[Bone tissue: rebuilding and inflammation]

In this review the author summarizes the knowledge related to structural elements of bone tissue. The process of bone reorganisation and knowledge about the special feature of bone metabolism in human are also discussed. It is noted that due to the reorganisation, there is a complete renewal of bone tissue in every 10 years, and this renewal lasts throughout the life. However, there are life periods when osteoclast activity is low, e.g. in childhood and the second decade of life when the gain of bone mass may be as much as 40% of the final bone mass. Overactivity of osteoclasts occurs at age 60 years in men and somewhat earlier in women. Reorganization of bone tissue is an elementary requirement for the physiological functions (locomotion, hemopoiesis, immune functions). The RANK-RANKL-osteoprotegerin axis plays an important role in the regulation of bone metabolism. Bone mass is dependent on osteocytes; osteoblasts are building up while osteoclasts are reabsorbing bone tissue. In this process transcription factors, hormone-like substances and a large number of cytokines are involved. In addition, the inflammatory process within the bone tissue as well as the defending, reparative inflammation and specific immune response are of great importance in bone reorganisation. This is particularly valid for α2-macroglobulin and transforming growth factor, although the exact role in bone reorganization has not been fully explored. It can be concluded that the elements, which participate in bone reorganization and in defending inflammatory and specific immunological processes, are essentially identical. Therefore, the existence of an osteo-immunological complex system has emerged.