Epidermal remodelling in psoriasis (III): a hexagonally-arranged cylindrical papilla model reveals the nature of psoriatic architecture

A geometric model of hyperproliferative psoriatic epidermis that is remarkably similar to psoriatic skin in vivo has been proposed. This model is based on hexagonally-arranged cylindrical dermal papillae, and it is constructed under the assumption that the total number of viable epidermal cells is proportional to the number of cells within proliferative compartment. Quantitative analyses indicate that at least two distinct conditions are consistent with this assumption, corresponding to psoriasis with and without a granular layer. Psoriasis without a granular layer (G-minus psoriasis) represents an expanding state, and psoriasis with a granular layer (G-plus psoriasis) represents a stationary and/or regressing state. There is also another relationship between G-minus and G-plus psoriasis, that they are exchangeable with each other during the disease evolution. For example, consistent with the regressing nature of G-plus psoriasis, G-minus psoriasis is exchangeable with G-plus psoriasis with less papillary height until the papilla has a certain height (360 μm). On the other hand, G-minus psoriasis with the papillae of more than the critical height is exchangeable with G-plus psoriasis with more papillary height, which ultimately results in a papillomatous architecture. G-minus psoriasis with the critical papillary height (360 μm) is exchangeable with G-plus psoriasis with the same papillary height. Collectively, the model reveals a self-organizing mechanism for the characteristically-ordered psoriatic architecture, one that is under dynamic control of epidermal remodelling.