Ultrastructural localization of lectin receptors on the bone-marrow sinusoidal endothelium of the rat.

The bone-marrow sinusoidal endothelium is a cellular barrier that separates developing blood cells in the extravascular space from the peripheral circulation. Mature blood elements enter the circulation via transendothelial migration pores. In the present study, monosaccharide constituents on the bone marrow endothelium were examined using lectin-affinity cytochemistry. With lectin-horseradish and lectin-ferritin conjugates, mannosyl, N-acetylglucosaminyl, galactosyl, N-acetylgalactosaminyl, and sialic acid were localized to the luminal plasmalemma, bristle-coated pits and diaphragmed fenestrae. These were conspicuously reduced on the abluminal plasmalemma. When the tissue was treated with biotinylated lectins followed by avidin-ferritin, only a localization with wheat-germ agglutinin (sialic acid; N-acetylglucosaminyl) was observed. Pretreatment of the bone marrow with neuraminidase enabled the localization of the other monosaccharide components by the biotin-avidin method. Accumulations of carbohydrate residues were identified near the endothelium subjacent to migrating cells. Fucosyl moieties marked by Ulex europaeus agglutinin ( UEA ) reagents on the endothelium were not present. All binding was abolished by incubation of tissue and lectin conjugates with specific hapten sugars. labeling was also not present after Pronase E treatment, indicating that the identified monosaccharides are components of glycoproteins rather than glycolipids. The possible function of endothelial-surface glycoproteins as receptors for the surfaces of mature blood cells and their role in transmural migration are discussed.