Functional analysis of the stratum corneum of scalp skin: studies in patients with alopecia areata and androgenetic alopecia.

Because of the presence of thick long hairs on the scalp, little information is available concerning the functional characteristics of the stratum corneum (SC) of scalp skin. We therefore conducted a functional study of the SC of lesional scalp skin of patients with alopecia areata and of patients with androgenetic alopecia. We compared the scalp with the cheek and the flexor surface of the forearm (volar forearm). The water barrier function of the scalp SC of both patient groups, in terms of transepidermal water loss (TEWL), was almost comparable to that of the volar forearm, and was far better than that of facial skin. However, hydration of the scalp skin surface, as evaluated by measurement of high-frequency conductance, was markedly higher than that of facial skin, and showed significantly higher values than the volar forearm. These characteristics seem to be dependent, at least to some extent, on the amount of sebum-derived skin surface lipids because these were abundant on the scalp skin. Moreover, removal of skin surface lipids led to a significant decrease in skin surface hydration. The superficial corneocytes, the size of which reflects the proliferative activity of the epidermis, were substantially smaller on the scalp than on the volar forearm but significantly larger than on the cheek. These findings suggest that the rate of turnover of the scalp epidermis is intermediate between that of the facial and volar forearm epidermis. We conclude that the SC of the scalp skin in humans is functionally distinct from that of the face and extremities.