Cartilage and Bone - Key Points

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Here are some points I want to emphasize about cartilage and bone.

1. Cartilage is avascular. Therefore it heals poorly and the damaged cartilage is replaced by dys/non-functional scar tissue.

2. Cartilage can grow both by apposition (deposition on surface) and by interstitial growth (expansion from within)

3. In growing cartilage, the perichondrium is the source of new chondroblasts/chondrocytes

4. Both fibrocartilage and articular hyaline cartilage entirely lack perichondrium.

5. Shrinkage of the nucleus pulposus of the IV disk occurs as a normal part of aging and results in a decrease in height of the individual.

6. Bone is highly vascular. Therefore it heals well and damaged bone is (generally) replaced by new fully functional bone.

7. Bone deprived of its normal blood supply will undergo avascular necrosis and die. This has important implications in the management of certain fractures which interrupt the normal blood supply of the bone.

8. Bone deposition and growth is always appositional, never interstitial

9. The highly vascular nature of bone also facilitates its role as a reservoir or sink of important metal ions. These can be rapidly absorbed from blood, or resorbed from bone and secreted into the blood.

10. The internal cavity of bones (medulla) is where bone marrow is found. Bone marrow is the source of stem cells from which all blood cells are derived. Some bone marrow stem cells can also give rise to other tissues. The medullary microenvironment is an important regulatory factor controlling the behaviour of these stem cells.

11. The rates of resorption and bone deposition are highly regulated and interlinked. Normally about 10% of the total bone mass is undergoing remodelling at any given time. When the rates of resorption and deposition become unlinked, or the relative kinetics change, bone disease can occur. Two such (unrelated) diseases are osteoporosis and Paget's disease. In osteomalacia, a disease of defective bone mineralization, the primary cause is lack of Ca due to vitamin D deficiency (rickets).

12. In surgical procedures involving bone, every effort is made to preserve the periosteum as it is an important source of osteoprogenitor cells from which osteoblasts are derived.