Concept of osteoblast
The osteoblast, cell found within bone tissue, is responsible for bone synthesis and mineralisation over the formation of the skeleton and further bone remodelling.
Morphology
Osteoblasts are mononuclear cells, with eccentric nucleus, which are side by side, on the surface of the bone in an arrangement similar to simple epithelium. They present a cuboid shape, when very active, or slightly flattened.
They have a well developed rough endoplasmic reticulum with dilated cisterns. The Golgi apparatus is large and has a characteristically clear paranuclear area.
Origin and regulation
Osteoblasts originate from mesenchymal progenitor cells of bone marrow stroma. These mother cells also originate from chondroblasts, fibroblasts, adipocytes and myocytes depending on the expression of specific transcription factors.
Osteoblastic differentiation is accompanied by the expression of characteristically osteoblast phenotypic markers: collagen type I (COL1), alkaline phosphatase (AP), bone sialoprotein (BSP) and osteocalcin (OC) genes.
For precursors to differentiate themselves in osteoblastic lineage, the presence of a specific transcription factor, Cbfa 1 (core binding factor a1) is required. This factor, the earliest and specific marker of osteogenesis, induces osteoblasts differentiation, controlling bone formation by differentiated osteoblasts and regulates the expression of osteocalcin.
Osteoblasts differentiation and function are regulated by growth factors which include BMPs (bone morphogenetic proteins), which are part of the superfamily TGF-β (transforming growth factor-beta). In addition of the stimulation of osteoblast precursor differentiation, BMPs also induce non-osteogenic cells to differentiate into cells of the osteoblastic lineage.
Function
The functions of osteoblasts are bone matrix synthesis and mineralisation control. The mineralised extracellular matrix is composed mostly by type I collagen and small amounts of osteocalcin, osteopontin, bone sialoprotein, BMPs, TGF-β and the inorganic mineral hydroxyapatite.
The differentiation of osteoblasts goes through 3 stages:
- The proliferation phase, in which several extracellular matrix proteins are synthesized: TGF-β (factor that promotes the differentiation of progenitor cells in pre-osteoblasts), procollagen I (collagen type I precursor) and fibronectin (adhesion protein with a role in cell adhesion and subsequent function);
- The maturation phase, characterized by a peak of expression of alkaline phosphatase, essential for digestion;
- The mineralization, which is promoted by OC (osteocalcin), OPN (osteopontin) and BSP (bone sialoprotein) gene expression.
At the end of the bone formation activity, osteoblasts, wrapped by the calcified matrix, differentiate into osteocytes. Alternatively, they can die by apoptosis or differentiate in lining cells.
References:
- Boissy, P., Malaval, L. and Jurdic, P. (2000). Ostéoblastes et ostéoclastes: une coopération exemplaire entre cellules mésenchymateuses et cellules hématopoïétiques. Hématologie. 6 (1), p6-16.
- Couret, I. (2004). Biologie du remodelage osseux. Médecine nucléaire. 28 (2), p57-65.
- Harada, S-I and Rodan, G.A. (2003). Control of osteoblast function and regulation of bone mass. Nature. 423, p349-355.
- Junqueiro, L. and Carneiro, J. (2004). Histologia Básica. Rio de Janeiro: Guanabara Koogan. p136-139.
