Biomedical Engineering Reference
In-Depth Information
7.3 Articular Cartilage
7.3.1 Structure
Articular cartilage is a hyaline cartilage covering the ends of bones in joints. It is
histologically composed of a few chondrocytes, an abundant ECM, and a moderate
amount of interstitial fluid. Chondrocytes constitute less than 5 % of cartilage
structure. They do not directly interact; rather each single cell acts as a functional
unit that is responsible for production and organization of its surrounding ECM.
Each chondrocyte possesses high metabolic activity; however, cartilage tissue
exhibits low metabolic activity due to the scarcity of its cell content [
26
-
28
].
Cartilage matrix includes collagen, proteoglycan, and noncollagenous proteins.
Collagen constitutes about two-thirds of cartilage's dry weight in an adult and it
forms an interwoven stroma responsible for the tissue's tensile strength. Collagen
type II is the dominant collagen in cartilage. Other collagens include collagen IX,
XI, and VI. Collagen IX and XI are interwoven with collagen II to make a complex
network that physically entraps other macromolecules. Collagen type VI binds
chondrocytes to the matrix through an integrin receptor that is involved in
mechanical signal transduction [
29
].
Proteoglycans such as hyaluronan, keratin sulfate, and chondroitin sulfate are
responsible for cartilage's compressive strength. Keratin sulfate and chondroitin
sulfate are joined to a core protein to form a proteoglycan monomer, the most
important of which is aggrecan. More than 300 aggrecans are linked to a linear
molecule of hyaluronan to form the proteoglycan aggregates. The negative charge
seen in proteoglycans holds a considerable amount of water. Noncollagenous
proteins include anchorin and cartilage oligomeric protein that mediate chondro-
cyte binding to the matrix. Interstitial fluid plays a crucial role in exchanges of
nutrients and oxygen in the cartilage, which has no blood vessels [
30
].
There are four structural zones: superficial, middle, deep, and calcified distin-
guishable in articular cartilage. The superficial zone is composed of flattened
chondrocytes that secrete a superficial zone protein (SZP) which is also known as
lubricin, thus providing a lubricating surface for joint movement. The middle zone
mostly contains collagen II and proteoglycan aggregates. The deep zone is char-
acterized by small round chondrocytes that are arranged in short columns per-
pendicular to the free surfaces and the calcified zone is characterized by a calcified
matrix with the presence of small chondrocytes [
31
,
32
].
7.3.2 Repair
The process of tissue repair involves the stages of necrosis, inflammation, and
repair. The existence of blood vessels is necessary for triggering an inflammatory
response, which brings repair cells including monocytes and macrophages to the
