CROTWELL, P.L.*; MABEE, P.M.: Gdf5 expression in late-stage zebrafish (Danio rerio) median fins

The genetic mechanisms underlying formation of joints in tetrapod vertebrates may have developed from those involved in segmentation of basal vertebrate fin skeletons. To collect preliminary data addressing this hypothesis, expression patterns of Growth and Differentiation Factor 5 (Gdf5), a gene known to be critical in the formation of tetrapod joints, were examined in the segmenting dorsal and anal fin radials of late-stage (~6-45 days post fertilization) zebrafish, Danio rerio. Radials, or “pterygiophores,” are cartilaginous rods that support the dorsal and anal fins, and that undergo segmentation during fin development. In situ hybridization revealed dynamic expression of Gdf5 in the mesenchyme between cartilage condensations of the endoskeletal supports of the dorsal and anal fins. Gdf5 expression was first detected in 5.5 mm SL (standard length) zebrafish, prior to the onset of segmentation. As development proceeded, the expression domains expanded distally to surround tips of developing cartilages, consistent with a role for this gene in cartilage growth and differentiation. Gdf5 was later expressed in the segmenting regions of the dorsal and anal fin radials, which may indicate a role in segmentation. After 7.5 mm, Gdf5 transcripts were no longer detected in any of the median fins, nor was Gdf5 expression reinitiated in later development of the median fin endoskeleton. Both the serial expression patterns and timing of gene expression are consistent with a role for this gene in cartilage growth and differentiation, and potentially in segmentation.