Box 2 Aqueous pores within TJ strands
Claude found that transepithelial resistance (Rt) increases logarithmically with the number of "parallel" TJ strands (n) that must be crossed by a permeating ion in the transepithelial direction (29,30). To explain this exponential relationship, aqueous pores that fluctuate between open and closed states were postulated within TJ strands (A) (30). If each parallel strand bears one channel, and if the open probability (P0) of all channels is the same, Rt would depend on P0-n (B). However, along individual TJ strands, there would be a large number of channels, and under this condition Rt would increase linearly with n (C). To maintain the exponential relationship, Cereijido et al. suggested compartmentalization, corresponding to the freeze-fracture replica images of TJ strand networks38 (D).