[Continuous remodeling of the skeleton. Growth factors and cytokines direct the activity]

Bone is living tissue perpetually undergoing metabolism in a process known as remodelling, a sequence of cellular events occurring throughout the skeleton. The process is initiated in response to bone resorption by multinucleated osteoclasts. The capacity to stimulate osteoclastic activity is a property common to a multiplicity of hormones and cytokines--e g, parathyroid hormone, vitamin D, thyroxine, interleukin-1 and tumour necrosis factor. There is also a group of growth factors and cytokines, such as interleukin-6 and interleukin-11, that serve as stimulators of osteoclastic recruitment. Following bone resorption by osteoclasts, osteoblasts are recruited to the resorption lacuna, where they secrete osteoid which is then mineralised to form mature bone. The coupling of bone resorption and formation is governed by growth factors embedded in the mineralised bone matrix and released during resorption. These include transforming growth factor beta, insulin-like growth factor. Osteoporosis is caused by imbalance between the resorption and formation phases of the remodelling cycle.