Compartmentalization of the human stratum corneum by persistent tight junction-like structures

Several tight junction (TJ) proteins were detected in the living layers of adult human epidermis, and TJ-like membrane ridges were observed at the top of the stratum granulosum (SG) in freeze-fracture studies. We applied standard and immunoelectron microscopy to look for TJ-derived structures in the stratum corneum (SC) of human adult epidermis and in cornified envelopes purified from the plantar SC. Besides confirming claudin-1 labelling in the proximity of SG desmosomes, we also observed immunolocalization near corneodesmosomes in the lower SC. In addition, TJ proteins were consistently detected in the purified cornified envelopes. Lateral but not horizontal walls of the corneocytes showed frequent points of molecular fusion between lipid envelopes. These structural associations were very frequently localized at the top of the lateral corneocyte membranes, thus sealing the extremities of lateral intercorneocyte spaces. We propose that TJ-like structures persist in the SC and contribute to the reinforcement of lateral contacts and to the formation of membrane interdigitations between corneocytes. Their presence could contribute to subdivision of the extracellular spaces of SC into consecutive individualized compartments. Intercellular lipids, enzymes and other (glyco) protein content could thus evolve in the keratinized epidermal layer at different paces, as preprogrammed in the underlying living cells and influenced by the environment, e. g. humidity. Such situation might explain differences in the degradation rates between the 'peripheral' and the 'non-peripheral' corneodesmosomes observed during physiological desquamation, as previously suggested by us and others.