Harnessing Cell–Biomaterial Interactions for Osteochondral Tissue Regeneration - Tissue Engineering: Cell-Surface Interactions for Tissue Culture - page 70

briefly review the biological aspects and structure of cartilage and discuss how
cell-biomaterial interactions can be harnessed to aid in chondrogenesis and
regeneration of cartilage.
3 Cartilage
Cartilage is located throughout the human body in sites such as the ears, elbows,
knees, intervertebral discs, nose, and ribs. Articular cartilage, or hyaline cartilage,
is primarily located on the surface of load-bearing articulating joints (e.g., ankles,
elbows, knees, wrists) and enables the movement of the joints to occur smoothly.
Articular cartilage, unlike most tissue, is alymphatic, aneural, and avascular.
Consequently, the primary mechanism by which cells access nutrients and waste
products are removed is by diffusion through the synovial fluid [ 1 ]. Accordingly,
the transport of various molecules to and from cartilage is facilitated by the high
water content of the tissue. Water composes approximately 60-85% of the total
wet weight of cartilage [ 2 ]. When the tissue undergoes compression, some water is
expelled from the tissue, providing lubrication to the articulating surface and
allowing the joints to move with low friction. The remainder of the cartilage
weight comprises cells and the ECM. This section will briefly review the com-
ponents and architecture of cartilage.
3.1 Chondrocytes
Chondrocytes are the primary cell type in cartilage and compose 1-10% of the
total articular cartilage volume [ 3 , 4 ]. They originate from embryonic mesodermal
cells, which are responsible for limb development. Mesenchymal stem cells
(MSCs) derive from the mesodermal cells and can differentiate into a variety of
cell types, including chondrocytes. Chondrogenic differentiation occurs by a
process known as cellular condensation. The process begins with degradation of
the local ECM to allow the MSCs to aggregate and enhance the number of cell-
cell interactions, which are necessary for chondrogenesis to occur [ 5 - 8 ]. Early
differentiated chondrocytes are metabolically active, exhibit a high proliferation
rate, and start to secrete ECM proteins. During this process, the chondrocytes
delineate into two different zones: peripheral and central. In the peripheral zone,
the characteristic zonal architecture of cartilage begins to develop, whereas in the
central zone, endochondral ossification occurs [ 9 ]. The mature chondrocytes have
a rounded morphology close to the subchondral bone, but at the cartilage surface,
their shape is flatter and more discoidal. Mature chondrocytes have limited pro-
liferative ability and are commonly surrounded by pericellular matrix [ 9 , 10 ].
Chondrocytes interact with surrounding ECM through receptors, and they are