Abstract
The skeleton has structural and locomotor functions, and is a mineral
reservoir. Bone turnover by osteoclasts and osteoblasts is a lifelong process,
incorporating growth, modelling and remodelling to repair microdamage
and access the mineral reservoir.
Signalling between bone cells is essential for the coordination of these
processes. Osteoblasts regulate osteoclast activity through the receptor
activator of nuclear factor-kB (RANK)/RANK ligand/osteoprotegerin system,
and osteocytes regulate osteoblast activity through sclerostin secretion.
If resorption and formation are balanced there is no net change in bone
mass after each cycle, but with ageing and some disease states resorption
exceeds formation leading to remodelling imbalance, decreased bone
mass and loss of microstructural integrity.
The rate of remodelling is determined by loading and endocrine influences.
The most important endocrine regulator of bone turnover is probably
oestrogen, but other hormones regulating bone metabolism include
insulin-like growth factor-1, parathyroid hormone and gut and adipocyte
hormones.
Keywords Bone; bone turnover; hormones; oestrogen; osteoblast;
osteoclast; osteocyte