Ferritin labeling in the fixed muscle capillary. A doubt on the tracer-experiments as the basis for the vesicular transport theory.

On the basis of his observations on the occurrence of characteristic caveolae and vesicles within the capillary endothelium, PALADE (1953) proposed an original hypothesis that substances are transported through the capillary endothelium enclosed in these vesicular structures which were presumed to move rapidly. The mechanism of "vesicular transport" was later named cytopempsis by MOORE and RUSKA (1957). This process consists of (1) engulfing substances from one surface by invaginations of the plasmalemma, (2) intracytoplasmic transport by vesicles formed by pinching off of the plasmalemmal invaginations and (3) discharge of the substances on the opposite surface by opening of the vesicles. Experimental studies using tracers such as ferritin (WISSIG, 1958; PALADE, 1960; JENNINGS, MARCHESI and FLOREY, 1962; BRUNS and PALADE, 1968b) have been the most important support of this hypothesis, as labelings of caveolae and vesicles were found at various intervals after intravenous injections of tracers. The purpose of the present study is to examine whether the tracerexperiments give morphological indication of possible vesicular transport in the capillary endothelium.