the DC. It is conceivable that these cells play a role in the homing of T-cells and B-cells respectively. In addition they might create a microenvironment supporting differentiation and proliferation of T- and B-cells. The marginal zone does not contain a characteristic non-lymphoid cell type. However, in this compartment B-cells are directly exposed to the circulating blood. It is suggested that this factor constitutes one of the essentials of the microenvironment in the marginal zone.