**1. Introduction**

#### **1.1 Skin**

46 Psoriasis

Zouboulis, C.C. (2009). The skin as an endocrine organ. *Dermato-endocrinology,* Vol. 1, No. 5,

Skin is composed of three layers: epidermis, dermis and hypodermis (Sugihara *et al.*, 1991). Epidermis is divided into five layers namely, *stratum basale*, *spinosum*, *granulosum*, *lucidum*, and *corneum* (Bragulla & Homberger, 2009, Nagarajan *et al.*, 2009). The differentiation process implies that keratinocytes are transformed through the different cell layers to reach their complete maturation in the *stratum corneum* (Harding, 2004). In this process, various proliferation and differentiation markers are expressed in a well-orchestrated sequence of events (Fig. 1). When the differentiation process is negatively affected, skin pathologies such as psoriasis can appear (Rashmi *et al.*, 2009, Karlsson *et al.*, 2004).

Fig. 1. Differentiation process
