[Electron microscopic study of the permeability of terminal blood vessels of the M. triceps brachii to horseradish peroxidase in fetal, juvenile and adult rats].

At the basis of some ultrastructural differences in fetal, juvenile and adult rat triceps muscle capillaries as described in preliminary papers the permeability of these capillaries to horse radish peroxidase was investigated. Capillaries of fetal and juvenile rats up to the 3rd week after birth were impermeable to horse radish peroxidase (HRP) for 2 and 10 minutes after injection, which can be explained with the low pinocytotic activity and the more complex structure of the junctions. Capillaries of the triceps brachii muscle in adult rats aged 2 month and more show a low permeability to HRP 2 and 5 minutes and a high permeability 15 minutes after the injection of the tracer. In the capillary segment of the terminal bed the tracer was traversed especially by the means of vesicles and some transendothelial channels built by linked vesicles. The interendothelial junctions seemed to be tight. In the venular segment of th capillaries and in small postcapillary venules additional to the vesicular transport a high permeability of some junctions was stated. The significance of the different permeability of distinct segments of the terminal vascular bed to small macromolecules is discussed with respect to the theories of th physiologists and compared with the results of recent investigators.