Laser scanning confocal microscopy of cytokeratin and VCAM-1 expression in the human thymus. Frozen sections of human thymus were labeled with a directly-FITC labeled pan-cytokeratin MoAb (KL-1) shown in green and an anti-VCAM-1 MoAb (1G11) indirectly stained with a LRSC-labeled secondary antibody shown in red. The last panel of each series is the differential interference contrast image (DIC), which allows the localization of staining in the thymus based on morphological criteria. Series I depicts a representative field at the corticomedullary junction (CMJ) identified at the intersection of the cortex (C) and medulla (M). Series II depicts the cortex. The first image in each series is the pan-cytokeratin staining (Pan-CK, green). The second image is the computer-merged view of both pan-cytokeratin and VCAM-1 staining in which the yellow color represents colocalization of both antibodies (yellow = green + red). The third image of each series is the VCAM-1 staining (VCAM-1, red). The majority of cytokeratin staining in the CMJ and C is colocalized with VCAM-1 consistent with the expression of VCAM-1 by TECs. The fourth image is the DIC view merged with the VCAM-1 staining in red. In the second image of series 1 (Pan-CK + VCAM-1) an arrow marks a VCAM-1 positive cell in the medulla that is pan-cytokeratin negative.