Uptake of carbon and polystyrene particles by the sinusoidal endothelium of rabbit bone marrow and liver and rat bone marrow, with special reference to multiparticle-pinocytosis.

The endocytosis of tracer particles by sinusoidal endothelial cells was studied in the bone marrow and liver of the rabbit and in the bone marrow of the rat after intravenous injection. The tracer particles used included carbon particles and polystyrene particles of different sizes ranging from 0.11 to 5.7 micron in diameter. The sinusoidal endothelial cells of the rabbit liver and rat bone marrow took up carbon through the formation of bristle-coated vesicles at the luminal surface, but they eventually failed to ingest polystyrene particles of 0.11 micron or larger. On the other hand, the sinusoidal endothelial cells of the rabbit bone marrow took up carbon and polystyrene particles varying from 0.11 to 2.02 micron in diameter. These endothelial cells were found to sequester circulating tracer particles in the cytoplasm through the formation of two kinds of vesicles at the luminal surface: bristle-coated vesicles and "multiparticle-pinocytotic vesicles." This special type of pinocytotic vesicle was larger in diameter than the bristle-coated vesicle and variable in size according to the size or amount of the particles to be sequestered. Carbon and 0.11 micron polystyrene particles were taken up by means of both the bristle-coated vesicles and multiparticle-pinocytotic vesicles, whereas particles 0.30 micron in diameter or larger were taken up by the vesicles similar in nature to multiparticle-pinocytotic vesicles. A sequence for the morphological events occurring in the formation of multiparticle-pinocytotic vesicles in the sinusoidal endothelial cells of rabbit bone marrow shall be suggested and discussed.