The ability of the BBB to regulate flux into and out of the CNS relies heavily on tight junctions. Tight junctions are important in the BBB because:
A: they permit cell-to-cell communication between cells in the CNS and those in the PNS.
B: the adherens protein is overexpressed at the BBB to ensure tight associations between cells.
C: they create a chemical barrier that inhibits all active transport across the BBB membrane.
D: they seal off extracellular pathways, requiring passage through highly-selective cell membranes.