EVIDENCE OF A TUBULAR SYSTEM FOR TRANSENDOTHELIAL TRANSPORT IN ARTERIAL CAPILLARIES OF THE RETE-MIRABILE

The arterial endothelial cells of the rete capillaries of the eel were examined by transmission electron microscopy on thin sections, on fre eze-fracture replicas, by scanning electron microscopy, after cytochem ical osmium impregnation and perfusion with peroxidase. The study reve aled the existence of membrane-bound tubules and vesicles that open at both the luminal and abluminal poles of the cell and at the level of the intercellular space. The tubules are straight or present successiv e dilations and constrictions. They branch in various directions and i ntrude deeply into the cell cytoplasm, forming a complex tubular netwo rk within the cell. Immunocytochemical techniques were applied on imme rsion-fixed tissues and on perfusion of the capillaries with albumin a nd insulin. These demonstrated that the tubular-vesicular system is in volved in the transport of circulating proteins. Furthermore, protein A-gold immunocytochemistry has revealed the association of actin with the membranes of this system. On the basis of these results, we sugges t that the transendothelial transport of serum proteins takes place by a transcytotic process through a membrane-bound tubular-vesicular sys tem and is equivalent to the large pore system presumed from functiona l studies.