Sequential effects of spadetail, one-eyed pinhead and no tail on midline convergence of nephric primordia during zebrafish embryogenesis.

Midline convergence of organ primordia is an important mechanism that shapes the vertebrate body plan. Here, we focus on the morphogenetic movements of pronephric glomerular primordia (PGP) occurring during zebrafish embryonic kidney development. To characterize the process of PGP midline convergence, we used Wilms' tumour 1a (wt1a) as a marker to label kidney primordia, and performed quantitative analyses of the migration of the bilateral PGP. The PGP initially are approximately 350 μm apart in a wild type embryo at 10h post fertilization (hpf). The inter-PGP distance decreases exponentially between 10 and 48 hpf, while the anterior-posterior (A-P) dimension of each PGP increases linearly between 10 and 12 hpf, then decreases substantially between 12 and 24 hpf. Using mutants in the Nodal receptor cofactor one-eyed pinhead (oep) and the T-box transcription factors spadetail (spt) and no tail (ntl), we were able to define distinctive regulation underlying these sequential phases of PGP midline migration. Zygotic oep mutants (Zoep(-/-)) exhibited defects in midline convergence after 16 hpf. Spt is necessary for PGP convergence from 10 hpf, whereas ntl's effect on convergence does not begin until 24 hpf. Notably, we observed normal cardiac convergence in spt(-/-) and ntl(-/-) embryos implying that these novel roles of spt and ntl in PGP migration cannot be explained simply by generalised effects on midline convergence. These findings demonstrate that quantitative approaches to developmental migration allow the parsing of early patterning events, and in this instance suggest that the zebrafish may offer insights into midline urogenital migration anomalies in humans.