Bone development and its relation to fracture repair. The role of mesenchymal osteoblasts and surface osteoblasts

Abstract

Bone development occurs by two mechanisms: intramembranous bone formation and endochondral bone formation. Bone tissue forms by eventual differentiation of osteoprogenitor cells into either mesenchymal osteoblasts (MOBL), which synthesize woven bone in random orientation, or surface osteoblasts (SOBL), which synthesize bone on surfaces in a well oriented lamellar array. Bone repair uses the same formation patterns as bone development but the specific mechanism of repair is determined by the biomechanical environment provided. Bone synthesis and maintenance are highly dependent on the blood supply of bone and on cell-cell communication via the lacunar-canalicular system. Recent investigations highlight the molecular cascades leading to cell differentiation, the components of the structural proteins such as the various collagens, and tissue vascularization. The patterning of bone matrix from an initial woven to an eventual lamellar orientation is essential for bone to develop its maximum strength. This review demonstrates the repetitive nature of woven to lamellar bone formation as mediated by MOBLs and SOBLs in both normal vertebrate bones and bone repair. Repair, using endochondral, primary, direct and distraction osteogenesis mechanisms, is reviewed along with the associated molecular, vascular, and biophysical features. Introductory concepts and terms Bone formationBone formation is complex but the three-dimensional positioning of cells and matrices is straightforward. As in any discussion of bone formation it is important to keep in mind the distinction between bone as a tissue (bone cells and the mineralized matrix) and bone as an organ (including several tissues such as bone, cartilage, fibrous tissue, marrow and blood vessels). Normal bone develops using only 2 mechanisms: a) Intramembranous bone formation is mediated by the inner periosteal osteogenic layer with bone synthesized initially without the mediation of a cartilage phase. b) Endochondral bone formation describes the synthesis of bone on a mineralized cartilage scaffold after epiphyseal and physeal cartilage have shaped and elongated the developing organ. These mechanisms are also used in fracture and osteotomy repair with the specific mechanism dependent on the mechanical environment provided during repair. With intramembranous bone repair, mesenchymal cells differentiate along a preosteoblast to osteoblast line while endochondral bone repair is characterized by the initial synthesis of cartilage followed by the endochondral sequence of bone formation. The terms intramembranous and endochondral refer to the tissue being replaced, not to the eventual bone synthesized which is the same in both mechanisms. Matrix orientationIn bone tissue formation osteoblasts synthesize and deposit type I collagen, the main protein constituent of bone matrix, in only 2 basic conformations, a) woven and b) lamellar. In woven bone the collagen fibrils are randomly oriented while in lamellar bone they are clustered in parallel arrays. The fibrils in lamellar...

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