Interaction of Cells with Decellularized Biological Materials - Tissue Engineering: Cell-Surface Interactions for Tissue Culture - page 107

induced cross-linkings between ECM proteins of these vessels would weaken the
immunological responses of the recipient, while preserving graft structure/shape
and prolonging its storage time. However, it emerged that dialdehyde starch was a
bad choice, as all grafts demonstrated severe calcification and degradation.
Another chemical agent—glutaraldehyde—was discovered and was found to be a
much better choice. Many different tissues were impregnated very successfully
with this agent, which, as a result, became readily available off-the-shelf. In 1973,
Dardik and Darkik [ 18 ] evaluated glutaraldehyde-preserved human umbilical
veins as an alternative biological bypass material, i.e., for lower extremity
bypasses. However, as already postulated by Kunlin [ 19 ] in 1949, these grafts
failed long term and, again, the greater saphenous vein was found to be the most
suitable graft in this anatomic area.
Reviewing the underlying mechanisms of graft deterioration, on find two
factors in particular which seem to play a major role in this phenomenon:
(1) immunological reactions in the sense of subliminal tissue rejection [ 20 , 21 ],
which seem to be induced by the antigeneity of resident allogeneic cells, and
(2) the method of preservation/fixation of these tissues with glutaraldehyde.
Initially, this latter agent was used to reduce the immunogenicity of tissues via
the cross-linking of collagen fibres to prolong its durability. In the meantime,
however, it had become clear that glutaraldehyde increases the risk of calcifi-
cation, potentially amplifies immunological reactions, and inhibits processes
of in vivo regeneration [ 22 ]. Today it is believed that the antigeneic properties of
allogeneic prostheses in the sense of histocompatibility differences are respon-
sible for immunological responses and their resulting tissue rejection. Although,
at least in theory, it is possible to modify immunological differences between
donor tissue and the recipient immune system, e.g., via various methods of tissue
preservation or low-dose immunosuppressive therapy, current clinical application
of allogeneic grafts is mostly limited to special cases such as infections or
elderly patients. In contrast, autologous vessel grafts, e.g., the greater saphenous
vein, are still the first choice for reconstructive and substitutional interventions,
especially in small- and medium-caliber vessels.
The clinical restriction of alloplastic vascular grafts to mainly large-vessel areas
is explained by the clinical observation that autologous vessel grafts such as the
greater saphenous vein still show superior patency rates. However, contrary to the
assumption that the greater saphenous vein may represent a universally applicable
vessel graft, it should be noted that this vessel is not available in every patient
because of prior surgical interventions, varicosis, or deep vein thrombosis.
Furthermore, it belongs to the venous and, consequently, low-pressure part of the
cardiovascular system, predisposing ectatic and degenerative deformation when
exposed to arterial/higher blood pressure load. Other autologous venous grafts,
e.g., the femoral vein, or those obtained from the upper extremity exhibit the same
structural disadvantages and are reported to be even less suitable than the greater
saphenous vein. Autologous arteries, too, are not available in every patient and are
shorter in length, so bypass grafts, e.g., at the lower extremity, are difficult to
perform.