Engineering Fibrous Tissues and Their Interfaces with Bone - Structural Interfaces and Attachments in Biology - page 322

modulus, and strain energy density were significantly increased over the unseeded
implant controls. Results also revealed an increased number of tenocytes and the
formation of a larger collagen fiber bundle in the MSC-seeded implants [ 75 ]. These
studies concluded that MSCs delivered to an injury site can improve the biome-
chanics, structure, and overall function of the tissue [ 75 , 78 ].
MSCs can also adhere to and proliferate on scaffolds and secrete ECM
molecules found in native tendon and ligament tissue [ 79 - 82 ]. Human MSCs
seeded on RGD-modified silk scaffold and incubated in vitro for 21 days demon-
strated upregulation of gene expression for collagen type I, collagen type III, and
tenascin-C by day 14 [ 81 ]. MSCs have also been shown to improve wound healing,
as seen by the increased mechanical properties and increased cell and fiber organi-
zation 26 weeks after implantation into a patellar tendon gap defect in a rabbit
model [ 80 , 83 ].
In sum, these findings suggest that MSCs are good candidates for tendon and
ligament tissue engineering [ 84 ]. However, specific genes and/or surface markers
that define a MSC from other stromal-derived cells have not been identified, which
may pose problems in confirming purity of cell source [ 85 ]. Similarly, before MSCs
can be used consistently for tendon/ligament tissue regeneration, specific protocols
must be developed to promote MSC differentiation toward a fibroblastic lineage,
while suppressing differentiation towards other cell types.
Another cell source for fibrous tissue engineering is MSCs derived from human
embryonic stem cells (hESCs) [ 86 ]. hESCs are cells from an early embryo that can
be expanded nearly indefinitely while in the undifferentiated state and have the
ability to develop into any cell type in the body [ 87 ]. One study investigated
the potential of hESC-derived MSCs (hESC-MSCs) for tendon regeneration [ 88 ].
The cells were formed into sheets in vitro , creating engineered tendons that were
implanted into defect patellar tendons in rats. After 4 weeks, hESC-MSC tendon
implants showed more collagen fiber formation than in fibrin-treated controls and
the tissue appeared to be dense with higher cell numbers with spindle-like
morphologies [ 88 ]. Using cell-labeling techniques, hESC-MSCs were seen to be
distributed around the injury site 2 weeks after implementation. These cells, under
microscopy, were characterized as having spindle-like morphologies and were
oriented along the direction of mechanical force. From real time-polymerase
chain reaction (RT-PCR) analysis, it was seen that collagen type III, collagen
type XIV, and tenascin-C had high gene expression after 2 weeks, indicating that
these cells were able to improve tendon repair through upregulation of ECM
molecules [ 88 ]. Gene expression of two key growth factors for tendon healing,
TGF- 3 and basic fibroblast growth factor (bFGF), was also detected 2 weeks after

implantation, suggesting that hESC-MSC are able to promote repair in part through
expression of soluble factors [ 88 ]. One major challenge of using hESCs themselves
in fibrous tissue engineering is that their pluripotency and self-renewal causes
concern for spontaneous differentiation and the resulting formation of teratomas,
a benign tumor composed of various cell types [ 89 ]. Therefore, an intermediate step
to differentiate hESCs into multipotent cells, such as the MSCs examined in this
study, may be required before this cell type can be applied in vivo .

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