Biomedical Engineering Reference
In-Depth Information
In comparison to the epidermis, the dermis is relatively non-cellular, com-
posed primarily of collagen, elastic fibers and ground substance. The dermis
contains all of the nerves, vessels, lymphatics and most of the glandular ele-
ments of the skin. The dermis is between 15 to 40 times thicker than the
epidermis; however, given that it is less cellular, the dermis consumes less
energy than the epidermis. Its embryologic origins are mesodermal and like
most mesodermal derivatives, the dermis is incapable of true regeneration in
postnatal life. In contrast to the epidermis, the dermis heals through a process
known as scarring.
Structurally, the dermis is a complex network of cellular and acellular compo-
nents. Collagen is the major acellular structural component of the dermis,
constituting approximately 70% of the skin's dry weight. Type I and type III
collagens represent 80% and 15% of the total dermal collagen, respectively.
Elastic fibers comprise approximately 2% of the dry weight of the skin. They
play a crucial role in wound contraction after a loss of skin integrity. Finally,
glycosaminoglycans (GAGs) are the third major acellular component of the
dermis. The principal GAGs in the dermis are heparin and heparan sulfate.
Major functions of GAGs include the binding of water and cationic molecules,
serving as cofactors for multiple enzyme pathways and participating in cell
adhesion and basement membrane formation (Cuono, 1988). In addition, two
important dermal GAGs, chondroitin sulfate and dermatan sulfate have been
implicated in endothelial proliferation during wound healing (Faham
et al
.,
1996).
The major cell type of the dermis is the fibroblast. It is responsible for both the
synthesis and the degradation of dermal proteins. In addition, the dermis contains
a small number of hematopoietic derived cells including macrophages, mast cells,
lymphocytes and eosinophils. Other cell types found in the dermis included those
associated with the vascular, lymphatic and nervous systems, and the epidermal
appendages.
The mature dermis can be divided into two main layers: the superficial papil-
lary layer and the deeper reticular layer. The papillary dermis contains a
disorganized collection of collagen bundles, elastic fibers, fibroblasts and
ground substance. A highly developed microcirculation in the papillary dermis
provides a blood supply for the overlying epidermis (the epidermis has no
inherent blood supply). The papillary dermis and the basal layer of the epider-
mis are intimately related at a region known as the basement membrane zone
(BMZ). Here at the dermal-epidermal junction, projections of the dermis,
known as dermal papillae, interdigitate with in-pockets of epidermis known as
rete ridges. In contrast, the reticular dermis is less cellular. Within the dermis is
a combination of mucopolysaccharides, chondroitin sulfate and hyaluronic acid,
which forms the ground substance and takes on a gel-like consistency; with age
it is gradually replaced by fibrous tissue.
An understanding of the biology and physiology of skin is paramount to
