Biomedical Engineering Reference
In-Depth Information
Sweat gland
pore
Hair shaft
Epidermis
Basement
membrane
Dermis
Sweat gland
duct
Subcutaneous
layer
Capillary
Touch receptor
Figure 18.1
Structure of skin consisting of three primary layers: the epidermis, dermis, and
subcutaneous layer. Reprinted by permission from Macmillan Publishers Ltd: [Nature] MacNeil, S.,
2007. Progress and opportunities for tissue-engineered skin. Nature 445, 874-880, © 2007.
The epidermis is composed of multiple layers that work together to create new skin cells and
bring them to the surface of the epidermis every 28 days [1, 3]. Within the innermost layer
of the epidermis is the stratum basal which consists of proliferating cuboidal cells. These
cells, keratinocytes, flatten as they move towards the surface of the skin, and gain more and
more keratin nearer to the surface. Eventually these cells terminate and easily flake off the
surface of the skin. Melanocytes, also found in the epidermis, provide skin with pigment and
protection against harmful ultraviolent rays. Beneath the epidermis is the dermal layer,
which consists mainly of fibroblast cells [4]. The dermis constitutes a network of vasculature
and collagen fibers that provide the skin with support and nourishment.
A break or insult to the skin triggers cells, growth factors, cytokines, and other biological
factors into action (reviewed in [5]). Together these factors orchestrate a well-coordinated
response to repair the damaged skin. Healing can be categorized into three phases: inflam-
mation, proliferation, and remodeling. At the onset of an acute injury, platelets aggregate in
a matrix of cross-linked fibrin fibers to form a fibrin clot. The clot provides immediate
closure of open vessels and also acts as a reservoir of growth factors that provide chemo-
tactic signals that recruit inflammatory cells [5, 6]. During the inflammatory stage, white
blood cells, neutrophils and monocytes, assemble at the site of injury to essentially rid the
area of any foreign substances, pathogens, dead cells, and other cellular debris. During the
proliferative phase, fibroblasts produce extracellular matrix and begin laying down collagen
and creating granulation tissue. In the final stage, the collagen becomes more organized and
the wound is healed. The healed tissue can result in inferior mechanical properties, discolor-
ation from normal skin, and lack sebaceous glands and hair follicles [6, 7].
Often, due to traumatic injuries, extreme external conditions, or disease, the skin cannot
provide adequate healing, resulting in chronic wounds. Chronic wounds are believed to be a
