Figure 3.

Formation of tight junctions at the boundary between the apical and basal-lateral membranes. (A) Transmission electron microscopy of Caco-2 cells during the time course revealed the formation of localized electron-dense, closely opposing plasma membranes between cells at the apex of the lateral membrane characteristic of tight junctions by day 4 (solid circle). Dashed circle indicates the location of the future tight junction before its formation. As the time course progressed, desmosomes were observed as electron-dense plaques on the lateral membrane of differentiating Caco-2 cells (box). Scale bar, 100 nm. (B) To functionally assess tight junction formation, Caco-2 monolayers were assayed for permeability of a small molecule, [³H]inulin. At early time points apically applied [³H]inulin rapidly equilibrated between the apical and basal-lateral compartments. By day 4 [³H]inulin diffusion across the monolayer was restricted. Error bars, n = 2.