Transmission electron microscopy imaging of exogenous tracer challenges in 16 week old MRL/lpr mice. A. Illustration of the BCSFB, inset showing sites of HRP accumulation in the basal side, close to tight junctions and in epithelial vesicles (modified from9). B. Large blood vessels surrounded by epithelial cells with HRP at the basement membrane (middle image), HRP in intracellular spaces between epithelial cells (lateral labyrinth, right inset) reaching the level of tight junctions (left inset, arrow). C. Epithelial cell cytoplasm with HRP filed vesicles (right inset, arrows). Scale bars 2 µm and 0.5 µm in inset (B), 1 µm in inset (C).