G. Sekaly, J. Anat. 133:397–405, 1981. with permission from blackwell publishing.) osteoclasts are Controlled by Signals From osteoblasts
The osteoblasts that make the matrix also produce the signals that recruit and activate the osteoclasts to degrade it. Disturbance of the balance can lead to osteoporosis, where there is excessive erosion of the bone matrix and weakening of the bone, or to the opposite condition, osteopetrosis, where the bone becomes excessively thick and dense. Hormonal signals have powerful effects on this balance. Chronic use of corticosteroid drugs, for example, can cause osteoporosis as a side effect; but this can be treated by other drugs that redress the balance, including agents that block the factors that osteoblasts secrete to recruit osteoclasts.
Local controls allow bone to be deposited in one place while it is resorbed in another. Through such controls over the process of remodeling, bones are endowed with a remarkable ability to adjust their structure in response to longterm variations in the load imposed on them. It is this that makes orthodontics possible, for example: a steady force applied to a tooth with a brace will cause it to move gradually, over many months, through the bone of the jaw, by remodeling of the bone tissue ahead of it and behind it.
Bone can also undergo much more rapid and dramatic reconstruction when the need arises. Some cells capable of forming new cartilage persist in the connective tissue that surrounds a bone. If the bone is broken, the cells in the neighborhood of the fracture repair it by a process that resembles the way bones develop in the embryo: cartilage is first laid down to bridge the gap and is then replaced by bone. The capacity for self-repair, so strikingly illustrated by the tissues of the skeleton, is a property of living structures that has no parallel among present-day man-made objects.
The family of connective-tissue cells includes fibroblasts, cartilage cells, bone cells, fat cells, and smooth muscle cells. Some classes of fibroblasts, such as the mesenchymal stem cells of bone marrow, seem to be able to transform into any of the other members of the family. These transformations of connective-tissue cell type are regulated by the composition of the surrounding extracellular matrix, by cell shape, and by hormones and growth factors. Cartilage and bone both consist of cells and solid matrix that the cells secrete around themselves—chondrocytes in cartilage, osteoblasts in bone (osteocytes being osteoblasts that have become trapped within the bone matrix). The matrix of cartilage is deformable so that the tissue can grow by swelling, whereas bone is rigid and can grow only by apposition. While osteoblasts secrete bone matrix, they also produce signals that recruit monocytes from the circulation to become osteoclasts, which degrade bone matrix. Through the activities of these antagonistic classes of cells, bone undergoes a perpetual remodeling through which it can adapt to the load it bears and alter its density in response to hormonal signals. Moreover, adult bone retains an ability to repair itself if fractured, by reactivation of the mechanisms that governed its embryonic development: cells in the neighborhood of the break convert into cartilage, which is later replaced by bone.