There are 3 types of cells in bone: osteoblasts, osteoclasts and osteocytes.


Osteoblasts are mononucleated cells derived from pluripotential stem cells (mesenchymal cell line) present in periostium and bone marrow. They may also be differentiated as muscle, adipocyte, cartilage or fibrous tissue. Initially, osteoblasts synthesize collagen (which will constitute the osteoid matrix) and produce alkaline phosphatase and osteocalcin, as well as macrophages/monocyte stimulating factors and cytokines (especially interleukin). All these factors, acting in paracrine manner, facilitate bone remodeling and favour osteoclast growth, maturation and function.
Initially, osteoblasts synthesize collagenous and non-collagenous proteins on the bone surface. Mineralization takes place some days later. When matrix deposition is complete, osteoblasts become  trapped within it and are called Osteocytes. Osteoblasts and osteocytes are interconnected, forming a true sincytium. When their function ceases, osteoblasts remaining on the bone surface (on the interface with bone marrow) become quiescent lining cells before undergoing apoptosis.

Osteoblasts have receptors for factors influencing growth and bone remodeling. These include receptors for PTH, calcitriol, glucocorticoids, sex hormones, growth hormones and thyroid hormones. In addition they have receptors for IL-1, TNFa, prostaglandin, Insulin Growth Factor (IGF), Transforming Growth Factor (TGF-b), Bone Morphogenic Protein (BMP), Fibroblast Growth Factor (FGF) and Platelet-derived Growth Factor (PDGF).
Factors produced by osteoblasts, which probably act locally, include prostaglandins, IL-6, IGF, TGF-b, BMP, PDGF and Vascular-Endothelial Growth Factor.


Osteoclasts are large mono- or multinucleated cells whose main function is to dissolve mineral and degrade the osteoid matrix. Bone resorption takes place on their ruffled borders, producing cavities called Howship´s Lacunae formed by digestion of the underlying trabecular bone. Their life span is 3-4 weeks; once inactive they undergo apoptosis.
The osteoclast cell line is related with the macrophage-monocyte series and is derived from granulocyte-macrophage colony-forming units (GM-CFU). The macrophage colony-stimulating factor (M-CSF), whose origin is the osteoblast, is required to initiate osteoclastogenesis. Likewise, this also depends on TRANCE/RANK interaction (receptor activator of nuclear factor kappa B ligand) or osteoclast differentiating factor.
Various hormones and local factors may either stimulate (calcitriol, PTH, TNFa, PG E2, IL-1, IL-11, e IL-6) or inhibit osteoclastogenesis (IL-4, IL-13, IFN, Osteoprotegerin). Estrogen and TGFb may reduce bone resorption by stimulating apoptosis of the osteoclast.

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