Biomedical Engineering Reference
In-Depth Information
(a) (b)
A
o
A
o
S
R
C
C
14.1
Schematic representation of two tissue blocks that have been
excised following closure of a full-thickness skin wound in the guinea
pig. (a) Tissue block following wound closure by spontaneous healing.
A
0
, initial wound area;
S
, fraction of
A
0
which has closed by formation
of scar tissue;
C
, fraction of
A
0
that has closed by contraction. (b) Tissue
block following closure by regeneration.
R
, fractional coverage of
A
0
by
regenerated skin;
C
, fractional coverage of
A
0
by contraction.
contraction (and secondarily by scar formation) (Fig. 14.1a) and one that has
closed primarily by induced regeneration (and secondarily by contraction) (Fig.
14.1b) illustrate the use of the wound closure rule and pose the methodological
problem of separating tissue types from each other prior to determination of the
area occupied by each.
Representative estimates of
C
,
S
and
R
, tabulated in
Table 14.1
, provide insights
into the relative importance of these processes in wound closure. In rodents, which
have a mobile integument, contraction accounts for almost all of wound closure. In
the human, where the skin is tethered to subcutaneous tissues, contraction accounts
for little more than one-third of the closure process. The last entry in Table 14.1
illustrates the phenomenon of induced regeneration. For example, spontaneous
healing in the guinea pig was described by the outcome [92, 8, 0]; however,
following grafting with the keratinocyte-seeded DRT scaffold (dermis regenera-
tion template), the outcome was [28, 0, 72], showing a marked reduction in
contraction and a large increase in original area eventually occupied by regener-
ated skin. The reduction in scar to zero is a rough estimate based on available
histological data. We stress here that the entries for
S
in Table 14.1 are rough
estimates, useful in identifying large effects, rather than being detailed quantitative
data.
14.5 Scar formation may be secondary to wound
contraction
A relationship has been shown to exist between fibroblast axis orientation and
orientation of collagen fibers synthesized during synthesis of stroma (Birk and
Trelstad, 1985). Following synthesis by fibroblasts, collagen fibers are extruded
outside the cell with an orientation of the fiber axis that coincides with that of the
cell. In a wound that is subject to a tensile field, as in a contracting wound,
