Engineering Fibrous Tissues and Their Interfaces with Bone - Structural Interfaces and Attachments in Biology - page 321

15.4.1 Cells
A main component of the tissue engineering approach to regeneration of fibrous
tissues is the availability of appropriate cells. The presence of cells is imperative to
remodeling because they promote proliferation, cell-cell signaling, and ECM for-
mation. Fibroblasts are the main cell type found in tendons and ligaments. Many
laboratories have developed fibroblast-seeded scaffolds and have successfully
shown that fibroblasts can adhere and proliferate on a variety of materials
[ 65 - 68 ]. These cells secrete typical tendon and ligament ECM molecules, such as
collagen type I, III, and proteoglycans. One concern with using this cell type is the
availability of native tendon and ligament fibroblasts because they are difficult to
harvest without using invasive surgery [ 27 ].
Another potential cell source for tissue regeneration is dermal fibroblasts. This
cell type can be easily obtained from the skin, which eliminates availability issues.
It has been shown that these cells can adhere to and proliferate on collagen fiber
scaffolds and silk sheets [ 69 - 71 ]. One study has shown that dermal fibroblasts have
the ability to proliferate faster than native ligament counterparts when cultured
in vitro and in vivo [ 68 ]. Autologous ACL and skin fibroblasts were retrieved from
rabbits, labeled, seeded onto a collagen fiber scaffold, and incubated in vitro . The
results revealed that after 3 days, the number of skin fibroblasts attached to the fiber
scaffold was significantly more than the numbers of attached autologous fibroblast,
suggesting increased cell proliferation [ 68 ]. After incubation, fibroblast-seeded
collagen fiber scaffolds were implanted into the knee joints of the rabbits that
served as cell donors. Viable dermal fibroblasts remained attached to the scaffold
for at least 4 weeks. Dermal fibroblasts have also been shown to upregulate gene
expression and secrete tendon and ligament ECM components like collagen type I
and III when treated with growth factors [ 72 , 73 ]. Although experiments involving
dermal fibroblasts have shown positive results for cell proliferation and ECM
formation [ 74 ], concerns remain about whether dermal fibroblasts will behave
similarly to native tendon/ligament fibroblasts after transplantation into a new
environment.
Stem cells have also been extensively explored for orthopedic tissue engineering
applications. For fibrous tissue regeneration, mesenchymal stem cells (MSCs) from
bone marrow sources have been widely investigated [ 9 , 75 ]. MSCs can differentiate
into osteoblasts, chondrocytes, myoblasts, and adipocytes, as well as tendon/liga-
ment fibroblasts [ 76 , 77 ]. One study has shown that a collagen type I construct
seeded with bone marrow-derived MSCs had greater structural and material
properties than constructs without cell seeding [ 78 ]. These constructs were
implanted in defect rabbit Achilles tendons and evaluated 4, 8, and 12 weeks
later via biochemical analysis and examination of fiber structural organization.
Over time, MSC-seeded constructs resulted in better alignment of cells and colla-
gen fibers than the unseeded controls [ 78 ]. Another experiment studied the mechan-
ical properties of MSC-seeded collagen type I gels implanted in injured rabbit
patellar tendons [ 75 ]. Four weeks after implantation, maximum stress, Young's