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Osteoprogenitor cell condensations can aggregate, dissipate or condense depending on the signals present, however these still remain largely unknown. Depending on the different effects, the cellular condensations may differentiate into osteogenic or chondrocytic condensations.
The initiation of endochondral ossification starts by proliferation and condensation of mesenchymal cells in the area where the bone will eventually be formed. Subsequently, these mesenchymal progenitor cells differentiate into chondroblasts, which actively synthesize cartilage matrix components.
Bone tissue is removed by osteoclasts, and then new bone tissue is formed by osteoblasts. Both processes utilize cytokine (TGF-β, IGF) signalling.In osteology, bone remodeling or bone metabolism is a lifelong process where mature bone tissue is removed from the skeleton (a process called bone resorption) and new bone tissue is formed (a process called ossification or new bone formation).
Osteoblasts and osteocytes are derived from osteoprogenitor cells, but osteoclasts are derived from the same cells that differentiate to form macrophages and monocytes. [21] Within the marrow of the bone there are also hematopoietic stem cells. These cells give rise to other cells, including white blood cells, red blood cells, and platelets. [22]
These growth factors modulate the differentiation of progenitor cells into osteoprogenitor cells, which are responsible for bone and cartilage formation. As a result of the demineralization process, DBM is more biologically active than undemineralized bone grafts; conversely the mechanical properties are significantly diminished.
Intra-cellular features are characteristic of a synthetically active cell. The cell density of full-thickness, human, adult, femoral condyle cartilage is maintained at 14.5 (±3.0) × 10 3 cells/ mm 2 from age 20 to 30 years. Although chondrocyte senescence occurs with aging, mitotic figures are not seen in normal adult articular cartilage.
Stromal cell-derived factor 1 (SDF-1) and CXCR4 mediate recruitment of mesenchymal stem cells. IL-1 and IL-6 are the most important cytokines for bone healing. IL-1 promotes formation of callus and of blood vessels. IL-6 promotes differentiation of osteoblasts and osteoclasts. [4] All cells within the blood clot degenerate and die.
At this stage of development, changes in the morphology of the osteoprogenitor cells occur: Their shape becomes more columnar and the amount of Golgi apparatus and rough endoplasmic reticulum increases. Eventually, all of the cells within the nidus develop into, and display the morphologic characteristics of, an osteoblast.