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 freeze-fracture replicas, by scanning electron microscopy, after cytochemical osmium impregnation and perfusion with peroxidase. The study revealed 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 successive dilations and constrictions. They branch in various directions and intrude deeply into the cell cytoplasm, forming a complex tubular network within the cell. Immunocytochemical techniques were applied on immersion-fixed tissues and on perfusion of the capillaries with albumin and insulin. These demonstrated that the tubular-vesicular system is involved 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 suggest that the transendothelial transport of serum proteins takes place by a transcytotic process through a membrane-bound tubular-vesicular system and is equivalent to the large pore system presumed from functional studies.