Page 40 - Read Online

Page 6 of 10 Polykandriotis et al. Plast Aesthet Res 2018;5:37 I http://dx.doi.org/10.20517/2347-9264.2018.52

computed tomography. We noticed an undisturbed perfusion of the extremities during the whole period
[13]
with a general patency rate of 83.3% of the AV-loops as previously shown . Pushing on, we found out that
the AV loop was able to vascularize a fibrin matrix. However, neovascularization seemed to emerge mainly
[13]
from the venous part and the interpositional graft of the AV-loop and less from the arterial segment .
In another line of experiments, after addition of vasoactive growth factors (VEGF and FGF) at the time of
implantation we could observe that the angiogenic events were significantly accelerated in the group with
[24]
the growth factors, in terms of onset of sprouting as well as progression into the phase of remodelling .

With micro-CT it was possible to visualize the patency of the vascular axis which could be confirmed both
as was macroscopically seen at the time of explantation. When we compared the findings to long term
observations after 6 weeks, the results at two weeks demonstrated initial arteriovenous shunting through the
graft. This indicates that the neoformation of the capillary network within the constructs in the chamber
forms an organoid like structure with an artery and a draining vein.

In the same time, the prevascularized matrices were secondarily loaded with a plethora of different types
of stem cells, including osteoblasts and liver stem cells. It was a natural evolution that the AV loop was
[30]
evaluated in the big animal model. Beier et al. performed the AV-loop operations on sheep with great
success.

Currently the AV-loop is utilized in selected cases in the clinical setting [9,11,16,31] .

IMPORTANCE OF THE AV LOOP MODEL FOR RECONSTRUCTIVE MEDICINE
In general, phenomena of neovascularization and neoangiogenesis play a crucial role in Plastic Surgery
[32]
and have led to the prefabrication and preformation of customized flaps that are created in situ in the
patient [33,34] . Flap prefabrication is a clinically applied method to generate custom made transplants with the
[35]
help of arterializing tissue either with arteriovenous bundles or arteriovenous loops . It has been described
for various tissues, including skin, fasciae, muscle, periosteum and/or bone [36-39] . In the clinical scenario
both arterial and venous supercharging have been postulated to potentially improve the survival area of
prefabricated flaps and to extend the possibilities of creating custom made tissue flaps [40-42] .

The combination of prefabrication and tissue engineering has been investigated intensely [39,43,44] . From
this clinically proven method we introduced the technique of prevascularizing scaffolds and constructs
in the context of Tissue Engineering with the help of an arteriovenous loop incorporated into a given
matrix [3,23] . In small and large animal models we observed that the arteriovenous loop triggers a vivid and
rapid angiogenetic response [7,11,17-19] . In our experiments male imbred Lewis rats were used. In personal
communications with other groups we were intrigued to hear that on Wistar rats the phenomenon of limp
ischaemia was common. Our data shed light on the early phase of angiogenesis between day 2 and day 4, as
we could not find vessel sprouting. On the other side between the 10th day up to day 14 a vivid angiogenetic
response was seen that led to a rapid induction of a dense neocapillary microvascular network. In our AV-
loop model we had expected an early sprouting from capillaries and venules as has been described generally
[45]
in the context of angiogenesis as soon as 27 h after the angiogenetic trigger . The influence of the fibrin
matrix that we used might be causative for this observation.

Tissue engineering (TE) has been a promising research field for more than two decades by now. Nevertheless
despite recent advances the translation of TE into daily clinical practice has not yet been achieved. One
major obstacle is the step of transplanting a generated tissue construct into a recipient site, where it is
dependant from the ingrowth of small vessels into the scaffold. Since this process of revascularization takes
several days any cell within a matrix rely on nourishing by diffusion only. Intrinsic prevascularization