Restricted expression of cdc25a in the tailbud is essential for formation of the zebrafish posterior body

The vertebrate body forms from a multipotent stem cell-like progenitor population that contributes newly differentiated cells to the posterior end of the embryo. Here, in vivo analyses show that proliferation is compartmentalized at the posterior end of the zebrafish embryo via regulated expression of mitotic factor Cdc25a. Furthermore, compartmentalization of proliferation during embryogenesis is critical to both body extension and muscle cell fate. This study reveals an unexpected link between precise regulation of the cell cycle and differentiation from multipotency in the vertebrate embryo.

The vertebrate body forms from a multipotent stem cell-like progenitor population that progressively contributes newly differentiated cells to the most posterior end of the embryo. How the progenitor population balances proliferation and other cellular functions is unknown due to the difficulty of analyzing cell division in vivo. Here, we show that proliferation is compartmentalized at the posterior end of the embryo during early zebrafish development by the regulated expression of cdc25a, a key controller of mitotic entry. Through the use of a transgenic line that misexpresses cdc25a, we show that this compartmentalization is critical for the formation of the posterior body. Upon misexpression of cdc25a, several essential T-box transcription factors are abnormally expressed, including Spadetail/Tbx16, which specifically prevents the normal onset of myoD transcription, leading to aberrant muscle formation. Our results demonstrate that compartmentalization of proliferation during early embryogenesis is critical for both extension of the vertebrate body and differentiation of the multipotent posterior progenitor cells to the muscle cell fate.

Postembryonic neuronal addition in zebrafish dorsal root ganglia is regulated by Notch signaling

Background

The sensory neurons and glia of the dorsal root ganglia (DRG) arise from neural crest cells in the developing vertebrate embryo. In mouse and chick, DRG formation is completed during embryogenesis. In contrast, zebrafish continue to add neurons and glia to the DRG into adulthood, long after neural crest migration is complete. The molecular and cellular regulation of late DRG growth in the zebrafish remains to be characterized.

Results

In the present study, we use transgenic zebrafish lines to examine neuronal addition during postembryonic DRG growth. Neuronal addition is continuous over the period of larval development. Fate-mapping experiments support the hypothesis that new neurons are added from a population of resident, neural crest-derived progenitor cells. Conditional inhibition of Notch signaling was used to assess the role of this signaling pathway in neuronal addition. An increase in the number of DRG neurons is seen when Notch signaling is inhibited during both early and late larval development.

Conclusions

Postembryonic growth of the zebrafish DRG comes about, in part, by addition of new neurons from a resident progenitor population, a process regulated by Notch signaling.

Formation of the asymmetric pineal complex in zebrafish requires two independently acting transcription factors

The pineal complex of zebrafish consists of a pineal organ and a left-sided parapineal organ. Mutation of the floating head (flh) gene, which encodes a homeodomain protein, causes premature termination of pineal cell division without affecting specification or asymmetric placement of the parapineal. The from beyond (fby) mutation, a premature stop codon in the T-domain-containing protein Tbx2b, disrupts formation of the parapineal while leaving the pineal largely intact. However, flh is reported as being required for tbx2b transcription. To resolve the paradox that flhand tbx2b mutants have opposite phenotypes but have been placed in the same genetic pathway, we have examined transcriptional cross-regulation in single flh or fby mutants and genetic epistasis in double mutants. Careful analysis shows that flh is not required for tbx2b transcription and double mutants exhibit an additive phenotype. We conclude that Flh and Tbx2b regulate separate programs of pineal and parapineal development.

Building an asymmetric brain: development of the zebrafish epithalamus

The human brain exhibits notable asymmetries. Little is known about these symmetry deviations; however scientists are beginning to understand them by employing the lateralized zebrafish epithalamus as a model. The zebrafish epithalamus consists of the pineal and parapineal organs and paired habenular nuclei located bilateral to the pineal complex. While zebrafish pineal and parapineal organs arise from a common population of cells, parapineal cells undergo a separate program that allows them to migrate left of the pineal anlage. Studying the processes that lead to brain laterality in zebrafish will allow a better understanding of how human brain laterality is established.

Tbx2b is required for the development of the parapineal organ

Structural differences between the left and right sides of the brain exist throughout the vertebrate lineage. By studying the zebrafish pineal complex, which exhibits notable asymmetries, both the genes and the cell movements that result in left-right differences can be characterized. The pineal complex consists of the midline pineal organ and the left-sided parapineal organ. The parapineal is responsible for instructing the asymmetric architecture of the bilateral habenulae, the brain nuclei that flank the pineal complex. Using in vivo time-lapse confocal microscopy, we find that the cells that form the parapineal organ migrate as a cluster of cells from the pineal complex anlage to the left side of the brain. In a screen for mutations that disrupted brain laterality, we identified a nonsense mutation in the T-box2b (tbx2b) gene, which encodes a transcription factor expressed in the pineal complex anlage. The tbx2b mutant makes fewer parapineal cells, and they remain as individuals near the midline rather than migrating leftward as a group. The reduced number and incorrect placement of parapineal cells result in symmetric development of the adjacent habenular nuclei. We conclude that tbx2b functions to specify the correct number of parapineal cells and to regulate their asymmetric migration.