| Summary: | Abstract Barrier function of hair follicles (HFs) is of great interest because they might be an entry port for allergens/pathogens, but could on the other hand be used for drug delivery or vaccination. Therefore we investigated tight junction (TJ) barrier function in human HFs. We show that there is a TJ barrier in the outermost living layer bordering to the environment from the infundibulum to the lower central part and between Henle’s and Huxles layer of anagen HFs. In club hair typical for catagen and telogen HFs a TJ barrier is found surrounding the club. This demonstrates that there is a continuous TJ barrier along interfollicular epidermis and HFs in different phases of HF cycle. However, interestingly, in cell culture experiments we can show that barrier is less tight in HF keratinocytes compared to interfollicular keratinocytes. Knock-down of the TJ protein claudin-1, which we demonstrate here to be less expressed in HFs of lesional atopic dermatitis skin, results in impaired barrier function, decreased proliferation and increased apoptosis of hair keratinocytes. This is in line with a hair growth phenotype in claudin-1 deficient patients (NISCH syndrome) and corresponding knock-out mice and indicates an important role of claudin-1 in HF barrier function and growth.
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