Osteoclasts and osteoblasts

The major cell types involved in bone resorption and deposition are osteoclasts and osteoblasts (Figure 48-11). The former are associated with resorption and the latter with deposition of bone. Osteocytes are descended from osteoblasts they also appear to be involved in maintenance of bone matrix but will not be discussed further here. 

An in vitro cell model resembles the remodeling process of osteoclastic bone resorption followed by osteoblastic bone formation investigate the influence of lead on the communication between these cells determine if lead in the medium (i.e. extracellular fluid) or in the matrix (i.e. in the bone) has different effects on osteoclasts and osteoblasts... 

The principal cells in bone are the osteoclasts and osteoblasts. Osteoclasts, the cells responsible for resorption of bone, are derived from hematopoietic stem cells. Osteoblasts are derived from local mesenchymal cells. They are the pivotal bone cell, responsible for bone formation. Skeletal tissues are remodelled throughout a lifetime, alternating resorption phases by osteoclasts with periods of intense collagen synthesis. This balance is under the control of mechanical and hormonal stimuli, which ensure the appropriate performance of the bone. Skeletal tissues have three... 

The adult skeleton is periodically remodeled by transitory anatomic structures that contain juxtaposed osteoclast and osteoblast teams and that replace old bone with new bone. The purpose of this remodeling is both to prevent bone aging and repair the damage that occurs as well as to guarantee a contribution of minerals, especially calcium, to body cells for their correct function. In the last few years, due mainly to the research in molecular biology and cellular differentiation and to studies of genetically manipulated mice, it has been possible to discover many aspects both of the cellular and molecular bases of this bone remodeling as well as of the differentiation and function of the two main implied cell types osteoblasts and osteoclasts. 

The most important cells implied in bone remodeling are the osteoclasts and osteoblast, although in this process different cellular types such as endothelial... 

Taranta A, Brama M, Teti A, De 1, V, Scandurra R, Spera G, Agnusdei D, Termine JD, Migliaccio S (2002) The selective estrogen receptor modulator raloxifene regulates osteoclast and osteoblast activity in vitro. Bone 30 368-376... 

For bone substitutes, it is very important that bioceramics have a considerable degree of porosity and particularly interconnected pores so that living bone grows rapidly into the pores. Special bone remodeling cells called osteoclasts and osteoblasts play an extremely important part of the process of rebuilding or repairing the bone. 

In addition to repairing and remodeling bone in the body, osteoclasts and osteoblasts continually turn over bone in the body by replacing bone with new liv-... 

Here, it should also be stated that there are bone growth factors and bone growth proteins that trigger the dispatch of osteoclasts and osteoblasts to an injury or im-... 

Bone contains two distinct enzymes, acid and alkaline phosphatases, which are associated with osteoclasts and osteoblasts, respectively (118). Further study of acid phosphatase and bone should be rewarding. Many... 

Hypoparathyroidism manifestates as osteomalacia with a low number and activity of both osteoclasts and osteoblasts. Osteoid formation is reduced, and its... 

The implantation experiments correlate well with the observations for skeletal development under the three dietary conditions. The osteopenia observed in the rats raised for 12 months on the L or D diets could be a manifestation of a disequilibrium between the rates of osteoclastic and osteoblastic activity. Both of these cellular activities are influenced by the trace element status of the animal. If the osteoblastic activity were more strongly inhibited by... 

Calcitriol behaves like a steroid hormone. It is transported to the nucleus of renal distal tubule cells, intestinal epithelial cells, osteoclasts, and osteoblasts where it induces calbindins, vitamin D-dependent calcium binding proteins. Calbindins mediate the intracellular movement of calcium, from diet to blood, from blood to osteoid matrix, or from bone to blood. There are two calbindin proteins, each encoded by separate genes, one of molecular weight about 9 kDa (calbindin-D. ) and one of molecular weight 28 kDa (calbindin-D28K). Each binds micromolar amounts of calcium and each disappears from animals that are... 

Vitamin D is a fat soluble vitamin related to cholesterol. In the skin, sunlight spontaneously oxidizes cholesterol to 7-dehydrocholesterol. 7-Dehydrocholesterol spontaneously isomerizes to cholecalciferol (vitamin D3), which is oxidized in the liver to 25-hydroxy cholecalciferol and, under the influence of PTH in the kidney, to 1,25-dihy-droxy cholecalciferol (calcitriol), the active form of vitamin D. Vitamin D induces the expression of calcium ion transport proteins (calbindins) in intestinal epithelium, osteoclasts, and osteoblasts. Calbindins and transient receptor potential channels (TRPV) are responsible for the uptake of calcium from the diet. In children, the absence of sunlight provokes a deficiency of vitamin D, causing an absence of calbindins and inadequate blood calcium levels. Osteoid tissue cannot calcify, causing skeletal deformities (rickets). In the elderly, there is a loss of intestinal TRPV receptors and decreased calbindin expression by vitamin D. In both cases, the resultant low blood calcium levels cause poor mineralization during bone remodeling (osteomalacia). Rickets is the childhood expression of osteomalacia. Osteoclast activity is normal but the bone does not properly mineralize. In osteoporosis, the bone is properly mineralized but osteoclasts are overly active. 

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