In vivo and In vitro Models of Psoriasis

1.1 Skin The evolution of life in the terrestrial environment required the development of a waterproof integument: the skin (Loden and Maibach, 2006). Skin is an extensive organ covering the entire exterior of the body (Stevens and Lowe, 2005). It provides the primary barrier against chemical and biological external agents and water loss (Hadgraft, 2001). The skin also plays an important role in thermoregulation, sensory perception and vitamin D metabolism (McKay and Leigh, 1995). The skin is composed of three main layers: the epidermis, the dermis and the hypodermis. The epidermis is the protective skin layer in contact with the external environment (Stevens and Lowe, 2005). This skin layer consists mainly of a stratified squamous keratinized epithelium (Junqueira and Carneiro, 2005). The epidermis cells, the keratinocytes, divide in the basal layer and differentiate throughout their migration to the surface. The epidermis is divided in 5 different layers (stratum basale, stratum spinosum, stratum granulosum, stratum lucidum and stratum corneum). The dermis is the feeder layer of the epidermis and provides most of the skin's mechanical resistance and elasticity. It is mainly composed of fibroblasts, epidermal appendages, blood vessels, nerves and nerve endings (Stevens and Lowe, 2005). Finally, the hypodermis is the deepest layer of the skin. It varies in size and content, but is usually composed of adipocytes which form the adipose tissue (Stevens and Lowe, 2005). Many severe skin diseases can be observed in human beings such as psoriasis.