In specialized parts of the body, free exchange of substances between blood and
tissue cells is hindered by the presence of a barrier cell layer@). Specialized milieu of the
compartments provided by these “blood-tissue barriers” seems to be important for
specific functions of the tissue cells guarded by the barriers. In blood-tissue barriers,
such as the blood-brain barrier, blood-cerebrospinal fluid barrier, blood-nerve barrier,
blood-retinal barrier, blood-aqueous barrier, blood-perilymph barrier, and placental
barrier, endothelial or epithelial cells sealed by tight junctions, or a syncytial cell layer@),
serve as a structural basis of the barrier. A selective transport system localized in the
cells of the barrier provides substances needed by the cells inside the barrier. GLUTl , an
isoform of facilitated-diffusion glucose transporters, is abundant in cells of the barrier.
GLUT1 is concentrated at the critical plasma membranes of cells of the barriers and
thereby constitutes the major machinery for the transport of glucose across these barriers
where transport occurs by a transcellular mechanism. In the barrier composed of doubleepithelial
layers, such as the epithelium of the ciliary body in the case of the
blood-aqueous barrier, gap junctions appear to play an important role in addition to
GLUT1 for the transfer of glucose across the barrier.