Bioreactors - Articular Cartilage Tissue Engineering - page 69

Improvements to the engineered tissue are not limited to increased matrix production. The
morphology of the constructs shows more uniform deposition of matrix than in other bioreac-
tors [ 353 , 550 ]. Collagen and proteoglycan accumulation occurs in both the peripheral and central
regions of the scaffold, and fibrous encapsulation is minimal or even non-existent [ 555 ]. These re-
sults indicate that oxygen and nutrients reach the construct center in sufficient amounts, which is
important for growing large tissues. While low oxygen concentrations are more representative of
the physiological environment in articular cartilage, anaerobic conditions have been shown to cause
poor matrix production [ 556 , 557 ]. The mass transfer enhancements of the rotating bioreactor are,
therefore, critical to its success. Oxygen and nutrients move further into the scaffold, facilitating the
growth of constructs as thick as 5 mm after 40 days of in vitro culture [ 353 ]. The culture system has
also been shown to increase cell proliferation/viability and decrease nitric oxide production, both of
which indicate a good growth environment [ 558 ].
Bioreactors for cartilage tissue engineering should provide an environment that is conducive
to retaining the chondrocytic phenotype. A number of studies have investigated rotating bioreactors,
or modified versions of these devices, for their capability to re-differentiate chondrocytes that have
non-ideal expression patterns [ 559 - 562 ]. De-differentiated chondrocytes transfected with BMP-2
were cultured in a “rotating shaft” bioreactor to induce chondrogenic changes in gene and protein
expressions, as well as stimulate rapid matrix accumulation [ 559 ]. Results in static culture were
inferior to those in the bioreactor as were results using non-transfected cells. This indicates that a
synergistic relationship exists between biochemical and mechanical stimuli, which can be facilitated
for cartilage growth by the rotating bioreactor. Constructs grown for three weeks in vitro and then
implanted in vivo for eight weeks showed good histological characteristics and integration with
surrounding tissue [ 560 ]. Another research group also found the rotating bioreactor conducive to
stimulating the chondrocytic phenotype [ 561 , 562 ]. Cells from aged subjects (

∼

84 years old) were
inoculated into the bioreactor without a scaffold and analyzed after twelve weeks in culture. The
resulting constructs formed a cartilaginous matrix that was rich in collagen type II and proteoglycans.
The preferred cell type for early cartilage tissue engineering studies was the chondrocyte.
However, difficulty with obtaining healthy chondrocytes from patients has driven interest towards
other cell types, like stem/progenitor cells (See Chapter 5). Many different types of cells have
been used in the rotating bioreactor because of its apparent conduciveness to the chondrocytic
phenotype. Bone marrow-derived mesenchymal cells have been used successfully to create cartilage
and osteochondral constructs exhibiting good protein accumulation over weeks culture [ 563 - 565 ].
These cells have been characterized as being more metabolically active than either static or simple
perfusion environments [ 566 ]. Another progenitor cell type, synovium-derived stem cells, has shown
an ability to secrete matrix rich in glycosaminoglycans and collagen type II after a month in the
rotating bioreactor [ 567 , 568 ]. Several other cell types including amniotic mesenchymal cells [ 569 ],
umbilical cord blood cells [ 570 ], and embryonic stem cells [ 571 ] have all successfully differentiated
down the chondrogenic lineage when cultured in the rotating bioreactor. Numerous researchers are