Osteoblasts synthesize and mineralize the collagenous extracellular matrix of bone. In membranous bone formation this occurs directly (eg., frontal and parietal bones of the skull vault). In endochondral ossification of the long bones (eg., tibia, femur), the initial deposition of calcified cartilage is resorbed by osteoclasts, making way for osteoblastic bone formation in the vascularized primary spongiosa.
Osteoblasts are derived from the mesenchymal stem cell lineage, and the differentiation pathway is regulated by increased expression of the transcription factors Runx2/Cbfa1 and Osterix that target promoters of osteoblast-specific genes. Bone morphogenetic proteins
(BMP's) such as BMP-7/OP-1 are strongly osteoinductive in vivo and in vitro, causing commitment and differentiation of osteoprogenitor cells and pre-osteoblasts into mature osteoblasts.
Osteoblast responses to mechanical strain and shear stress in vitro serves as a paradigm for exercise-dependent regulation of bone mass and skeletal microarchitecture
Pathway of Osteogenesis
The biomechanical strain response of cyclically stretched osteoblasts on silicone membranes initially involves ionic Ca2+ entry through a Gd3+-sensitive stretch-activated channel, with rapid activation of PI-3kinase, Akt, and downstream targets. Constitutive nitric oxide synthase (NOS) expression (eNOS, nNOS) by osteoblasts appears to play a role in mechanical signal transduction. NO is a potential regulator of bone formation and bone resorption that has widespread actions in other tissues. Expression of cytokine-inducible iNOS in osteoblasts is a major factor in TNF-a triggered osteoblast apoptosis in bone inflammation (eg., periodontitis, osteomyelitis).
Osteoclasts are responsible for the normal remodeling and vascularization of bone. The osteoclast is a highly specialized catabolic cell that seals tightly to bone matrix, acidifies its apical extracellular compartment, dissolves the mineral phase, and degrades bone proteins with lysosomal enzymes. In a single day, one active osteoclast is capable of degrading the total amount of bone that can be produced by about 150 osteoblasts. The osteoclast and osteoblast have a 'ying-yang' relationship. Osteoclast activity is regulated by stromal/osteoblastic cells, while a homeostatic process known as "coupling" regulates new osteogenesis in sites of osteoclastic activity. Bone matrix sequesters abundant mitogenic (growth factor) activity, including PDGF's, FGF's, TGFb's and BMP's. Osteoclastic release and activation of these and other factors is probably involved in the local coupling of osteoblastic new bone formation.
Osteoclasts originate by fusion of macrophage-like mononuclear precursors. Sequential expression of critical genes and contact with RANKL-expressing stromal cells or osteoblasts is required for differentiation of functional osteoclasts. The specific osteoclast differentiation factor RANKL, its receptor RANK, and its inactivating soluble decoy receptor osteoprotegerin (OPG), locally regulate the populations of osteoclasts in various regions of the skeleton.
Our studies of osteoclasts include:
- mechanisms of differentiation and activation,
- regulation by osteoblast-derived proteins in the extracellular matrix, and
- pathological tumor-mediated activation.
TRAP+ Osteoclasts Resorbing Bone Mineral in vitro
TRAP+ Osteoclasts Resorbing Bone Mineral in vitro Specific growth factors produced by osteoblasts, breast cancer cells, and many other cell types, has been discovered to have direct actions on osteoclasts [Yang QL, McHugh KP, Wunderlich L, Hauschka PV., manuscript]. Models for in vitro osteoclast differentiation and analysis have been developed. Precipitated hydroxyapatite monolayers are an excellent high-resolution resorption substrate, allowing single osteoclast resorptive trails to be imaged. Chemotaxis and other signaling mechanisms by matrix proteins are being analyzed. Do osteoclasts exhibit any selectivity in their resorption of mineralized substrates? We are now able to engineer biomimetic substrates with pure matrix proteins in order to analyze matrix protein signaling to osteoclasts.