bone remodeling

Bone Modeling and Remodeling
Osteoblasts and osteoclasts act in coordination and are considered the functional unit of bone known as the basic multicellular unit (BMU). The processes of bone formation and resorption are tightly coupled, and their balance determines skeletal mass at any point in time.[13] As the skeleton grows and enlarges (modeling), bone formation predominates. Once the skeleton has reached maturity, the breakdown and renewal of bone that are responsible for skeletal maintenance is called remodeling and is likely initiated at sites experiencing fatigue and microdamage. In adults, approximately 1 million BMUs are active at one time, and they remodel, or replace, 10% of the skeleton annually.
Peak bone mass is achieved in early adulthood after the cessation of modeling, and it is determined by a variety of factors, including the type of vitamin D receptor inherited, state of nutrition, level of physical activity, age, and hormonal status. Beginning in the fourth decade, however, the amount of bone resorbed by the BMUs exceeds that which has been formed, resulting in a steady decrease in skeletal mass. Eventually, this results in senescent osteoporosis, which is exacerbated in women by the rapid bone loss that occurs during the decade following menopause (postmenopausal osteoporosis).