Differences in involucrin immunolabeling within cornified cell envelopes in normal and psoriatic epidermis.

Epidermal keratinocytes form a cornified cell envelope (CE) beneath the plasma membrane during the late stages of differentiation. This CE is stabilized by cross linking of several precursor proteins, including involucrin. In psoriasis, the expression pattern of the precursor proteins is known to be deranged; involucrin expression is increased and loricrin expression is decreased. However, these changes have not been previously evaluated ultrastructurally. In the present study, we performed light and electron microscopic immunohistochemistry in conjunction with conventional transmission electron microscopy to assess the nature of involucrin involvement in normal and psoriatic CE. In normal epidermis, CEs were observed from the deepest cornified cells or, when present, from the transitional cells, increasing in thickness and changing electron densities with maturation. In psoriatic epidermis, CE formation started earlier, one to several cells below the cornified layer. Psoriatic CEs were generally thinner and showed a constant high electron density. Immunoelectron microscopy revealed that the normal CE was involucrin positive only at a very early stage, whereas psoriatic CE showed persistent involucrin immunoreactivity. These results suggest that in normal skin, involucrin is the major constituent of the CE only in its early stages of assembly. In contrast, CE formation seems to be initiated prematurely in psoriatic skin, where involucrin remains the major constituent of the CE during maturation.