Cellular and molecular alterations of osteoblasts in human disorders of bone formation.

Osteogenesis is a complex process characterized sequentially by the commitment of precursor cells, the proliferation of osteoprogenitor cells, the differentiation of pre-osteoblasts into mature osteoblasts and the apposition of a calcified bone matrix. Recent advances in cell and molecular biology have improved our knowledge of the cellular and molecular mechanisms controlling the different steps of bone formation in humans. Using ex vivo/in vitro studies of disorders of bone formation, we showed that the recruitment of osteoprogenitor cells is the most important step controlling the rate of bone formation in both rodents and humans. Accordingly, treatments stimulating osteoblast recruitment were found to increase bone formation in experimental models of osteopenic disorders. Using models of human osteoblastic cells, we identified the profile of phenotypic markers expressed during osteoblast differentiation, and found that hormones and growth factors control osteoblastic cell proliferation and differentiation in a sequential and coordinate manner during osteogenesis in vitro. Our recent evaluation of the phenotypic osteoblast abnormalities induced by genetic mutations in the Gs alpha and FGFR-2 genes led to the characterization of the role of these genes in the alterations of osteoblast proliferation and differentiation in humans. These studies at the histological, cellular and molecular levels provided new insight into the mechanisms that are involved in pathological bone formation in humans. It is expected that further determination of the pathogenic pathways in metabolic and genetic abnormalities in human osteoblasts will help to identify novel target genes and to conceive new therapeutic tools to stimulate bone formation in osteopenic disorders.