CFSE is a fluorescent dye which can track the cell division. In Vitro: CFSE is a fluorescent dye which can track the cell division^[1]. To examine the ability of immune cells to migrate into live slices, a single-cell suspension of autologous splenocytes or PBMC is isolated, labeling them with CFSE, and adding them to the top of the slices on day 2 of culture. After 6 additional days of culture, the CFSE-labeling immune cells (green) are found to have migrated throughout the slices. No positive cells are found in the slices without the addition of CFSE-labeling immune cells^[2].