The skin provides a vitally important protective separation between the internal and the external environments1. There are three structural layers to the skin: the epidermis, the dermis and subcutis. The epidermis is the outer layer, serving as the physical and chemical barrier between the interior body and the exterior environment. The dermis is a deeper layer providing the structural support of the skin. Subcutis is made up of loose connective tissue and fat, which can be up to 3 cm thick on the abdomen.
Epidermis is a stratified squamous epithelium consisting of several cell types. The most abundant cell type of epithelial layer of the skin is the keratinocytes which synthesize the protein keratin2. Protein bridges called desmosomes connect the keratinocytes, which are in a constant state of transition from the deeper layers to the superficial. The epidermis varies in thickness based on the tissue of origin. The four separate layers of the epidermis are stratum basale (basal or germinativum cell layer), stratum spinosum (spinous or prickle cell layer), stratum granulosum (granular cell layer) and stratum corneum (Fig. 1). These layers are formed by the differing stages of keratinocyte maturation. In addition to their structural and barrier function, keratinocytes also play a crucial role in influencing immune response by secreting inhibitory cytokines and activating Langerhans cells, in response to injury.
Dermal fibroblasts are cells within the dermis layer of skin which are responsible for generating connective tissue and allowing the skin to recover from injury. Dermal fibroblasts generate and maintain the connective tissue which unites separate cell layers. Furthermore, these dermal fibroblasts produce the protein molecules including laminin and fibronectin consisting of the extracellular matrix. By creating the extracellular matrix between the dermis and epidermis, fibroblasts allow the epithelial cells of the epidermis to affix the matrix, thereby allowing the epidermal cells to effectively join together to form the top layer of the skin.
Skin cells release inflammatory mediators (cytokines and chemokines) in response to chemically-induced tissue and cell damage. These cytokine and chemokines dilate and increase permeability of blood vessels and attract immune cells to the injury site for antigen clearance and tissue repair. Inflammatory mediators also stimulate nerve endings leading to itching and stinging sensations.