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The vertebrate skeleton is composed of bony and cartilaginous structures that are developed under the control of numerous genetic networks. The transcription factor Nkx3.2 and the signaling molecule Gdf5 play a fundamental role during joint development and chondrogenesis, a process whereby mesenchyme cells form precartilaginous condensations followed by chondrocyte differentiation. Mutations in these genes can lead to some rare human skeletal diseases and are furthermore thought to play a role during osteoarthritis, whereby the articular cartilage in synovial joints degrades. Both genes are fairly well studied in amniotes, but their full function and regulation are not completely understood. This thesis focuses on further characterization of Nkx3.2 and Gdf5 function, by using the zebrafish Danio rerio, a small vertebrate, as a model organism.

We generated a CRISPR/Cas9 nkx3.2 mutant zebrafish line and detected broad phenotypes in the axial skeleton. Nkx3.2 deficiency in knockout zebrafish confirms previously reported jaw joint loss, but also revealed new phenotypes in the occipital region, the Weberian apparatus, the vertebrae and some fins.

By identifying a cis-regulatory element of nkx3.2 in zebrafish, we were able to generate a transgenic zebrafish line labelling the developing jaw joint and jaw joint progenitor cells. This line enables detailed documentation of jaw joint development and paves the way for a better understanding of joint development. Knockout of this nkx3.2 enhancer sequence in zebrafish did not result in any phenotypic differences, indicating a redundant function. Besides the identification of a nkx3.2 enhancer in the zebrafish genome, we identified homologous nkx3.2 enhancer sequences in the genomes of multiple gnathostome species and found that they display a high degree of functional conservation.

To study the role of Gdf5, we generated a CRISPR/Cas9 gdf5 mutant line. gdf5 mutant zebrafish displayed abnormalities in endoskeletal elements of all median and the pectoral fins showing truncation of median fin endoskeletal elements and partial absence of pectoral fin radials.

Finally, we developed an optical projection tomography (OPT) based automated workflow to generate 3D reconstructions of in situ and skeletal-stained zebrafish embryos and larvae. The acquired imaging data of skeletal-stained larval zebrafish was subsequently used to quantify phenotypic differences between mutant and wild-type zebrafish groups. This technique allows for the identification of even subtle phenotypic differences at early stages of development.

To conclude, the work presented in this thesis provides further understanding of the role of Nkx3.2 and Gdf5 during skeletogenesis in zebrafish and contributes to the development of zebrafish imaging techniques.