Mini-reviewTargeted and nontargeted bone remodeling: relationship to basic multicellular unit origination and progression
Introduction
Most discussions of bone remodeling focus on how it is influenced by hormones, cytokines, growth factors, and other molecules, and pay no attention to why it occurs. All physiologic processes are aimed at something, however, and it is impossible to understand how they are regulated without some criteria of success or failure. In this mini-review, I examine the purposes and instruments of bone remodeling and how they are related.
Section snippets
Purposes of bone remodeling
Bone remodeling is the mechanism of bone replacement in the vertebrate skeleton. Considering the reasons for replacement, it must be required to preserve the functional capacity of bone, which is in some way compromised if it is allowed to become too old.28 The primary function of bone is mechanical load bearing, which is carried out by cortical bone throughout the skeleton and by peripheral cancellous bone. Subsidiary functions are to participate in plasma calcium homeostasis and to support
Instruments of bone remodeling
Bone replacement is initiated by osteoclastic resorption followed soon after by osteoblastic formation. These are commonly regarded as independent processes, but in reality resorption and formation are closely linked within discrete temporary anatomic structures, first described by Frost who gave them the name “basic metabolizing units,”5 a term he later changed to “basic multicellular units,” usually abbreviated BMU.6 The individuality of the BMU is most readily demonstrated in cortical bone
How the instruments of bone remodeling are used to accomplish its purposes
Each remodeling project begins with the recognition of a target — bone needing to be replaced — which defines the need for a new BMU. The next step is to select the site of BMU origination, which will be reasonably close to the nearest convenient blood vessel, where the components of origination just enumerated will unfold. Target recognition, site selection, and directional control probably involve the ubiquitous network of osteocytes.16 Thinking about bone remodeling in terms of projects
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