Endocytosis by endothelial phagocytes: uptake of bovine serum albumin-gold conjugates in bone marrow.

The endocytic function of the endothelial cells of the venous sinuses ("sinusoids") in rat bone marrow was investigated. Marrow sinusoidal endothelial cells (MSEC) were exposed to pulse injections of bovine serum albumin-gold (BSA-Au) conjugates in vivo and examined after timed intervals by electron microscopy. BSA-Au particles were rapidly taken up by MSEC. Within 1 min, BSA-Au was internalized by means of coated pits and vesicles (exclusively) and processed through pleomorphic endosomes to dense bodies known to be secondary lysosomes. In this processing, no involvement of the Golgi apparatus was observed. The continuous association of BSA with gold throughout endocytic processing to lysosomes was verified by immunolabeling with anti-BSA antibody. Although the nature of the affinity mediating endocytosis of BSA-Au by MSEC, in particular the possibility of specific adsorption of BSA and receptor involvement, remains to be established, it is suggested that marrow sinusoidal endothelial cells internalize the albumin-coated particles and process them through to lysosomal structures for purposes of degrading the albumin component.