Harnessing Cell–Biomaterial Interactions for Osteochondral Tissue Regeneration - Tissue Engineering: Cell-Surface Interactions for Tissue Culture - page 90

architectural changes such as specific surface area could be related to chondrocyte
phenotype, ECM protein synthesis, and chondrogenic differentiation [ 192 , 195 , 198 ].
It has been demonstrated that a rounded morphology and disorganized cytoskeletal
structure of cells were observed in nanosized fiber meshes, whereas well-spread
chondrocytes with organized cytoskeletons were seen in microfibers [ 195 ]. This
observation is correlated to another report demonstrating that rounded cell shape was
retained when chondrocytes attached on fibers with a smaller diameter than the size
of the cell [ 192 ]. In addition to morphological changes caused by altering the fiber
diameter, GAG production and qualitative immunostaining for type II/IX collagen,
aggrecan, and cartilage proteoglycan link protein were higher in nanofibrous
poly(lactic acid) scaffolds than in microfiber scaffolds [ 195 ]. Moreover, type II
collagen gene expression in a PCL fibrous scaffold with a diameter of 500 nm was
also higher than in a PCL fibrous scaffold with a diameter of 1,000 nm [ 198 ].
Therefore, it can be speculated that changes in fiber diameter and subsequent
modulation of architecture in nanofibrous scaffolds can regulate the cell-material
interaction and chondrocyte behavior can be optimized.
8 Osteochondral Tissue Regeneration
8.1 Zonal Cartilage Engineering
As previously described, articular cartilage is an avascular tissue with a single cell
population and dense ECM that has a zonal organization. Each zone has a unique
distribution of chondrocytes, biochemical composition, and mechanical properties
[ 118 , 204 ]. To closely mimic the native phenotype and formation across articular
cartilage tissues, chondrocyte subpopulations in two or more distinct layers of
hydrogel have been engineered.
Superficial and deep zone chondrocytes from bovine articular cartilage have
been encapsulated in photopolymerized bilayered poly(ethylene oxide) diacrylate
hydrogels [ 204 ]. In this bilayer co-culture system, deep zone cells produced
more collagen and proteoglycan than superficial cells after 6 weeks of in vitro
culture. In addition to the inhomogeneity of ECM production, deep zone cells also
exhibited higher shear and compressive strength than the homogeneous cell
control. This research showed the heterotrophic cell interaction and modulated
biological/mechanical properties of engineered cartilage tissues. Another study
also demonstrated that engineered agarose hydrogels containing zonal chondro-
cytes exhibited depth-varying cellular and mechanical inhomogeneity similar to
that of native tissue [ 205 ]. Following 42 days of in vitro culture, the data indicated
that production of GAG and collagen from superficial and middle/deep zone
chondrocytes was enhanced when they were layered with the other subpopulation
in a bilayered construct. One of the recent approaches to mimic the highly orga-
nized zonal architecture of articular cartilage investigated a variety of hydrogel
formulations with a combination of CS, MMP-sensitive peptides, and HA in PEG