Bone remodeling

The Knochengeweberemodellierung (English bone remodeling ) describes a process in the course of degraded old bone by osteoclasts and subsequently replaced by newly formed bone by osteoblasts.

Task

Bone Remodeling serves the organism to maintain a stable and functional skeletal system, which would wear out quickly without this repair mechanism. The main purpose of this mechanism is thus to structural damage ( micro-cracks ) caused by everyday stresses and movements to repair and thus to respond to changing stresses on the bone. Thereby, the microarchitecture of bone is adapted to the respective conditions. In the same part of the bone remodeling is also responsible for restoring fully functional after the first bone fracture healing. Here, the callus is gradually replaced by fully load-bearing bones.

Activity

The process of bone remodeling takes place continuously and shows that bone is not rigid, but is continually adapted to changing conditions. About 3 percent of the cortical bone and 25 percent of trabecular bone can be reconstructed in this way each year. Thus, it is assumed that within 7-10 years, the equivalent of the total bone mass of the people will once degraded and resynthesized.

Operation

" Dig " During the remodeling osteoclasts with the help of various lytic enzymes ( cathepsin K, MMP- 3 and 9, ALP) in the bone matrix and form resorption lacunae ( lacunae Howship ). This absorption below osteoblasts secrete in large fields of at least 50 cells new collagenous bone skeleton ( osteoid ) from which gradually calcified in the course, thus forming new bone. Some of the osteoblasts are hereby incorporated by the mineralizing matrix and differentiate into osteocytes in the further course.

Receiving a plurality of osteoblasts, the bone-building counterparts of osteoclasts. In the TEM image of the synthesized osteoid by the osteoblasts as bright, the mineralized bone matrix down sharply demarcated zone (black borderline ) with two embedded osteocytes ( former osteoblasts) is clearly visible.

TEM image of a built-in into the bone matrix osteocytes.

Regulation

Since this Bone remodeling is a process that requires the simultaneous presence of osteoclasts and osteoblasts, he must be subject to a certain control. This is called the " Coupling". However, the exact mechanism of this control is still unknown. The osteocytes an important, albeit unknown role is attributed here, however. Most probably is an important role of the parathyroid hormone (PTH), which has both a stimulating effect on the differentiation of osteoclasts as well as by osteoblasts. Furthermore, vitamin D and cytokines osteoprotegerin and RANKL are attributed an important role.

In the adult procreative people keep in bone remodeling resorption and the de novo synthesis the scale. In the growth outweighs the structure in postmenopausal absorption (resulting in senile osteoporosis ).

Role in calcium homeostasis

Since bone is the same to his function in the locomotor apparatus, the largest reservoir of the body of calcium and phosphate, the remodeling is further attributed an important role as a factor regulating the homeostasis of these two substances. Since this mechanism when necessary must not be started until the organism can quickly respond to fluctuations in the calcium and phosphate levels in the blood.