Chondroblast

Chondroblasts, or perichondrial cells, is the name given to mesenchymal progenitor cells in situ which, from endochondral ossification, will form chondrocytes in the growing cartilage matrix. Another name for them is subchondral cortico-spongious progenitors. They have euchromatic nuclei and stain by basic dyes. These cells are extremely important in Chondrogenesis due to their role in forming both the Chondrocytes and cartilage matrix which will eventually form cartilage. Use of the term is technically inaccurate since mesenchymal progenitors can also technically differentiate into osteoblasts or fat. Chondroblasts are called Chondrocytes when they embed themselves in the cartilage matrix, consisting of proteoglycan and collagen fibers, until they lie in the matrix lacunae. Once they embed themselves into the cartilage matrix, they grow the cartilage matrix by growing more cartilage extracellular matrix rather than by dividing further. Chondroblasts, or perichondrial cells, is the name given to mesenchymal progenitor cells in situ which, from endochondral ossification, will form chondrocytes in the growing cartilage matrix. Another name for them is subchondral cortico-spongious progenitors. They have euchromatic nuclei and stain by basic dyes. These cells are extremely important in Chondrogenesis due to their role in forming both the Chondrocytes and cartilage matrix which will eventually form cartilage. Use of the term is technically inaccurate since mesenchymal progenitors can also technically differentiate into osteoblasts or fat. Chondroblasts are called Chondrocytes when they embed themselves in the cartilage matrix, consisting of proteoglycan and collagen fibers, until they lie in the matrix lacunae. Once they embed themselves into the cartilage matrix, they grow the cartilage matrix by growing more cartilage extracellular matrix rather than by dividing further. Within adults and developing adults, most chondroblasts are located in the perichondrium. This is a thin layer of connective tissue which protects cartilage and is where chondroblasts help to expand cartilage size whenever prompted to by hormones such as GH, TH, and glycosaminoglycans. They are located on the perichondrium because the perichondrium, located on the outside of developing bone, is not as heavily ensheathed in cartilage extracellular matrix as the interior and because here where capillaries are located. The type of growth maintained by chondroblasts is called appositional bone growth and increases the girth of the affected tissue. It is important to note that perichondrium, and thus chondroblasts, are not found on the articular cartilage surfaces of joints. The extracellular matrix secreted by chondroblasts is composed of fibers, collagen, hyaluronic acid, proteoglycans, glycoproteins, water, and a host of macromolecules. Within finished cartilage, collagen fibers compose 10-20% of the volume, water 65-80%, and the proteoglycan-hyaluronic acid aggregates the remaining portion. Due to the proliferative nature of Chondroblasts, cells compose a larger portion of the composition than what is normally found within completed cartilage. Collagen Type II fibers are responsible for giving the future cartilage matrix its tensile strength. The structure of these fibers, like the majority of collagen fibers, forms a triple helix structure. Proteoglycans resist the compression generally put upon cartilage and generate the swelling pressure responsible for stress shielding the matrix from compression loading. They attach themselves to up to 100 Chondroitin sulfate molecules and up to 50 keratan sulfate glycoaminoglycan chains. These chains together are attached to a hyaluronic acid backbone which, in conjunction with the collagen fibrils, create an interstitial intrafibrillar space in which water is held in by the negative charge of the proteoglycans. As suggested in the name, mesenchymal progenitors originate from the mesoderm. These cells, when forming from the mesoderm, specifically form from embryonic stem cells via induction through BMP4 and fibroblast growth factor FGF2 while the fetus is inside the womb. It has been suggested that differentiating embryonic stem cells with these growth factors could prevent stem cells, once injected into potential patients, from forming teratomas, or stem cell caused tumors. An important genetic component of this process is Sox9, a HMG box transcription factor, which marks progenitor cells for chondrogenic differentiation. Inactivation of the Sox9 gene will result in the loss of all Cartilage, and thus Chondroblast, formation. This factor is also expressed alongside Sox5 and Sox6. Runx2 is another important genetic component of Chondroblast formation. It has been found that expressing this gene will result in the suppression of the differentiation of chondroblasts. Expression of this gene will also prompt already formed cartilage to undergo endochondral ossification which will prompt the cartilage to form bone. It is important to note here that these genes are not the only factors which determine whether chondroblasts will form. General inactivation or activation of these gene doesn't turn all affected cells into one type or another. Extrinsic environmental factors act upstream in determining what cell type will form out of any particular mesenchymal progenitor cell.