Differences in permeability of microperoxidase and horseradish peroxidase into the alveolar bone of developing rats

Microperoxidase (small m.wt tracer) and horseradish peroxidase (large m.wt tracer) were used to investigate the existence of extracellular (between blood vessels and bone cells) and intracellular transport pathways in the alveolar bone cells of five-day-old rats. HRP directly penetrated the cytoplasm and nucleus of the osteocytes and osteoblasts, but MP did not. These findings suggest that HRP at high dosages is more toxic to bone cells than is MP, since the direct penetration of HRP probably results from alteration of plasma membrane permeability. If so, MP seems to be a more suitable tracer than HRP. The findings from MP tracing suggest that a main transport pathway of bone fluid exists between the external part of the unmineralized zone, which is located in the lacunar and canalicular spaces around bone cells, and the innermost edge of the mineralized matrix, which constitutes the lacunar and canalicular walls; in addition, minor pathways as accessory routes might diverge from the main pathway to go around the bone cells.