Cartilage is an important structural component of the body. It is a firm tissue but is softer and much more flexible than bone. Cartilage is a connective tissue found in many areas of the body.

Cartilage formation

Cartilage is part of the skeletal system and is formed from condensed mesochyme tissue that is mostly derived from the mesoderm germ layer while a fetus is developing inside the womb. This loose undifferentiated connective tissue undergoes differentiation to form the cells of the various connective tissues throughout the body such as bone and cartilage. The process of cartilage formation is called chondrification or chondrogenesis.
 

The precursor cells that become cells of bones and cartilage are called skeletal blast cells. Skeletal blast cells that express the transcription factor Sox9 along with the coexpression of Sox5 and Sox6 develop into chondroblast precursors and those with coexpression of transcription factors Runx2 and osterix become osteogenic precursors.

Differentiation of chondroblasts is favoured in areas under compressive force and low partial pressure of oxygen (pO2) as these downregulate the expression of morphogenetic protein 3, a protein which usually inhibits cartilage differentiation.

By contrast, osteogenic differentiation is favoured in regions under mild forces and with a relatively high pO2, conditions which upregulate the expression of BMP4, which induces bone formation.

Growth of cartilage is attributable to two processes:

1. Interstitial growth which includes:
   • Cell division of the chondrocytes
   • Synthesis of the extracellular matrix
   • Expansion of the cartilage matrix from within
2. Appositional growth which includes:
   • Differentiation of the chondroblasts or perichondrial cells
   • Synthesis of the extracellular matrix
   • Expansion of the girth of the cartilage