Bone remodeling

Bone remodeling or bone metabolism is a lifelong process where mature bone tissue is removed from the skeleton (a process called bone resorption) and new bone tissue is formed (a process called ossification or new bone formation). These processes also control the reshaping or replacement of bone following injuries like fractures but also micro-damage, which occurs during normal activity. Remodeling responds also to functional demands of the mechanical loading.

Process

In the first year of life, almost 100% of the skeleton is replaced. In adults, remodeling proceeds at about 10% per year.

An imbalance in the regulation of bone remodeling's two sub-processes, bone resorption and bone formation, results in many metabolic bone diseases, such as osteoporosis.

Bone resorption

Osteoclasts are multi-nucleated cells that originate from the fusion of pre-osteoclasts which are derived from hematopoietic precursors of the monocyte/macrophage lineage. The osteoclasts are located in what are called Howship's lacunae, pits in the bone surface which also have been resorbed by the osteoclasts.

Bone formation

New osteoblasts are derived from osteoprogenitor cells, which are derived from mesenchymal stem cells in the bone marrow. The new osteoblasts then create a layer of bone matrix, called osteoid, which is mineralized within a few weeks.

Regulation

The process of bone remodeling is tightly regulated by a variety of mechanisms, including the action of parathyroid hormone (PTH), calcitonin, and vitamin D.

Clinical significance

Diseases that are the result of alterations in bone remodeling are many, and include osteoporosis, Paget's disease, and osteopetrosis.

See also

• Osteoclast
• Osteoblast
• Osteocyte
• Osteoporosis
• Paget's disease
• Osteopetrosis