PERMEABILITY FOR HORSERADISH-PEROXIDASE IN STRIAL CAPILLARIES IN EACHTORN OF COCHLEA

The capillary permeability of the stria vascularis in each turn of coc hlea was examined at intervals of 10 and 30 min and I and 2h after an injection of horseradish peroxidase (HRP), using light and electron mi croscopic techniques. Ten minutes after HRP was administered, 18% of e xamined vessels showed HRP leakage. Thereafter, the level of HRP leaka ge increased with time. One hour after the injection, HRP had leaked f rom almost all vessels of the upper three turns. Tow hours after the i njection, the tracer in the vessel lumen became sparse with very thin staining or nonstaining, and a large amount of HRP was observed outsid e 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 capi llaries of these two regions. When the tracer disappeared from the ves sels (2 h after the HRP injection), labelled vesicles in endothelial c ells were frequently visible near the outside surface, whereas very fe w were visible near the inner surface. It is speculated, therefore, th at HRP which has leaked can be reabsorbed back into the blood circulat ion via a similar micropinocytosis down the concentration gradient.