Computational models of tissue homeostasis will facilitate a deeper understanding of many diseases. They link molecular networks, cellular differentiation and the spatial and temporal organization of tissues. Here we show an approach which is able to computationally turn a healthy in silico epidermis into one with four central properties of psoriatic epidermis. We achieve this by altering a single simulation parameter in the cellular differentiation program of the simulated epidermal keratinocytes: the fractional time period during which transit amplifying cells proliferate (tau). Prolonging tau results in the four main pathological characteristics of psoriatic skin: (1) an absolute increase of the germinative compartment, (2) an absolute increase of the differentiated compartment, (3) a higher proportion of germinative cells and (4) a marked reduction in turnover time. The prolongation of tau is able to increase the proliferation capacity of the epidermal tissue without altering the cell cycle frequency.

MOTIVATION: Systems biology is currently focused on integrating intracellular networks, although clinically, diseases are largely defined by their histological features. For example, no computational model can simulate today the formation of a horizontally layered epidermis. Since the epidermis is the most complex structured epithelial tissue, systems biology models could yield important insights in epithelial tissue, in which most of all human cancers arise. RESULTS: We describe the algorithms of a system, capable of simulating the tissue homeostasis in human epidermis leading to a horizontally layered tissue with cells of different differentiation stages. The system predicts epidermal morphology, tissue kinetics and 2D flow of Ca2+ ions. Predicted properties of an epidermis with a healthy and a disturbed barrier are compared with the literature. The system closely mimics the respecting physiological situations. AVAILABILITY: Additional information and films of the simulation are available at the website. Source code is available on request. http://www.zbh.uni-hamburg.de/research/ESB/index.php CONTACT: [email protected]