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Fig. 1 Established methods for assessing epithelial barrier function. Methods for measuring epithelial barrier function can be categorized as measuring global barrier function (top row) or local barrier function (bottom row). Results may be detected after a set waiting time, typically ranging from minutes to hours (left), or in real time (right). (A) Tracer permeability assays track the passage of macromolecules from the apical compartment to the basal compartment after a set period of time. Permeability measurements are averaged across the whole tissue. (B) TER measures the electrical resistance of a monolayer. Measurements can be recorded instantaneously. (C) Sandwich assays immobilize tracers once they cross the barrier, allowing sites of barrier breaches to be detected at the experiment end point. (D) ZnUMBA employs the fluorogenic dye FZ3, which increases in fluorescence at sites of barrier breaches, allowing real-time monitoring of barrier breaches and their repair.