Biomedical Engineering Reference
In-Depth Information
occurred in 2007 (American Cancer Society, 2007). In Europe, the British
Isles has been the highest rates of skin cancer in children and adolescents (de
Vries et al., 2006). The most common types of skin cancer are basal cell
carcinoma (BCC) and squamous cell carcinoma (SCC). In particular BCC
of the skin is the most prevalent type of cancer affecting Caucasians (Daya-
Grosjean and Couve-Privat, 2005; Holikova et al., 2004). However, the most
dangerous type of skin cancer is malignant melanoma. This form of skin
cancer can be fatal if it is not treated early but comprises only a small
proportion of all skin cancers. In Europe a recent study showed that in the
19-20 year period of the study, the age standardized rate of malignant
melanoma per million for adolescents in the British Isles increased from 5 to 29
(de Vries et al., 2006). In addition, in Europe as a whole it increased from 6 to 14
(de Vries et al., 2006).
1.1 Stem Cells in the Skin
The epidermis is a multilayered epithelium that covers the skin providing a
waterproof barrier that essentially controls the rate of water loss from the body.
In mammals, it comprises the interfollicular epidermis and the adexal struc-
tures, including the sebaceous glands and the hair follicles. The epidermis
contains several types of cells, including keratinocytes (95%), melanocytes,
Langerhans and Merkel cells. The different types of cells play an important
role in maintaining the normal functions in the skin. The ''stratum basale'' (the
basal layer) is the epidermis layer where most of the cells undergo rapid cell
division in order to replenish the cells lost through terminal differentiation from
the surface. Hair follicles are part of the skin epithelium, which is located in the
dermis and associated with a sebaceous gland and a tiny bundle of muscle fibre
called the arrector pili. Hair follicles are self-renewing structures that reconsti-
tute themselves through the cycle comprising anagen (growing phase), catagen
(regression phase) and telogen (resting phase). Melanocyte stem cells reside in
the bulge region of the hair follicle (Nishimura et al., 2002; Amoh et al., 2005).
The terminally differentiated cells in all regions of the epidermis are con-
tinually shed from the skin and must be replaced throughout adult life. The
replacement depends on the stem cells. These cells show an extensive self-
renewal capacity and produce progeny that undergo terminal differentiation
along the different epidermal lineages. The hierarchical model of cellular repla-
cement was originally described by Gilbert and Lajtha (Gilbert and Lajtha,
1965) for haematopoietic bone marrow and, more recently, for cutaneous
epithelium by Potten and Booth (Potten and Booth, 2002). In both models,
stem cells are thought to be quiescent or to cycle slowly and to be protected
within the tissue architecture. When stimulated, they should be capable of
considerable proliferation and some of these cells are multipotential and cap-
able of producing more than one differentiated lineage. Epidermis of mice was
