Biomedical Engineering Reference
In-Depth Information
papillary dermis. It comprises highly interlacing (reticular) collagen fibers that are
thicker and more closely packed than those in the papillary dermis. The mechanical
strength and substantial deformability of the dermis is enhanced by the presence
of elastin fibers. While the collagen fibers are highly crystalline microfibrils that
stretch to a modest extent, elastin fibers are much thinner, noncrystalline (amor-
phous), and deform extensively, almost as much as if they were rubber bands. The
combined mechanical reinforcement by these two types of fibers makes the dermis
a very robust tissue (Burkitt et al. 1993) with strongly nonlinear stress-strain behav-
ior that has been modeled in terms of the geometry of the collagen fibers (Comni-
nou and Yannas 1976).
The dermis supports the epidermis in at least two vital ways. First, it provides
a tough base that can repeatedly absorb substantial mechanical forces of various
types, including shear, tensile, and compressive forces, that would have caused an
unsupported epidermis to fail. Second, it incorporates a rich vascular system that is
required for the metabolic support of the avascular epidermis. The blood supply of
the dermis becomes intimately available to the epidermis at the dermal papillae. In
addition, the dermis provides thermoregulatory control to the organism, as well as
a tactile sensation.
There are several skin appendages in the dermis, including hair follicles, sweat
glands, and oil-secreting (sebaceous) glands, that are embryonically derived from
the epidermis (Burkitt et al. 1993). The adipose layer underneath the dermis, the hy-
podermis (subcutis) is often considered to be part of the dermis (Young et al. 2006).
In some areas of the body (e.g., scalp) the hypodermis contains the lower parts of
many hair follicles.
5.3.2
In Vivo Synthesis of the Dermis Using the Cell-Free Dermis
Regeneration Template
Dermis was partially synthesized when the DRT was grafted on a dermis-free defect
in the adult guinea pig either as a cell-free scaffold (Yannas and Burke 1980; Yannas
1981) or as a keratinocyte-seeded scaffold (Yannas et al. 1981, 1982b; Orgill 1983;
Yannas et al. 1989; Murphy et al. 1990). This observation was confirmed in the
adult swine model (Compton et al. 1998) and in clinical trials with humans (Burke
et al. 1981; Heimbach et al. 1988). In all these cases the dermis was imperfectly
regenerated as it lacked adnexa (hair follicles, sweat glands, etc.) In this section we
will focus on use of the cell-free DRT; ensuing sections will describe studies with
the cell-seeded DRT.
The commercially available product is Integra™, manufactured by Integra Life-
Sciences.
A dramatic delay in onset of contraction of dermal edges, amounting to about
20 days relative to the ungrafted defect, was observed in the guinea pig study when
cell-free DRT was grafted on it (Fig.
5.2
; curve labeled DRT)(Yannas 1981). Fol-
lowing this initial delay in onset of contraction, the cell-free scaffold was degraded,
contraction eventually started and was responsible for closure of most of the defect
