Long-term storage of endocytosed latex beads in keratocytes in vivo.

Endocytosis by keratocytes (corneal fibroblasts) is an important part of the host defense system. To investigate the long-term fate of endocytosed materials, we injected polystyrene latex beads into the corneal stroma of four rabbits. The corneal stroma was observed under a transmission electron microscope 4 and 800 days after the injection. After 4 days, the beads were found not only between the collagen fibers of the stroma, but also in some keratocytes. After 800 days, no extracellular beads were seen, but endocytosed beads remained, surrounded by limiting membranes, in the cytoplasm of keratocytes. These observations demonstrate that keratocytes endocytose latex beads and store them for a long time, isolating these foreign materials from the corneal stroma. These observations suggest that keratocytes, like some other fibroblasts perform a noninflammatory and nonimmunological defense function.