Skin grafts - Biomaterials for Treating Skin Loss - page 72

storage between +4 and +8 °C) or by cryopreservation, comprising storage in
special solutions (for example, 15% glycerol) and dry storage at -80 to -196 ºC
(van Baare et al ., 1998b; van Baare et al ., 1998a; Cameron et al ., 2000). The main
difference between the two methods is that refrigerated, glycerolized tissue is not
viable but maintains its structure and mechanical properties, whereas, cryopreserved
tissue consists of viable cells which can migrate into the wound bed.
Cryopreservation can enhance the safety of allografts beyond fresh specimens, but
cannot replace extensive donor screening (Pirnay et al ., 1997); whereas refrigerative,
glycerol treatment appears to inactivate extracellular and intracellular HIV (van
Baare et al ., 1998a; van Baare et al ., 1994; de Backere, 1994). Theoretically, the
risk of infection by allograft skin transplant is greater than for other forms of organ
or tissue transplantation given that one donor may be used to treat a large number
of recipients. In addition, recipients may have a depressed immune system for
pathologic (e.g. due to burns, cancer or severe trauma) or physiologic reasons (e.g.
extremes of age) which may predispose them to infectious agent transmission.
Thus, widespread use of allograft has been hampered, in part, by concerns over
disease transmission.

5.5 Principles of skin xenografts

5.5.1 Xenogeneic transplantation

Several issues unique to xenogeneic transplantation, not seen among allografts, are
observed. First the transplanted tissues may not function properly in the new
xenogeneic environment (Auchincloss and Sachs, 1998). Second, the presence of
preformed antibiodies to xenogeneic tissue in the recipient in the absence of prior
exposure is common and leads to hyperacute rejection (Hammer et al ., 1998). This
precludes xenogeneic transplantation of tissue into humans from most species
except non-human primates. However, the use of successful xenogeneic skin
grafts as temporary wound coverage continues despite these issues.

5.5.2 Xenogeneic skin grafts
Xenografts have been used as temporary wound covering with wide success. Since
the 1960s, the primary donors are pigs in the United States, by virtue of their
relative affordability and similar histologic structure compared to humans
(Bromberg et al ., 1965). Porcine skin grafts are dressings rather than grafts, as they
do not become vascularized (Chiu and Burd, 2005). Although the animal skin may
become extremely adherent and indeed occasionally incorporated into the healed
wound, xenografts are not true grafts or transplants (Sokolic et al ., 1960; Chang et
al ., 1973; Aronoff et al ., 1976). No capillary ingrowth or vessel to vessel connec-
tions occur after porcine xenograft placement (Pruitt, 1997; Lee, 1972). Passive
plasmatic nutrition maintains hydration and cellular viability for a period of time,