This high-magnification view of the blood-brain barrier (BBB) emphasizes its hierarchical organization. In the center, an arteriole is cross-sectioned, visualizing its cylindrical 3D structure.  The faint outline of purple cell bodies indicates that our transgenic mouse line does not perfectly confine Cre-recombinase expression to endothelial cells, a detail we soon plan to address toward our goal of screening BBB-penetrant AAV capsids.