Endocytosis of low density lipoproteins in human endothelial cells: typical morphological aspects of the high affinity receptor-mediated pathway as revealed by serial sections and acid phosphatase cytochemistry

We used electron microscopy serial sections and acid phosphatase cytochemistry to follow the endocytosis of low density lipoproteins (LDL) by cultured human umbilical vein endothelial cells. The surface LDL-receptors were labeled with LDL-colloidal gold conjugates at 4 degrees C and then, cells were incubated for 0, 5, 10, 20, 30 and 60 min at 37 degrees C before fixation. The organelles identified in this way and studied by serial sections corresponded to those described in endothelial cells and in other cell types, with a peripheral endosomal compartment and a juxtanuclear endosomal compartment, located in the vicinity of the Golgi apparatus and mainly composed of round vesicle-containing structures called multivesicular bodies. By acid phosphatase cytochemistry, the limits between the endosomal juxtanuclear compartment and the lysosomal compartment were also studied and it was shown that multivesicular bodies are probably the first site for the acquisition of acid hydrolases. Lastly, the observation of highly gold-labeled endothelial cells when cells were incubated for 90 min at 37 degrees C with the LDL-gold conjugate, suggested that LDL was degraded in the lysosomal compartment.