Fig. 3. Lack of correlation between intercellular junction length, pore number and TER. (A) Immunofluorescent localization of ZO-1 in cells grown on filters and used for TER was used to determine the length of tight junction contacts per monolayer area. Caco-2 cells are the smallest and thus have the most potential junction contact length to leak through. Bar, 12 µm. (B) Duplicate cell fields were photographed and measurements of perimeter lengths were determined using imaging software. The perimeter lengths differed by less than 30% among the three cell lines; Caco-2 monolayers contain the most and MDCK C7 the least. TER (C) was not necessarily inversely related to junction length (B) nor to pore number, as determined by Papp of the 3.5 Å PEG species (D).