Studies on intra-arterial cushions. II. Distribution of horseradish peroxidase in cushions at the origins of intercostal arteries in mice.

The distribution of horseradish peroxidase (HRP) in intraarterial cushions at the origins of intercostal arteries was studied in mice 30 min after intravenous injection of the tracer. HRP is found to be evenly distributed in the extracellular space of the entire arterial wall. Basal lamina material is penetrated by the tracer protein. This is not the case with collagenic fibrils, elastic material and intercellular granular deposits which occur regularly in cushions of adult mice but not in those of newborn animals (granules 300 to 800 A in diameter, probably cell debris). However, the elastic sheets do not act as diffusion barriers. HRP is not phagocytized in significant amounts by ramified smooth muscle cells within the time intervals tested. It is therefore assumed that granular deposits are also not phagocytized by smooth muscle cells.