Confocal microscopy of apotransferrin in a Caco-2 cell monolayer. Caco-2 cell monolayers were offered apo-Tf-TxRed from the basal side for 20 minutes. The cell layers were counterstained with ToPro-1 to show the nucleus in green and the monolayers observed by laser scanning confocal microscopy as described in Materials and Methods. Shown is a photomicroghraph of a section through a typical monolayer. As shown in the schematic (A), the cell layer was tilted so that the optical section (B) passed through the basal and apical portions of the cells constituting the monolayer. Note the vesicles above the nucleus at the right side of the figure. Because of the optical slicing power of laser confocal microscopy, it is possible to determine clearly in this figure if a vesicle is above or below the nucleus.