Biomedical Engineering Reference
In-Depth Information
well-formed rete ridges also increases the rate of transfer of metabolites from the
dermis to the avascular epidermis. A particularly effective feature by which such
proximity between the two tissues facilitates mass transfer is the presence of capil-
lary loops, part of the vascular plexi of the dermis, that approach the epidermis very
close by insinuating themselves into the dermal papillae (Fig.
5.2
, top left) (Burkitt
et al. 1993).
5.4.2
Simultaneous Synthesis of a Dermis and an Epidermis
with Keratinocyte-Seeded DRT
In a series of early studies with the guinea pig model, DRT was seeded, immedi-
ately prior to grafting, with autologous, uncultured KC at a mean density of 5 × 10
5
(± 10 %) cells/cm
2
graft area (Yannas et al. 1981, 1982a, b, 1984, 1987b; Orgill
1983). In these studies KC were separated from a small skin biopsy to yield a cell
suspension with an estimated basal cell content of about 40 % (Prunieras 1975; Reg-
nier et al. 1985); the balance of the cell population was not identified. The cells were
driven with mild centrifugation, under conditions of carefully controlled centrifugal
force and time, to the boundary of DRT with the silicone film, previously shown
to be the optimum site for the seeded KC. The cell-seeded bilayer device was then
grafted on dermis-free defects in the guinea pig. The entire optimized protocol for
cell harvest, seeding of the DRT and grafting, required about 3-4 h to implement
(Orgill 1983). These cells were not cultured for any significant period prior to im-
plantation (Yannas et al. 1981, 1982a, b, 1984; Orgill 1983).
Seeding of the DRT with uncultured KCs led to a delay of 13.5 days in half-life
for defect contraction relative to the ungrafted defect, significantly shorter than the
delay observed with the cell-free DRT (about 20 days). Contraction was arrested
between 35 and 50 days; at this point, the defect area was about 20 % of initial
area. After about 50 days, the defect area started expanding at a rate that was about
twice that for skin expansion due to normal growth of the animal. This observation
was consistent with an interpretation of significant synthesis of new tissue (Orgill
1983; Yannas et al. 1989). A confluent neoepidermis formed at day 12.6 ± 2 when
the density of seeded KC was 5.0 × 10
5
cells/cm
2
graft area. Numerous keratin cysts,
present approximately after day 10 in the subepidermal region, had been extruded
through the neopidermis by day 25. After about 90 days, the defect perimeter en-
closed an area of tissue over half that of the original defect, grossly appearing to be
very similar in color, texture, and touch to intact skin outside the scarred perimeter
with the exception that the new skin was totally hairless (Yannas et al. 1981, 1982a,
b, 1984; Orgill 1983). See also Fig. 8.11. Contraction kinetics for cell-free and cell-
seeded DRT are shown in Fig.
5.2
(compare curves labeled DRT and KC + DRT).
In summary, the cell-free scaffold led to synthesis of a small mass of dermis while
the defect area closed mostly by contraction. On the other hand, the keratinocyte-
seeded scaffold led to closure of the defect area to a relatively minor extent by
contraction and eventually closed largely by partial synthesis of skin over about two
thirds of the initial defect area, featuring an epidermis and dermis, as was confirmed
in later studies.
