Hyaline Articular Cartilage - Articular Cartilage Tissue Engineering - page 7

Figure 1.6: Chondrocytes (light gray) and their surrounding pericellular matrix (medium gray) form a
functional unit termed the chondron. The adjacent territorial and interterritorial matrices (dark gray) are
collectively termed the extracellular matrix [ 26 ].
splendens [ 29 - 31 ], and researchers are now trying to understand how it forms and what its function
might be.
The superficial, or tangential, zone of articular cartilage comprises the upper 10-20% of the
tissue. It is characterized by having small diameter, densely packed collagen fibers that are oriented
parallel to the cartilage surface [ 32 ]. The matrix has a relatively low proteoglycan content as well as
low permeability [ 33 , 34 ]. The cells in this layer are densely packed and exhibit flattened, discoidal
shapes that are oriented along the neighboring collagen fibers in a tangential direction [ 35 , 36 ].
Superficial zone cells secrete specialized proteins that are hypothesized to facilitate the wear and
frictional properties of the tissue [ 21 ].
The middle, or transitional, zone occupies approximately 40-60% of the total tissue thickness.
The collagen fibers in this region exhibit an arcade-like structure interspersed with randomly oriented
fibers [ 26 ]. Proteoglycan content reaches its maximum in the middle zone [ 37 ] (Figure 1.2). The
cell density is much lower in this region than the superficial zone, and the cells themselves are more
spherical in shape [ 35 , 36 ].
The deep zone is the last region of purely-hyaline tissue before reaching bone. Its collagen
structure is characterized by large fibers that form bundles oriented perpendicular to the articular
surface and are anchored in the underlying subchondral bone [ 33 ]. Proteoglycan content is much
lower than in the middle zone [ 37 ], and the cell density is also the lowest of the three cartilaginous
zones [ 35 ]. Cells in the deep zone often group together in a columnar organization. They are slightly
elongated and oriented in the direction of collagen fibers, perpendicular to the articular surface [ 36 ].
A thin line termed the “tidemark” is present between the deep zone and calcified zone of
articular cartilage [ 38 ].The calcified zone is a region of the tissue that transitions into the subchondral
bone, minimizing the stiffness gradient between the rigid bone and more pliable cartilage [ 39 ].
Underlying this region of the cartilage is the subchondral bone, which is the ultimate anchorage
point for cartilage tissue as a whole.