Biomedical Engineering Reference
In-Depth Information
the dermis and attached epidermis and extending over an area that is sufficiently
large to allow detection of the outcome, that is formation either of scar or of
regenerated dermis. An incision injures a very small mass of tissue and offers a
fleeting opportunity to distinguish between scar and a new dermis. A partial-
thickness wound is filled with a layer of residual dermis, making it difficult,
though not impossible, to distinguish newly synthesized dermis from the native
tissue. In the discussion that follows, the data presented are based exclusively on
full-thickness skin wounds, mostly in rodents but also in the swine and the
human.
The outcome of an experimental study of induced regeneration can be described
in terms of the type of tissue that is synthesized inside the wound. Three processes
of wound closure have been recognized: contraction, scar formation and regenera-
tion. Accordingly, the outcome of a wound healing process is simply cast in terms
/of the fractional extent to which the original wound area,
A
o
, has been closed by
contraction of skin edges, scar formation or regenerated tissue. Assigning a
percentage to the area closed by contraction (
C
), scar formation (
S
) and regenera-
tion (
R
), we arrive at the wound closure rule (Yannas, 2001):
C
+
S
+
R
= 100
(14.1)
The quantity
C
can be determined by monitoring the kinetics of change in the
wound area by contraction, optionally using India ink or another label to mark the
original wound boundary (identified as the boundary of the injured dermis) and
identifying the asymptotic (time-independent) value of the area,
A
, that remains
after contraction of the dermal boundary has ceased. It follows that
A
o
- A
∞
is the
total wound area that has been closed by contraction. This quantity is used to
calculate the percent of original area that has been closed by contraction, C = [(
A
o
- A
∞
)/A
o
] × 100.
In studies of repair, for example with full-thickness skin wounds in the adult
mammal, where the only outcomes are contraction and scar formation,
R
= 0, and
the above equation becomes
C
+
S
= 100. In such cases the quantity S is then
calculated simply as the percent of original area that has closed by scar formation,
S
= [
A
∞
/A
o
] × 100.
In cases where it is likely that regeneration has been induced, a distinction
between
S
and
R
has been traditionally made by histological methods. Conven-
tional histology can be supplemented and extended by quantitative study of
collagen fiber orientation, which leads to a clear distinction between scar tissue
(planar orientation of fiber axes) and dermis (quasi-random orientation) (Ferdman
and Yannas, 1993).
The detailed methodology for measuring
C
,
S
and
R
, which is based on
histological tissue sections, has been described elsewhere in detail (Yannas, 2001).
There is currently no methodology for quantitative tissue analysis of the three
tissue types on the surface of a tissue block that includes the entire wound. Nor is
there methodology to study separately the kinetics of each process
in vivo
.
∞
