Fig. 8. Passive transport of small molecules through the thymic medullary conduits. Thymic fragments were incubated with 10 kDa FITC-dextran (A,C,D1) or 500 kDa FITC-dextran (B) for 30 minutes (A,C,D) or 2 hours (B) at 4°C (A,B) or 37°C (C,D). After fixation, cryostat sections of the incubated fragments were labeled with antibodies against the laminin 2 chain (A,B,D2) or CD31 (C) followed by Cy3-labeled secondary antibody. All sections were also labeled with DAPI (blue staining) to identify the cell nuclei. (A) After 30 minutes at 4°C, the 10 kDa FITC-dextran can be detected in blood vessels and in conduits (identified by the LN-5 staining), whereas the 500 kDa FITC-dextran cannot be found in conduits (red staining) even after two hours of incubation (B). After 30 minutes at 37°C, the 10 kDa FITC-dextran is still restricted to blood vessels and to conduits. This is shown in (C) by a labeling of blood vessels with the CD31 antibody and in D1 by LN-5 labeling. D2, FITC-dextran; D3, merged image with DAPI staining. Note that there is no FITC-labeling visible outside the conduit system (C,D).