Osteoblasts are most commonly referred to as bone-building cells and osteoclasts as bone-degrading cells. This view is certainly too short-sighted. Rather, meaningful interaction between the two cell types is a prerequisite for balance in bone metabolism.
What are osteoblasts?
A living bone is constantly undergoing remodeling and requires the activity of both degrading and remodeling cells. A balance between dissolution and renewal of bone substance is enormously important to adapt the bone’s constitution to metabolic activity and stresses. In this context, the osteoblasts on the one hand take over the part of bone building, they form the components of the bone substance (matrix). On the other hand, they also regulate the activity of osteoclasts by inhibiting or stimulating them. In this way, the cooperation of the two cell types is perfectly coordinated and their activity is adapted to requirements. In the constant process of breakdown and build-up, the osteoblasts themselves also undergo transformation. They are transformed from their active form into an inactive one, the osteocytes. These are then an important component of the bone substance, but no longer take an active part in the regeneration process. At the same time, new active osteoblasts are constantly being regenerated in order to continue to have a sufficient number of building cells available.
Anatomy and structure
While osteoclasts belong to the macrophages (giant phagocytes), osteoblasts develop from undifferentiated stem cells of bone connective tissue. They are small bean-shaped cells and show the typical structure of very metabolically active cells. On the one hand, many mitochondria can be seen inside, the power plants that supply the energy for the increased work metabolism. Rough endoplasmic reticulum is also represented in large numbers. This is where the 3 important proteins are synthesized that are necessary for building bone substance. Collagen type I is important for the pliability of the bone. Osteocalcin and osteonectin are proteins responsible for the mineralization of bone. The distinctive Golgi apparatus with its membrane stacks takes over the transport of the synthesized substances to the cell membrane, from where they are released to the outside, into the intercellular space, and passed on to their destination. For the synthesis of the described substances, the presence of 3 vitamins is crucial. In collagen production, vitamin C is needed for the cross-linking of collagen fibrils, the prerequisite for the functionality of the protein. Vitamin K is needed for the incorporation of calcium. Finally, vitamin D ensures that sufficient calcium is absorbed into the blood via the intestine and is available to osteocalcin. Vitamin D needs sunlight to be produced in the skin. Calcium is needed for mineralization, or the strengthening of bone.
Function and tasks
Remodeling processes are constantly taking place in living bone. Sports, exercise and weight bearing make the bone thicker and stronger; if these stimuli are missing, it becomes thinner and weaker. Defects have to be repaired. The control center for these processes are the osteoblasts. They adjust their activity level and that of the osteoclasts to the demand. Even during normal stresses, microtrauma occurs as a result of incorrect stresses or movements, causing small cracks in the bone. These mini fractures need to be repaired, a process that is constantly taking place in the bone. The healing process always has the same sequence. First, the osteoclasts go into action. They eliminate the defective tissue together with healthy cell material. A wound cavity (lacuna) is formed, which is larger than the actual defect. This procedure is intended to ensure that really all the destroyed material is removed and that new intact bone tissue can actually develop. Then, the osteoblasts begin to re-close and strengthen the lacuna by forming bone tissue. The buildup takes much longer than the previous breakdown. When the bone is subjected to more intense stress from work activities or sports, compression or traction or both are produced. Increased compression results from weights, and increased tension results from the transmission of tendon traction to the bone. As mentioned earlier, osteoblasts act as a controller of this process so that buildup and breakdown processes are always in balance. They are able to slow down or promote the activity of the osteoclasts.They secrete substances (rank ligand) that can dock onto receptors of the osteoclast and activate them. The release of another molecule (osteoprogesterin) can interrupt this process and stop osteoclast activity.
Diseases
Several bone diseases can be attributed to a disturbance in the balance between the building and breaking down processes in bone metabolism, usually due more to a disturbance in the function of osteoblasts. Scurvy can be traced back to an insufficient supply of vitamin C. As a rule, malnutrition is responsible for this, which is why the disease now occurs mainly in underdeveloped countries. The lack of vitamin C leads to the osteoblasts not being able to produce the necessary cross bridges between the collagen chains. This results in defective collagen that can no longer perform its functions. Rickets in children, known as osteomalacia in adults, results from a deficiency of vitamin D due to reduced intake and too short exposure to sunlight. As a result, not enough calcium is absorbed through the intestine and is available to osteoblasts for incorporation into the bones. As a result, they lack strength, remain or become soft and deform, especially where they are exposed to pressure (bow legs). In osteoporosis, the balance of bone metabolism is thrown out of kilter. Either the anabolic activity of the osteoblasts is reduced or their control function on the activity of the osteoclasts is reduced. In both cases, there is an increased breakdown of bone substance, and bone density is reduced. Among other symptoms, increased fracture tendency with skeletal deformities is a typical feature of this disease.
Typical and common bone diseases
- Osteoporosis
- Bone pain
- Bone fracture
- Paget’s disease
