Tight junctions have turned up in the epidermis.

It isn't often that textbooks have to be revised because of a single research paper, but that appears to be the case for the work described by Furuse et al. on page 1099. The standard view of epithelial structure is that continuous tight junctions (TJs) are required only in simple epithelia like those that separate internal organs, but are not found in stratified epithelia like the epidermis. The authors' analysis of mice lacking the TJ component claudin-1, however, shows that TJs are indeed found in the skin and are at least as necessary there as in simple epithelia.

Claudin-1 belongs to a multigene family that contributes to the backbone of TJ strands, so its deletion should affect simple epithelia. Surprisingly, the major defect in claudin-1–deficient mice is lethally leaky skin: the mice are born normally but die within a day, apparently from dehydration. Permeability assays of the claudin-1–deficient skin show excessive water loss.

In contrast with previous reports, Furuse et al. found continuous TJs in the stratum granulosum, a subset of the epidermis, of both wild-type and mutant mice. The results suggest that functional TJs are required in both types of epithelia and that loss of claudin-1 increases the permeability of the epidermal TJs without disrupting the organization of the keratinocytes. ▪