The osteoblasts are one of the three cell types found in the specialized connective tissue in the structural support of the body: the bone. These cells are derived from other cells called osteoprogenitor cells and their main function is to synthesize the bone matrix.
Bone is composed of an extracellular matrix that hardens thanks to the deposition of calcium, giving the tissue strength and rigidity, and of three main classes of cells: osteoblasts, osteoclasts, and osteocytes.
Light micrograph of decalcified 'cancellous' bone showing active osteoblasts actively synthesizing osteoid (Source: Robert M. Hunt via Wikimedia Commons)
Osteoblasts are known as the bone-forming cells, while osteoclasts and osteocytes are the resorption and "gap" cells, respectively. Of these, the most abundant class corresponds to osteocytes (more than 90%), followed by osteoblasts (5%) and, to a lesser extent, osteoclasts (1%).
These cells have traditionally been identified as bone-forming cells. However, it is currently known for sure that they participate in many other events such as, for example, the synthesis of paracrine and autocrine factors such as cytokines, growth factors, proteases and others.
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Osteoblasts are derived from mesenchymal precursor cells, which also give rise to chondrocytes (cartilage cells), myoblasts (muscle cells), adipocytes (fat cells), and tendon cells, depending on the transcription factors that regulate their differentiation.
Since they belong to the stromal or mesenchymal cell system, osteoblasts are associated with the bone marrow, and they belong to a separate lineage from the hematopoietic cell system.
Among the elements involved in the formation of these cells are three transcription factors (Cbfa1, Osx and ATF4) and some proteins with specific functions in bone morphogenesis.
During skeletongenesis, osteoblasts participate in two forms of bone development: intramembranous, which gives rise to the skull, and endochondral, which is formed from a "mold" of cartilage.
However, this special class of bone cells is not completely differentiated, since they can "plunge" into the extracellular matrix to form osteocytes, whose secretory system is reduced; or, on the contrary, they can undergo apoptotic processes (programmed cell death).
The cellular fate of osteoblasts, as well as that of most cells in an organism, is genetically determined, and proliferation and differentiation events are strongly dependent on hormones and transcription factors.
characteristics
Osteoblasts are partially differentiated multinucleated secretory cells (with several nuclei), within which the organelles are spatially ordered so that the nucleus remains away from the prominent secretory region.
According to electron micrographs, osteoblasts have an abundant rough endoplasmic reticulum and a highly developed Golgi complex with numerous secretory vesicles, which accounts for the active secretory function of these cells.
They are known as "cuboidal" cells due to their morphological characteristics and are found forming unicellular layers adhered to bone surfaces.
Unlike other related cells such as osteocytes (in which they can differentiate), osteoblasts come into contact with their neighboring cells through short extensions and use longer ones to communicate with nearby osteocytes.
Both the osteoblasts and most of the osteocytes are separated from the mineralized bone matrix thanks to an organic substance in the bone matrix known as osteoid, synthesized by the osteoblasts.
In their cell membranes, osteoblasts have important factors such as integrins and hormone receptors, among which the receptors for parathyroid hormone stand out. This stimulates the secretion of the osteoprotegerin ligand, necessary for the differentiation of osteoclasts.
They are able to respond to estrogen, growth hormone, vitamin D3, and thyroxine, as well as other factors such as specific cytokines and transcription factors on which their differentiation depends.
Features
The functions of osteoblasts can be summarized in the maintenance of skeletal architecture, since they are responsible for the synthesis of the organic constituents of the bone matrix. These include collagen fibers, glycoproteins, and some proteoglycans.
Their functions are mainly related to their maturation, since from a common origin they can differentiate into bone matrix synthesizing osteoblasts, into bone lining cells and into osteocytes.
It is also in charge of the synthesis of certain enzymes and specific factors whose function involves the removal of the osteoid, contributing to the access of the osteoclasts to the calcified bone surface, thus controlling its function.
Along with osteoclasts, osteoblasts participate in bone remodeling processes by replacing regions of bone reabsorbed by osteoclasts in response to different types of mechanical stress applied to bone tissue.
Since they have the ability to regulate osteoclast activity, osteoblasts indirectly participate in body calcium homeostasis.
They participate not only in the secretion of the organic components of the bone matrix, but also in its calcification through the secretion of enzymes such as alkaline phosphatase, capable of regulating the phosphorylation of other phosphoproteins.
In addition, some of the glycoproteins produced by these cells, such as osteonectin / SPARC, tenascin C, fibronectin, and members of the thrombospondin family of proteins, are involved in the regulation of adhesion, migration, proliferation, and differentiation from others. bone cells.
Related pathologies
Many diseases in man are related to the function of osteoblasts, as a consequence of the direct involvement of these cells in the formation of bones.
Among the most common diseases associated with osteoblasts are osteoporosis, Paget's disease (which has to do with the deformation and fragility of bones) and osteoarthritis (wear of the protective tissues that line the ends of the bones).
Osteoporosis, for example, arises from a negative balance between the bone-forming activity of osteoblasts and the bone resorption activity in which osteoclasts specialize.
This negative balance seems to be related to deficiencies in the proliferation or differentiation of osteoprogenitor cells or to excessive apoptosis events.
References
- Caetano-López, J., Canhao, H., & Fonseca, J. (2007). Osteoblasts and Bone Formation. Acta Reum Prot, 32, 103–110.
- Gartner, L., & Hiatt, J. (2002). Text Atlas of Histology (2nd ed.). Mexico DF: McGraw-Hill Interamericana Editores.
- Johnson, K. (1991). Histology and Cell Biology (2nd ed.). Baltimore, Maryland: The National medical series for independent study.
- Mackie, EJ (2003). Osteoblasts: novel roles in orchestration of skeletal architecture. The International Journal of Biochemistry & Cell Biology, 35, 1301-1305.
- Martin, TJ, Fundlay, DM, Heath, JK, & Ng, KW (1993). Osteoblasts: Differentiation and Function. In Physiology and Pharmacology of Bone. Springer-Verlag Berlin Heidelberg.
- Tenenbaum, HC, & Heersche, JNM (1982). Differentiation of Osteoblasts and Formation of Mineralized Bone in Vitro. Calcif. Tissue. Int., 34, 76–79.