Harnessing Cell–Biomaterial Interactions for Osteochondral Tissue Regeneration - Tissue Engineering: Cell-Surface Interactions for Tissue Culture - page 72

3.3 Architecture
Cartilage has a zonal architecture with four main compartments: superficial/tan-
gential, middle/transitional, deep/basal, and calcified. Overall, when looking from
the superficial zone to the deep zone, one finds that the water content decreases,
oxygen pressure decreases, compressive strength increases, and tensile strength
increases [ 9 , 21 ]. The superficial/tangential zone is located at the articulating
surface and composes 10-20% of the total cartilage volume [ 22 ]. In the superficial/
tangential zone, chondrocytes are flat and disc-shaped with collagen fibrils densely
packed and parallel to the tissue surface. A limited number of proteoglycans are
located in this area with high amounts of fibronectin [ 3 ]. The superficial/tangential
zone receives the highest tension, compression, shear, and hydrostatic pressures
[ 22 ]. A large amount of superficial zone protein has been isolated primarily in this
zone [ 20 ]. The middle/transitional zone accounts for 40-60% of the total cartilage
volume [ 22 ]. In this zone, chondrocytes are more rounded in shape and the
proteoglycan content is greater compared with the superficial zone, with collagen
fibrils being more dispersed throughout the tissue. There is no specific orientation
of the cells and ECM in this zone. Within the deep/basal zone (30% of the total
tissue volume), chondrocytes are rounded, clustered, and aligned into columns that
are perpendicular to the subchondral bone, and the cells are more proliferative
compared with those in the other zones [ 22 , 23 ]. Additionally, the ECM molecules
are aligned perpendicular to the joint surface. The ECM contains more collagen
and GAG than in the superiorly oriented zones [ 21 ]. The collagen is larger in
diameter in this zone and transcends the tidemark which delineates the deep zone
from the calcified zone. The calcified zone is a thin layer that is located above the
surface of the subchondral bone. Here, chondrocytes are rounded but smaller than
in the other zones, and the cells are surrounded by calcified ECM [ 9 ]. The quantity
of proteoglycans and collagen fibrils is lower in this zone, with the collagen fibrils
anchored into the underlying calcified bone [ 9 ].
Chondrocytes are also surrounded by pockets of ECM proteins that are clas-
sified into three different regions defined as the pericellular matrix, territorial
matrix, and interterritorial matrix. The pericellular and territorial matrices allow
the chondrocytes to bind to matrix proteins and protect them during mechanical
loading [ 10 ]. A chondrocyte surrounded by the pericellular matrix is known as a
chondron, and the thickness of pericellular matrix is approximately 2 lm[ 12 ].
The pericellular matrix contains minimal collagen fibrils, with the exception of
collagen type VI, and there are abundant proteoglycans, such as aggrecan,
hyaluronan, decorin, and biglycan, and glycoproteins such as fibronectin, link
protein, and laminin [ 10 ]. The territorial and interterritorial matrices function
primarily for load bearing. The territorial matrix surrounds the chondrocytes
2-5 lm from the membrane surface [ 12 ]. In this area, type II collagen is present
with high amounts of aggrecan containing CS [ 12 ]. All these proteins wrap around
the chondrocytes to protect them from the mechanical stresses. Collagen fibers
from the territorial matrix adhere to the pericellular matrix. The interterritorial