Observations on the ultrastructure of sinusoids and reticular cells in human bone marrow

The ultrastructural characteristics of sinusoids in human bone marrow were generally similar to those previously reported in the rat. However, human sinusoids differed from rat sinusoids in displaying frequent tight junctions between adjacent overlapping or interdigitating endothelial cells. In both species, large areas of the sinusoidal walls were devoid of much subendothelial connective tissue and of an outer adventitial cell layer. The marrow sinusoids of humans resembled those of rabbits and rats in that processes of macrophage cytoplasm protruded through endothelial cells into the sinusoidal lumen; thin veils derived from such processes were apposed over the inner surfaces of some endothelial cells. Features observed in pathological human marrow not noted in rodent marrow are the phagocytosis of extruded erythroblast nuclei and of abnormal erythroblasts by perisinusoidal adventitial cells (reticular cells) and the presence of large secondary lysosomes and siderosomes in sinusoidal endothelial cells. When compared with mouse bone marrow, human bone marrow contained very few non-phagocytic reticular cells that were unassociated with sinusoids and other blood vessels. There were no absolute ultrastructural differences between perisinusoidal adventitial cells, intersinusoidal non-phagocytic reticular cells and macrophages lacking phagosomes or containing only a few small phagosomes. Consequently, on some occasions, these cell types could not be reliably identified from the study of a single thin section. Extracellular reticulin fibres were found adjacent both to non-phagocytic reticular cells and to macrophages. Mitosis was observed in macrophages, albeit very rarely.