An electron microscopic study on the reticuloendothelial system in the bone marrow.

Abstract

The reticulo-endothelial system and fat cells of red bone marrow of the femur of guinea pigs, rats and rabbits was examined with the electron microscope. 1. The reticulo-endothelial tissue in the bone marrow consisted of reticulum cells and sinal endothelium. 2. The reticulum cells were characterized by large pale cytoplasm and a number of slender cytoplasmic processes which were thought to be connected with those of adjacent reticulum cells to form a cellular network. They also projected thin sheet-like processes toward the sinal walls and fat cells to cover their outer surface. 3. The sinal wall consisted of a single layer of endothelial cells. Only rudiments of basement membrane were present surrounding the sinal wall. The sinal endothelium showed gaps of various sizes, which were thought to be formed by the fenestration of the cytoplasm. Pseudopod-like processes of reticulum cells were seen to project into the sinal lumens through the endothelial gaps. 4. The fat cells contained single large fat droplets occupying the most part of the cytoplasm. In the perinuclear portion of the cytoplasm, a number of small droplets and glycogen granules were present. Basement membranes were seen surrounding fat cells. 5. After irradiation of 60Co, a marked proliferation of reticulum cells and fat cells were observed. The proliferated reticulum cells showed more abundant cytoplasmic organelles than normal ones and possessed numerous irregular cytoplasmic processes. On their surface were formed fibrous materials probably corresponding to reticulin fibers, which were not observed in normal cases. 6. As compared with endothelial cells of normal sinuses, those of irradiated cases showed thicker cytoplasm, prominent tight junctions and more complete basement membranes. Endothelial gaps were not observed in later stages of irradiation.