**3.2** *In vitro* **models**

#### **3.2.1 Monolayer**

By using only a small skin biopsy, monolayer techniques allow the attainment of a large number of cells (normal or pathological) supporting the production of many experiments. In monolayer models, only one cell type is studied. Thus, keratinocytes (or fibroblasts) can be used to test different conditions or to observe psoriatic skin features such as hyperproliferation or abnormal differentiation of keratinocytes. These models allow the isolation of one cell type for step by step dissection of the implied mechanisms. Even if it was not possible to observe direct interaction between cell types, these models allowed the discovery of many interesting facts about psoriasis, and favoured a better understanding of the pathology (Jean & Pouliot, 2010).
