By IVCT, it has become clear that the injected BSA diffused out from blood vessel s of the corticomedullary area s but not from blood capillaries in the subcapsular areas of cortex. As shown in Fig. 17.2b, it did not leak out through the blood capillaries until 2 h after BSA injection. However, it leaked out earlier through the blood capillaries of connective tissues at 1 h with the same BSA concentration, as previously reported [19]. In addition, immunoreactive intensities of intravenously injected BSA in the interstitium were variously detected, depending on time intervals after the injection and also BSA concentration (Figs. 17.2 and 17.3). Therefore, these findings are compatible with the existence of functional barriers along blood capillaries of the subcapsular cortex , although molecular permeability through blood vessels in the corticomedullary areas is higher than that in the thymic cortex, as already reported [20]. As shown with albumin and BSA immunolocalizations in the thymic cortex of living mice prepared with IVCT, it is concluded that soluble serum protein s have difficulty in directly entering the thymic cortex because of the blood–thymus barrier , but they are diffused throughout the whole cortex from the corticomedullary areas.