EXPERIMENTAL FIGURE 18-49 Keratin intermediate filaments are dynamic, as soluble keratin is incorporated into filaments. Monomeric type I keratin was purified, chemically labeled with biotin, and microinjected into living epithelial cells. The cells were then fixed at different times after injection and stained with an antibody to biotin and with antibodies to keratin. (a) At 20 minutes after injection, the injected biotin-labeled keratin is concentrated in small foci scattered throughout the cytoplasm (left) and has not been integrated into the endogenous keratin cytoskeleton (right). (b) After 4 hours, the biotin-labeled keratin (left) and the keratin filaments (right) display identical patterns, indicating that the microinjected protein has become incorporated into the existing cytoskeleton.