Is Capillary Micropinocytosis of any Significance for the Transcapillary Transfer of Plasma Proteins?

It has been repeatedly suggested that micropinocytosis plays a decisive role in the transfer of macromolecules across capillary walls. The evidence for this hypothesis is, however, only indirect. Thus physiological data ( e.g. Carter et al. 1974, Renkin et al. 1974) cannot exclude that macromolecular transfer takes place by way of diffusion and/or filtration through e.g. “large pores” ( cf. Landis and Pappenheimer 1963, Crone 1974). Ultramicroscopic pictures ( e.g. Simionescu et al. 1973, 1975, Jennings and Florey 1967) no doubt illustrate uptake of macromolecular tracers within endothelial micropinocytotic vesicles, and their rapid appearance in the interstitial fluid space. It should be stressed, however, that such pictures cannot alone be taken as proof of any directed transendothelial vesicle passage and here opinions, also among experts on ultrastructure, seem to differ somewhat. The tracer appearance in the interstitial fluid may, for example, be the result of diffusion and/or filtration through nearby pores which, because they make out such a minor fraction of capillary walls, are very difficult to visualize. In fact, the ultrastructural pictures might rather illustrate e.g. an endothelial “phagocytotic activity”, of great importance in its own right but without relevance for “active” transcapillary passage.