Permeability for horseradish peroxidase in strial capillaries in each turn of cochlea.

The capillary permeability of the stria vascularis in each turn of cochlea was examined at intervals of 10 and 30 min and 1 and 2 h after an injection of horseradish peroxidase (HRP), using light and electron microscopic techniques. Ten minutes after HRP was administered, 18% of examined vessels showed HRP leakage. Thereafter, the level of HRP leakage increased with time. One hour after the injection, HRP had leaked from almost all vessels of the upper three turns. Two hours after the injection, the tracer in the vessel lumen became sparse with very thin staining or nonstaining, and a large amount of HRP was observed outside the vessels. It was noted that the capillary permeability to HRP at the basal turn was different from that of the upper three turns. This seems to be the result of the different pinocytic activity in the capillaries of these two regions. When the tracer disappeared from the vessels (2 h after the HRP injection), labelled vesicles in endothelial cells were frequently visible near the outside surface, whereas very few were visible near the inner surface. It is speculated, therefore, that HRP which has leaked can be reabsorbed back into the blood circulation via a similar micropinocytosis down the concentration gradient.