Fibroblast growth factor 18 gives growth and directional cues to airway cartilage

The majority of congenital airway anomalies arise from deficits in the respiratory tract cartilage, emphasizing the importance of this cartilage to the form and function of the upper airway. The primary objective of this study was to characterize molecular mechanisms that regulate rate and direction of chondrocyte growth in the larynx and trachea. Our hypothesis for this study was that fibroblast growth factor 18 (FGF18) provides proliferative and directional cues to the developing laryngeal and tracheal cartilage in the mouse by up-regulating the cartilage-specifying gene, Sox9.

Molecular genetic and histological analyses of gene expression and cartilage growth in a mouse model.

Controlled mating of wild-type FVB/N (Friend Virus B-type/NIH mouse) mice and FGF18 overexpressing mice were carried out, and embryos ranging from embryonic (E) day 10.5 to E18.5 were obtained. The respiratory tract, including the larynx, trachea, and lung, was removed through meticulous dissection, and subjected to whole-mount in situ hybridization with RNA probes, or was sectioned and subjected to immunohistochemistry. Respiratory tracts from FVB/N mice were grown in culture in the presence of exogenous FGF18 or known inhibitors of the FGF pathway, and then subjected to quantitative reverse transcriptase polymerase chain reaction to measure the expression of cartilage-specific genes.

The upper respiratory tract begins as a simple out-pouching from the ventral foregut endoderm at E10.5. The chondrocytes that form the cartilaginous structures of the upper respiratory tract are located at the junction of the respiratory tract out-pouching and the ventral foregut endoderm. This population of chondrocytes then undergoes directional proliferation to eventually assume the mature three-dimensional configuration of the upper respiratory tract cartilaginous framework. Immunohistochemical localization of extracellular signal-regulated kinases, a known modulator of FGF signaling, demonstrated the presence of this enzyme at the periphery of growing cartilage. Explants of larynx-trachea-lung grown in culture with exogenous FGF18 demonstrated hyperplastic growth and directed growth towards the FGF18 source. Finally, both FGF18 overexpressing tracheas and tracheas cultured with exogenous FGF18 demonstrated increased expression of the cartilage-specifying gene, Sox9.

FGF18 provided both directional and proliferative cues to chondrocytes in the developing upper respiratory tract. FGF18 exerted this effect on developing chondrocytes by up-regulating Sox9 expression. Laryngoscope, 2009.