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).