Immunoelectron microscopic study of plasma protein pathways through the different segments of the rat intestinal vasculature.

Nineteen Brown-Norway (BN) rats received intravenous injections of sheep anti-peroxidase (HRP) antibodies. Four BN rats were immunized to HRP. The anti-HRP antibodies were used to trace permeability pathways of large physiological molecules across different vessels of the small intestine. This organ was chosen because of the possibility of convenient "in situ" fixation and for the diversity of vessel types it contains. It was shown that: 1) There were no obvious transendothelial pathways in arteries with an elastic lamina. 2) The antibodies readily crossed fenestrated capillaries through the fenestrae. 3) There were two possible pathways through muscle capillaries and pericytic venules, namely transcytoplasmic vesicular "cactus-like" channels and interendothelial junctions. 4) Interendothelial permeability was a possible factor in veins with an elastic lamina. 5) Lymphatics were readily permeable through intercellular junctions and cytoplasmic vesicles.