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Chondrocytes are the prevalent cells of cartilage, with the number per matrix ratio differing with cartilage type. Chondrocytes arise from chondroblasts, which are proliferating cells that originate from mesenchymal cells after exposure to the transcription factor SOX9 [sex determining region Y (SRY)‐box 9]. Chondroblasts produce type II collagen, aggrecan, proteoglycans, and glycosaminoglycans, and therefore, the cartilaginous matrix. They become embedded in individual lacunae within the matrix that they produce (Figure 3.15); once embedded, they become chondrocytes. Chondrocytes then maintain the cartilage matrix throughout life, although the numbers of chondrocytes reduce with age. Joint trauma, inflammation, and stress fractures that extend into the cartilage can lead to chondrocyte damage and severe structural damage of the cartilage (Xiong & O'Brien, 2012).

Table 3.4 Summary of Cells, Extracellular Matrix (ECM), Subtypes, and Function of Cartilage and its Subtypes Under Normal Conditions

Characteristic	Description
Tissue type	Dense pliable connective tissue
Cells	Main cell types: Chondrocytes, chondroblastsAdditional cell types: Mesenchymal stem cells (low in number)
ECM	Hyaline cartilage: Collagen II (15–20%), water (60–80%), GAGs (e.g., hyaluronic acid)Fibrocartilage: High collagen content, lower water content than hyalineElastic cartilage: High elastin fiber content
Subtypes	Hyaline (and its subtype, articular cartilage), fibrocartilage, elastic
Function	Hyaline: Protection of bony surfaces, especially at points of movementFibrocartilage: Strength and rigidity, joint support and fusionElastic cartilage: Resilience and pliability