Analysis of the zebrafish segmentation clock

Abstract

In vertebrates, the repeating vertebrae in the vertebral column are the clearest indicators of the segmented body plan. The embryonic precursors of the vertebral column and skeletal musculature are bilaterally symmetric blocks of tissue flanking the notochord called somites. Somites are generated sequentially and periodically from an unsegmented tissue called the presomitic mesoderm (PSM) by a process called somitogenesis. Underlying the periodicity of somitogenesis are transcriptional oscillations of cyclic genes in the cells of the PSM. On a tissue level, these oscillations manifest as travelling waves, departing from the posterior and arresting in the anterior. The position of arrest prefigures the position of the new somite boundary. The molecular network that comprises the cyclic genes and their regulation in the PSM is termed the segmentation clock. Retinoic acid (RA) has been previously proposed to be a differentiation signal that acts to arrest the oscillations at the anterior of the PSM. This thesis shows the zebrafish RA catabolism mutant giraffe has an altered cyclic gene wave pattern, an observation that suggests that rather than stop their oscillations, cells tune their frequencies in response to RA signalling, introducing a novel function for RA in the zebrafish segmentation clock. In amniotes, the segmentation clock instructs the metamery of the vertebral column, but in zebrafish, the relationship is not established. This thesis demonstrates that the segmentation clock is not required in zebrafish for the development of a periodic vertebral column by using a novel segmentation clock mutant, thereby supporting a role for the notochord in the development of vertebral column metamery. Therefore, two periodic patterning processes establish zebrafish body pattern – one segments the somites and musculature, and the second segments the vertebral column. This thesis advances the understanding of the mechanisms of body pattern establishment by way of these novel insights.