Transcytosis is prominent in brain microvascular peptide and protein transport,[19] because the brain microvascular endothelium, which forms the blood-brain barrier (BBB) in vivo, expresses unique, epithelial-like, high-resistance tight junctions.[20] The brain endothelial tight junctions virtually eliminate the paracellular pathway of solute transport across the microvascular endothelial wall in brain. In contrast, the endothelial barrier in peripheral organs does not express tight junctions, and solute movement through the paracellular pathway is prominent at the endothelial barrier in organs other than the brain or spinal cord.[21]