Biomedical Engineering Reference
In-Depth Information
Dendritic cells (DCs), which appear in the perivascular region, in close proxim-
ity to fibroblasts, make up a diverse subclass of immune cells. These cells are
capable of acting as antigen-presenting cells (APCs), acquire phagocytic potential
and express an assortment of epitopes, including CD34 (hematopoietic progenitor
antigen) and factor XIIIa (activated fibrin stabilizing factor, also expressed by
platelets.)
Hence, they may play an important role in the early stages of wound
healing by virtue of having the ability to cross-link fibrin and fibronectin. With
considerable plasticity among subpopulations, these cells can transform their
phenotypes from one subtype to another, depending on necessities within the local
microenvironment.
As in the epidermis, certain dendritic cells in the dermis are responsible for
patrolling against environmental (and possibly endogenous) antigens. Dendritic
antigen-presenting macrophages (plentiful in the dermis) subserve a function of
Langerhans cells by initiating immune responses directed against locally injected
(as well as systemic) antigens. Non-antigen-specific (innate) immune responses
are also mediated by macrophages in the dermis; however, these cells tend to be
less dendritic and more phagocytic (in dealing with offensive particulates by
phagocytosis and subsequent enzymatic degradation within phagolysomes.)
The dermis is thought to harbor a subpopulation of mesenchymal cells with a
potential for self-renewal and differentiation plasticity.
10
Recent evidence impli-
cates dermal stem cells having capacity to differentiate along neurogenic and
myogenic lines.
10
A
similar subpopulation (adipocyte stem cells) has been identi-
fied in the subcutaneous fat (subcutis).
11
The contribution of these subpopulations
to wound repair and regeneration is yet to be elucidated. Moreover, it remains
possible that circulating hematopoietic cells (with stem cell capacities) may seed
wounds and play an important role in the healing process (further discussion to
follow.)
12
Dermal extracellular matrix (ECM) is mainly composed of collagen and elastic
fibers, embedded within ground substance.
1
All three components are synthesized
by fibroblasts. These components are distributed heterogenerously throughout the
dermis. Of the three, collagen (the main constituent of leather) is by far the most
abundant. Ranging from 2 µm to 15 µm in diameter, collagen fibers are either
found in a finely woven meshwork (as seen in papillary dermis) or as thick, dense
bundles (as seen in
reticular dermis.) The papillary dermis (smaller in size) is
dominated by collagen III, whereas the reticular dermis (larger in size) is domi-
nated by collagen I. Overall, the ratio of collagen I to collagen III is 4:1. However,
it is variably increased after wound repair. Ranging from 1 µm to 3 µm in diameter,
elastic fibers are found intertwined among collagen fibers. Like collagen, elastic
fibers tend to be smaller in the papillary dermis and thicker in the reticular dermis.
Spaces not occupied by collagen or elastic fibers are filled by an amorphous
material called ground substance - composed mainly of glycosaminoglycan. Also
known as acid mucopolysaccharides, glycosaminoglycans (GAGs) are found in
sulfated and unsulfated forms. Of the two, unsulfated forms, such as hyaluronic
