Generation of Apical-Out Intestinal Organoids and Assessment of Organoid Proliferation Rate Using EdU-Labeling

Here, we describe the generation of floating cultures of apical-out intestinal organoids from hydrogel-embedded intestinal organoid cultures. Concurrently, floating basal-out organoid cultures are established for direct comparison between apical-out and basal-out organoids. Apical-out and basal-out organoids are subsequently subjected to the thymidine analog 5-ethynyl-2'-deoxyuridine (EdU), which is integrated into the newly synthesized DNA during the S-phase of cell division. This incorporation into DNA can be visualized in morphologically intact organoids using laser scanning confocal microscopy. Cells labeled with Hoechst33342 and EdU are then quantified in a semiautomatic manner using image analysis software. Calculation of the percentage of EdU-positive cells of the total number of cells allows for the analysis of cell proliferation in three-dimensional (3D) organoids. Despite being used here for the analysis of proliferation in intestinal organoids, the protocol is applicable to the analysis of nucleus-specific stainings of various sorts in other organoids or two-dimensional cell cultures as well.