Horseradish peroxidase uptake into the rat choroid plexus epithelium, with special reference to the lysosomal system

Abstract

Uptake of horseradish peroxidase (HRP), lysosome formation, and acid phosphatase (AcPhase) activity in the rat choroid plexus epithelium were studied. HRP was administered intravenously or intraventricularly for various times, and the choroid plexuses were fixed by ventriculo—cisternal perfusion of aldehydes. AcPhase reaction was performed both on control material and on choroid plexuses from the HRP experiments. The uptake of HRP from the cerebrospinal fluid (csf) side of the epithelium was, in general, more pronounced than that from the blood side. HRP became micropinocytozed and sequestered in a well-developed lysosomal apparatus situated in the apical half of the epithelial cells. The main pathway for endocytozed HRP was from micropinocytic vesicles to large endocytic vacuoles which may appear as multivesicular bodies. These eventually developed into dense bodies or dense multivesicular bodies, representing secondary lysosomes. HRP labeling was also seen in elongated tubular structures, which may be chains of fused micropinocytic vesicles or smooth endoplasmatic reticulum cisterns. AcPhase activity was located in distinct Golgi cisterns within the Golgi complexes. Small Golgi vesicles (primary lysosomes), some (dark) multivesicular bodies, and most dense bodies were also AcPhase positive. The AcPhase activity of HRP-exposed choroid plexus epithelium and of control specimens was largely the same. However, the number of AcPhase-positive dense bodies appeared to increase after prolonged administration of a large dose of HRP into the csf. The results provide evidence that the choroid plexus epithelium is able to remove protein from the ventricular system.