Biomedical Engineering Reference
In-Depth Information
average diameter of the collagen fibers (Fig.
4.8
). Analysis of the azimuthal inten-
sity distribution of scattered light yields numerical average values of the degree
of fiber alignment in the plane of the epidermis, expressed in terms of an orienta-
tion index,
S
, a simple trigonometric function that varies between 0 for randomly
oriented fibers and 1 for a perfectly aligned arrangement. The average diameter of
collagen fibers is calculated from the scattering angle at which the intensity reaches
its first minimum (Ferdman and Yannas 1986, 1993; Ferdman 1987). The results
of a light-scattering study, in which data from guinea pig scar were compared with
intact skin adjacent to it, are presented in Table
4.1
. The data show the presence of a
small degree of orientation in sections of intact dermis cut either parallel to or per-
pendicular to the major contraction direction, itself perpendicular to the long axis of
the scar in Fig.
4.7
(top). In contrast, fibers in scar are highly, though not perfectly,
oriented in the plane in sections cut parallel to the direction of contraction, though
not perpendicular to it. Data in Table
4.1
also show that the collagen fibers in intact
dermis are thicker than those in scar. A deformation-field theory of scar formation
is presented in Chap. 8.
Methods for quantitative differentiation between dermis and scar can, in prin-
ciple, also be based on the noncollagenous components of these tissues. The pro-
teoglycan content in scar differs significantly from that in the dermis, amounting,
in essence, to a 16 % higher content of dermatan sulfate and a 35 % lower content
of hyaluronic acid in scar (Swann et al. 1988; Garg et al. 1989, 1990, 2000). Scar
also differs from dermis in the size of the glycosaminoglycan (GAG) side chains,
the degree and location of sulfation of GAGs, the size of the protein core of the pro-
teoglycans, the degree of D-glucuronic acid to L-iduronic acid epimerization, and
the ratio of biglycan to decorin (Garg et al. 2000). The presence of elastin fibers has
been confirmed in skin scars in human and in rodents (Davidson et al. 1992); how-
ever, scar from skin wounds has been reported to contain a smaller proportion of
elastic fibers (elastin) than in normal dermis (Peacock and Van Winkle 1976). The
paucity of elastic fibers and the orientation of collagen fibers preferentially along
lines of tension during wound healing have been cited as the structural basis for the
observation that scar is stiffer (less extensible) than skin adjacent to it (Peacock and
Van Winkle 1976).
Neuroma formation characterizes the spontaneous closure of stumps generated by
transecting a peripheral nerve. Closure of a stump by capping prevents the reconnec-
tion of the severed axons and leads to a total loss of electrophysiological function along
the transected nerve. Capping of stumps also stems the flow of exudate and restores
some of the homeostatic balance that was lost after transection. As with the spontane-
ous healing of a skin defect that leads to formation of functionally inferior dermal scar,
the price of spontaneous healing of a transected nerve wound is loss of specialized
organ function; however, there is also chronic pain (Hazari and Elliot 2004).
The tissue mass that caps the proximal nerve stump at the end of the healing pro-
cess comprises, as described in Chap. 2, disorganized connective tissue that is poor-
ly vascularized and interspersed with Schwann cells together with a large number
