ABSORPTION ACTIVITY AND BARRIER PROPERTIES IN THE ENDOLYMPHATIC SAC -ULTRASTRUCTURAL AND MORPHOMETRIC ANALYSIS

A constant volume of horseradish peroxidase (HRP) was injected directl y into the endolymphatic sac (ES) lumen of the guinea pig to investiga te the detailed absorption activity and the barrier properties of the ES. The reaction products were analyzed using an ultrastructural and m orphometric method 1 to 10 h after the injection of this tracer. The e pithelial cells in the proximal portion did not absorb the intralumina l HRP at any intervals after the tracer injection. The epithelial cell s of the intermediate portion were classified clearly into two types a ccording to their absorption activity: active-absorptive, and non-acti ve cells. Uptake in the active-absorptive cells reached its maximal ra te 8 h after the injection and then decreased. The active-absorptive c ells in the intermediate portion are considered to play a major role i n the macromolecular absorption in the ES epithelium. The non-active c ells scarcely absorbed the intraluminal HRP, suggesting that these cel ls are not involved in the macromolecular absorption. The absorption a ctivity in the distal portion was lower than that in the active-absorp tive cells in the intermediate portion 1 to 8 h after the HRP injectio n, while higher 10 h after the injection. Not only the intermediate po rtion but also the distal portion may play an active role in the macro molecular absorption. In no portion of the ES did intraluminal HR-P pe netrate beyond the junctional complexes between epithelial cells or th rough the cytoplasm. It is conceivable that there is a tight barrier t o the intraluminal macromolecules in the epithelial linings of the ES.