THE SPECIES-SPECIFIC STRUCTURE OF MICROANATOMICAL COMPARTMENTS IN THEHUMAN SPLEEN - STRONGLY SIALOADHESIN-POSITIVE MACROPHAGES OCCUR IN THE PERIFOLLICULAR ZONE, BUT NOT IN THE MARGINAL ZONE

The microanatomical structure of human and rat splenic white pulp is c ompared, with special emphasis on the localization of the marginal zon e occupied by immunoglobulin M (IgM)(+) IgD(-/dull) B lymphocytes and its specialized macrophages. Our study reveals that in contrast to rat s, the marginal zone of humans primarily exists in the vicinity of pri mary and secondary splenic follicles and that it is almost absent arou nd the periarteriolar T-cell zones. We demonstrate that in humans ther e is an additional compartment, the perifollicular zone, located betwe en the marginal zone and the red pulp. The perifollicular zone is a dy namic region of variable cellular and phenotypic composition, which ca n be regarded either as a part of the red pulp or of the follicles. In most cases the perifollicular zone appears as a compartment of the re d pulp containing erythrocyte-filled spaces which differ from the typi cal red pulp sinusoids. Similar to the splenic cords, the perifollicul ar zone mostly harbours scattered B and T lymphocytes. However, someti mes B lymphocytes clearly predominate in the perifollicular area. In a ddition, strongly sialoadhesin-positive macrophages form sheaths aroun d capillaries in the perifollicular zone. Such capillary sheaths are n ot observed in rats. In humans weakly sialoadhesin-positive macrophage s are also present in the perifollicular zone and in the red pulp. In some specimens sialoadhesin is, however, strongly expressed by a large number of dispersed perifollicular macrophages. Interestingly, in str iking contrast to rats, the human marginal zone does not contain sialo adhesin-positive macrophages and marginal metallophilic macrophages ar e also absent in humans. Thus, sialoadhesin-positive macrophages and I gM(+) IgD(-) memory B lymphocytes both share the marginal zone as a co mmon compartment in rats, while they occupy different compartments in humans. We show that the human splenic marginal zone does not contain a marginal sinus and assume that in humans the perifollicular region i s the compartment where antigen and recirculating lymphocytes enter th e organ.