Growth/differentiation factor-5 (GDF-5) and skeletal development

Background: Growth/differentiation factor-5 (GDF-5) has been shown to be es sential for normal appendicular skeletal and joint development in humans an d mice. In brachypod, a Gdf-5 gene mouse mutant, the defect is first appare nt during early chondrogenesis, with the cartilage blastema already reduced in size by E12.5. This defect is associated with changes in the expression of cell surface molecules. Methods: To understand further how GDF-5 controls cartilage formation, we f irst mapped the expression of the Gdf-5 gene during skeletal development (p lease note that the abbreviation for the gene is given in italics and the a bbreviation for the protein expressed by the gene is given in capital lette rs). Subsequently, we over-expressed GDF-5 in the developing chick embryo u sing a replication competent retrovirus, RCAS(BP). We determined its effect s on skeletal development by histological examination and its effects on ea rly growth by autoradiography of proliferating cells. In addition, we exami ned the effect of GDF-5 on chondrogenic differentiation using micromass and single cell suspension cultures of limb mesenchymal cells. Results: These studies show that the Gdf-5 gene is expressed in the early c artilage condensation, the perichondrium, and the joint interzone. Over-exp ression of GDF-5 in chick limb buds, during the condensation stage or later when the skeletal elements have formed, increased the size of the affected elements. In both cases, the increase in size was associated with an incre ase in cell number and, at later stages, this was correlated with an increa se in S-phase cells. In vitro studies showed that GDF-5 could increase cell adhesiveness, and this may be a mechanism through which GDF-5 initiates co ndensation formation. Conclusion: These studies show that GDF-5 acts at two stages of skeletal de velopment and by two distinct mechanisms. First, GDF-5 promotes the initial stages of chondrogenesis by promoting cell adhesion, which is consistent w ith the expression of Gdf-5 in the cartilage condensation. Second, GDF-5 ca n increase the size of the skeletal elements by increasing proliferation wi thin the epiphyseal cartilage adjacent to its expression within the joint i nterzone.