Bone physiology: Bone cells, modeling, and remodeling

Abstract

The skeleton undergoes constant change through the processes of modeling and remodeling. Modeling is defined as change in the size or shape of skeletal elements that occurs when formation and resorption are independent, e.g., during growth or in an adaptive response to a change in mechanical loading. Remodeling is defined as resorption and formation that occur essentially at the same site and in a programmed manner in which formation is temporally and spatially coupled to resorption. Both modeling and remodeling are achieved by the concerted action of osteoclasts and osteoblasts, either working independently (modeling) or in sequence (remodeling). Both processes are regulated by osteocytes. Osteocytes also act as mechanosensors, contribute to calcium homeostasis and regulate serum phosphate by secretion of fibroblast growth factor 23. Bone resorption and formation are regulated a host of endocrine and paracrine factors, including, but not limited to, parathyroid hormone, vitamin D, insulin-like growth factor, receptor activator of nuclear factor kappa B, and sclerostin. Good nutrition (calcium, magnesium, vitamin D, protein, etc.) is essential for skeletal well-being during growth, adulthood and, particularly, in older age.