Final answer:
When biotin-labeled keratin is injected into an epithelial cell and then visualized, the labeling pattern appears as yellow fluorescence.
Step-by-step explanation:
When biotin-labeled keratin is injected into an epithelial cell and then visualized, the labeling pattern appears as yellow fluorescence. Biotin is a fluorescent tag that binds specifically to the keratin protein, allowing it to be visualized under a microscope. Therefore, when the keratin is labeled with biotin and injected into the cell, it will emit a yellow fluorescence when visualized. This helps researchers study the distribution and behavior of keratin in epithelial cells.