psoriasis regulates epidermal development in zebrafish.

The zebrafish epidermis completely envelopes the embryo by 14 hours postfertilization, providing an essential barrier between the internal organs and the environment. As the embryo increases in size, keratinocytes in the epidermis must proliferate and differentiate to form the three epidermal layers present in the adult. The mechanisms controlling growth, differentiation, and maintenance of the fish epidermis are mostly unknown. Here, we describe psoriasis, an epidermal mutant that exhibits widespread overproliferation of the epidermis at 3 days postfertilization and a defect in keratinocyte differentiation. Based on mosaic analysis, we show that psoriasis acts non-cell-autonomously, suggesting that psoriasis encodes a secreted factor. Our analysis of the psoriasis mutant indicates that keratinocyte differentiation and proliferation are tightly regulated to maintain a cohesive epidermal sheet around the embryo and that disruptions in these processes result in the formation of epidermal aggregates.