Fig. 2.

Comparative characteristics of the structure of the blood-brain barrier (BBB) in normal and blood–tumor barrier (BTB) is in brain tumors. BTB is generally considered « leakier » than BBB. BTB is characterized by an aberrant distribution of pericytes, and loss of astrocyte end foots and neuronal connections. T cell subpopulations and peripheral monocytes are found in brain tumors, indicating permeability to circulating immune cells. In addition, junctional proteins are reduced in endothelial cells (ECs) of BTB, and the intratumoral vasculature never fully restores normal BBB in brain metastases. Although BTB is "defective", it retains important aspects of the BBB, including the expression of active efflux transporters in ECs and tumor cells.