Immunoelectron microscopic analysis of cornified cell envelope formation in normal and psoriatic epidermis.

The cornified cell envelope (CE) is an insoluble, highly resistant structure formed beneath the plasma membrane of differentiating keratinocytes. It consists of various crosslinked precursor proteins, including involucrin and loricrin. However, neither the normal assembly process of CE nor its alteration in skin disorders has been fully characterized. In this study we analyzed CE formation in normal skin and in lesional psoriatic skin by immunoelectron microscopy. In the superficial granular cells of normal epidermis, involucrin labeling was detected along the plasma membrane, whereas loricrin staining was mostly distributed diffusely within the cells, occasionally with some additional granular aggregates within the cytoplasm and nucleus. Loricrin was also present predominantly on the desmosomal attachment plaques, colocalizing with desmoglein. In the cornified cells, involucrin labeling was reduced, whereas loricrin labeling was retained and continuously decorated the CEs, with relative sparing of the desmosomal areas. In typical psoriatic epidermis, involucrin staining was very intense but loricrin labeling was markedly reduced. The involucrin-positive CEs were formed precociously and further maturation of CE did not occur. These results suggest that formation of CE occurs sequentially, initially involving involucrin deposition and subsequently involving loricrin incorporation. Psoriatic epidermis demonstrates a lack of proper CE maturation.