Extracellular and transcellular transport of horseradish peroxidase (HRP) through the hypothalamic tanycyte ependyma.

Abstract

Penetration of the median eminence and ventrolateral walls of the III ventricle by intraventricularly injected horseradish peroxidase was studied in rats. Experimental times varied between 10 to 70 min (short-term experiments) and 12 hrs to 30 days (long-term experiments). In short-term experiments, the median eminence was found to be completely stained whereas the lateral walls of the III ventricle were penetrated only up to 1 mm in depth. Spreading of the tracer takes place predominantly through the extracellular space and cellular uptake and transport do not seem to play a role during the first 70 min following the injection. In long-term experiments, the tanycytes exhibit a variety of intracellular inclusions marked by HRP precipitate. Tanycytic perikarya contain dense bodies and lipofuscin-like aggregates. Lipoprotein granules are thought to arise from these and are interpreted as lysosomal residual bodies. In tanycytic processes and perivascular endfeet, accumulations of HRP-containing tubules and polymorphous granules are encountered suggesting a transport towards the blood vessels of the portal plexus or of the arcuate and ventromedial areas, respectively. Sporadic tanycytes which are completely and evenly stained from the ventricular surface to the end of their processes were observed in the arcuate and ventromedial area. Whether this appearance can be taken as a sign of normal cell function seems doubtful. Some possible routes of transport through the median eminence—extracellular and transcellular—are summarized in a schematic drawing, taking into account the present findings and those published elsewhere in the literature.