The heterogeneity of the osteoblast phenotype

Developmentally, anatomically and histologically, bones are not all the same. A number of studies over several years has raised the issue of osteoblast heterogeneity. These include observations on the diverse behavior of clonal cell lines from both tumor and normal osteogenic tissue, on the heterogeneous occurrence of disease in different parts of the skeleton, and on the response of different parts of the skeleton to therapeutic interventions. More recent observations on the biochemical make up of different bones and different regions of bones and on the molecular profiles of osteoblasts associated with different skeletal sites are giving new insights. The accumulating evidence indicates that osteoblasts in different maturational states, in different environments, and in different microenvironments, express diverse gene repertoires. The diversity extends to expression of bone matrix molecules, transcription factors, hormones and cytokines, and their receptors. The data support the view that there is not a single unique phenotype characterizing all osteoblasts, but rather that different expressed gene repertoires are associated with distinct osteoblast subpopulations and that this molecular diversity may reflect a need for functional diversity in osteoblasts engaged in different activities.