The attack may originate from the external environment or be endogenous. The epidermis is now recognized as being an active player in inflammatory reactions of the skin. The epidermis produces several cytokines, in particular Il-1 which is made constitutively and is present in large amounts in the stratum corneum. Sometimes, the epidermal reaction may be very marked, as in chronic hyperplastic dermatitis seen in any chronic pruritic skin disease, but the lesions may not be diagnostically helpful. The trick is to separate those epidermal reactions that are part of the dermatitis reaction and those that indicate a specific disease.
Acanthosis refers to an increase in width of the stratum spinosum as a result of hyperplasia.
Hypertrophy of the epidermis, as occurs with cell swelling, is much less common. Acanthosis may occur as early as 36-48 h after a superficial physical insult, such as tape stripping of the stratum corneum. Thus, hyperplasia is not necessarily a sign of chronicity. The increase in proliferative rate in the epidermis may be reflected by both an increase in the amount of stratum corneum and in alterations to the rate and degree of differentiation. Orthokeratotic hyperkeratosis refers to an increase in the amount of normally differentiated stratum corneum.