Identification of important interactions between subchondral bone ...

CHAPTER 2: Introduction
Cartilage is divided into different zones that have different chondrocyte phenotypes,
compositions, structures, and biomechanical properties (fig. 7). From the articular cartilage
surface and down to the subchondral bone, we find the superficial zone, the middle zone, the
deep zone and the calcified cartilage zone (fig. 7A). Since the superficial zone is exposed to
compression, tensile forces and diffusion, it has flattened chondrocytes and is rich in collagen
fibres that are parallel arranged to the surface (fig. 7B). In the middle zone, chondrocytes have a
spherical shape and are randomly distributed. The collagen fibres are arranged in an angulated
manner at which the compression load is better dispersed. The deep zone is responsible for
providing the greatest resistance to compressive forces. The collagen fibres are arranged in a
radial disposition, with the highest proteoglycan content, and the lowest water concentration. The
chondrocytes are still spherical, but are typically arranged in columnar orientation parallel to the
collagen fibres. The tide mark distinguishes the deep zone from the calcified cartilage zone. The
calcified layer plays an integral role in securing the cartilage to bone by anchoring the collagen
fibrils of the deep zone to the subchondral bone. In the calcified zone, the cell population is
scarce and chondrocytes are hypertrophic 57,58 .
Fig. 7. Schematic illustration of the zones observed within
articular cartilage. A) 3D view of articular cartilage divided in
different zones. B) The arrangements of collagens in the different
zones are illustrated in the boxes next to the cartilage zones. Adapted
with modifications from Sah et al. 59 .
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