Osteoblast
Osteoblasts and osteoclasts on trabecula of lower jaw of calf embryo.
Part of the growing edge of the developing parietal bone of a fetal cat.
Gray's	subject #18 87
MeSH	Osteoblasts

An osteoblast (from the Greek words for "bone" and "germ" or embryonic) is a mononucleate cell that is responsible for bone formation. Osteoblasts produce osteoid, which is composed mainly of Type I collagen. Osteoblasts are also responsible for mineralization of the osteoid matrix. Bone is a dynamic tissue that is constantly being reshaped by osteoblasts, which build bone, and osteoclasts, which resorb bone. Osteoblast cells tend to decrease as individuals become elderly, thus decreasing the natural renovation of the bone tissue.^[1]

[edit] Osteogenesis

Osteoblasts arise from osteoprogenitor cells located in the periosteum and the bone marrow. Osteoprogenitors are immature progenitor cells that express the master regulatory transcription factor Cbfa1/Runx2.

Osteoprogenitors are induced to differentiate under the influence of growth factors, in particular the bone morphogenetic proteins (BMPs). Aside from BMPs, other growth factors including fibroblast growth factor (FGF), platelet-derived growth factor (PDGF), transforming growth factor beta (TGF-β) may promote the division of osteoprogenitors and potentially increase osteogenesis.

Once osteoprogenitors start to differentiate into osteoblasts, they begin to express a range of genetic markers including Osterix, Col1, ALP, osteocalcin, osteopontin, and osteonectin. Although the term osteoblast implies an immature cell type, osteoblasts are in fact the mature bone cells entirely responsible for generating bone tissue in animals and humans.

[edit] Morphology and histological staining

Hematoxylin and eosin staining reveals that the cytoplasm of osteoblasts is basophilic due to the presence of a large amount of rough endoplasmic reticulum. A large Golgi apparatus is also present in the centre. The nucleus is spherical and large. Active osteoblasts synthesize, and stain positively for, Type-I collagen and alkaline phosphatase.

Osteoblasts, several displaying a prominent Golgi apparatus, actively synthesizing osteoid containing two osteocytes.

[edit] Osteoblasts and osteocytes

Main article: osteocyte

Osteoblasts that become trapped in the bone matrix become osteocytes. They cease to generate osteoid and mineralized matrix, and instead act in a paracrine manner on active osteoblasts. They are believed to act in a mechanosensory manner. ^{[2] [3]}

Osteoblasts (pointer) lining bone and osteocytes within lacunae of bone

[edit] References

1. ^ D’ippolito, Gianluca; Schiller, Paul C.; Ricordi, Camillo; Roos, Bernard A.; Howard, Guy A. (1999). "Age-Related Osteogenic Potential of Mesenchymal Stromal Stem Cells from Human Vertebral Bone Marrow". Journal of Bone and Mineral Research 14 (7): 1115-1122. doi:10.1359/jbmr.1999.14.7.1115, http://www.jbmronline.com/doi/full/10.1359/jbmr.1999.14.7.1115. Retrieved on 26 October 2008. 
2. ^ Ehrlich, P. J.; Lanyon, L. E. (2002). "Mechanical Strain and Bone Cell Function: A Review". Osteoporosis International 13 (9): 688-700. doi:10.1007/s001980200095, http://www.springerlink.com/content/vpy4j72t710tgx83/. Retrieved on 25 October 2008. 
3. ^ You, J.; Yellowley, C. E.; Donahue, H. J.; Zhang, Y.; Chen, Q.; Jacobs, C. R. (2000). "Substrate Deformation Levels Associated With Routine Physical Activity Are Less Stimulatory to Bone Cells Relative to Loading-Induced Oscillatory Fluid Flow". Journal of Biomechanical Engineering 122 (4): 387-394. doi:10.1115/1.1287161, http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JBENDY000122000004000387000001. Retrieved on 25 October 2008. 

[edit] See also

• Bonesetter

[edit] External links

• MedEd at Loyola Histo/practical/cartilage/hp9-49.html

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