Blood vessels all over the body have an inside lining made up of a continuous layer of cells called endothelial cells. In some parts of the body, such as the brain and the retina, the endothelial cells are so close together that there are essentially no gaps between them. This is important, because the fluid around the cells of the nervous system has to be different from the blood in order for these cells to work correctly. In the nervous system the gaps between endothelial cells are sealed off by tight junctions, which are aggregates of proteins on the surfaces of cells that bring the cells very close together. Some examples of proteins that we know are in tight junctions are occludin and zonula occludens-1.
Protein leaks into the retina during diabetes.
FITC-labeled protein was injected into rats before sections were prepared for analysis of fluoresence in the retina. In 3-month diabetic rats the protein can be seen throughout the retina. In this case we injected 100mg per kg body weight of the protein albumin, a major component of normal blood, labeled with the fluorescent tag FITC, which shines bright green in the microscope. After 15 minutes the eyes were rapidly fixed and prepared for histological sectioning so that we could measure the FITC that had gone into the retina. As shown in Figure 1, by three months of diabetes the blood vessels of rats are much more leaky to the FITC-labeled protein compared to rats of the same age that are not diabetic.