Roles of FGF-10 on the development of diathrodial limb joints

Members of the fibroblast growth (FGF) family of signaling proteins are known to play important roles in limb skeletal patterning and in chondrocyte proliferation and maturation. Recent work from this laboratory showed that FGF members are expressed in limb developing joints. Thus, the present project focused on what roles these proteins may have in joint development.

Heparin-coated beads precoated with recombinant FGF-10 or GDF-5 were implanted around incipient proximal and distal joints of digits 3 and 4 in Day 6-8 chick limb buds in organ culture. Specimens were processed for whole mount in situ hybridization using antisense riboprobes encoding chick GDF-5 and FGF-10 or for histology analysis at indicated time points.

Whole mount in situ hybridization revealed that FGF-10 is expressed, and its transcripts are present, during interzone formation. Gain-of-function experiment revealed that exogenous FGF-10 caused down-regulation of expression of FGF-10 as well as GDF-5. In specimens continuously treated with exogenous FGF-10, joint formation was markedly impaired and often resulted in fusion of contiguous cartilaginous phalanges.

The study provides evidence for the first time that FGF-10 is expressed during joint development in addition to FGF-2 and FGF-4. The precise roles of these signaling molecules will require further work. However, it is possible to speculate that these proteins, singly or in concert, may favor proliferation of mesenchymal cells during interzone formation. Our data also show that prolonged treatment with exogenous FGF-10 leads to joint impairment and fusion. Similar defects were observed previously when other joint-associated proteins were experimentally manipulated, indicating that a fine balance among distinct regulatory molecules is needed for normal joint formation.