151performed to combine various drugs, cells, and biomaterial implants in the brain,
researchers have attempted many more combinatorial approaches in the spinal cord.
The administration of VEGF following transplantation of NPSCs in the rodentmodel of stroke , for example, has demonstrated signifi cant motor recovery com-
pared to groups that were only treated with VEGF or NPSCs alone [ 406 ].
Combinatorial treatment with NPSCs has also been extended to an experimental
model of hypoxic-ischemia , where transplantation of progenitor cells combined
with chABC signifi cantly decreased infarction size compared to groups without
combined therapy [ 407 ]. Additionally, growth factors have also been encapsulated
in hydrogels in attempts to increase migration of neurons and have even been uti-
lized to engineer specialized scaffolds to deliver stem cells [ 408 , 409 ]. Nonetheless,
there are limitations to consider when experimenting with the simultaneous treat-
ment of several techniques. For example, VEGF co-delivered with FGF after closed
head injury had no signifi cant effects compared to groups that received a single
growth factor alone [ 410 ]. The authors of this study hypothesize that signaling path-
ways may become oversaturated in response to elevated concentrations of various
signaling factors, which could be potentially problematic for many combinatorial
approaches [ 410 ].
As a proof of concept for the use of combinatorial treatments in the spinal cord,Johnson et al. investigated the effi cacy of NPSCs transplanted in fi brin scaffolds
impregnated with growth factors to enhance cell survival and promote neuronal dif-
ferentiation [ 411 ]. The authors report that the combination of NT-3, PDGF and fi brin
scaffold supported NPSC activity up to 8 weeks after transplantation and was suc-
cessful in signifi cantly increasing NPSC retention in vivo as compared to bolus and
growth factor-free scaffold transplant conditions [ 411 ]. Later studies would then
investigate similar combination treatment methods on the regenerative capabilities
of the spinal cord in addition to cellular behavior, with varying results. For example,
Kim et al. loaded PLGA microspheres with dbcAMP and separately cultured NPSCs
on fi brin scaffolds, which were then both seeded onto chitosan microconduits to
study the effects of NPSC behavior in vitro and in vivo [ 315 ]. Although it was found
that transplanted NPSC/microsphere loaded microconduits were successful in pro-
moting NPSC survival and neuronal differentiation , the data suggest that pretreat-
ment with dbcAMP, but not microsphere treatment, increased in vivo survival and
neuronal differentiation, suggesting that dbcAMP alone may be suffi cient to induce
these effects [ 315 ]. Nonetheless, the full combination strategy of stem cell and
microsphere loaded chitosan channels was still effective in promoting NPSC sur-
vival and differentiation, and promoted extensive host axonal regeneration and
improved recovery 6 weeks after full transection of the spinal cord [ 315 ]. In a similar
study, Wilems et al. modifi ed fi brin scaffolds with PLGA microspheres and encapsu-
lated progenitor motor neurons (pMNs) in a model of rat sub-acute SCI [ 392 ]. PLGA
microspheres were designed to sustain delivery of chABC and/or NEP1-40, a small
myelin-associated inhibitor antagonist, for two weeks. While in vitro experiments
confi rmed that pMN viability was unaffected when cultured with chABC and/or
NEP1-40, in vivo experiments with both molecules and encapsulated pMNs yielded
reduced cell survival and increased macrophage infi ltration. Further, scaffolds loaded
7 Regenerative Strategies for the Central Nervous System