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Analyses of Noto expression and function during neural tube and notochord development in the tail bud of mouse embryos – questions about tail bud mesenchymal stem cell potential

The fact that embryo tail morphogenesis and elongation occur long after gastrulation is completed resulted in controversy about the mechanisms of the tail bud differentiation. The formation of embryo tail structures, in particular the neural tube, the notochord, and the tail gut, could be outcome of the interactions among gastrulation - predetermined germ layers, or alternatively, origin of the tail structures could be traced to the mesenchymal stem cells of the tail bud. To analyze genetic markers of the tail bud development in the mouse embryo and to get insight in the mechanisms of the tail bud differentiation, mouse mutant embryos with loss of function of Noto gene were compared to their wild type counterparts, and to the Noto spontaneous mutant referred as truncate. In addition, in frame- inserted GFP within the Noto gene served as a marker of developing notochord In the wild type 11.5-days old embryos, the tail bud mesenchyme was continuous to the two cords ; the medullary cord located dorsally, and the tail cord located centrally within the embryo tail. The medullary cord extended into the neural tube by the process of secondary neurulation, i.e. rearrangement of mesenchymal cells during formation of the neural tube. The tail cord extended into both notochord and the tail gut. To test whether morphological continuum of the notochord and the tail bud cells was related to its origin from the tail bud mesenchyme, the expression of Noto was analysed. Indeed, the Noto expression was present specifically in the tail notochord and extended well into the tail bud. In contrast to the narrow notochord, the expression spread in the wider region of tail bud cells. In order to get insight in Noto function during tail bud development, the homozygous Noto mutants were analysed. Instead of the tail notochord, which was missing in the long segments of the mutant tails. the notochord-like cells arose from the medullary cord together with cells involved in the secondary neurulation. Instead of the notochord, part of the tail cord gave rise to the additional tail gut. To clarify which of the observed malformed structures expressed the notochord markers, Noto expression was analyzed in the homozygous Noto mutants. Noto expression was still present within undifferentiated tail bud mesenchyme and subsequently confined to the notochord-like cells within the medullary cord. In conclusion, the presented data suggest potential of the tail bud cells to generate notochord. Formation of the notochord from the medullary cord, and the additional tail gut from the tail cord support hypothesis of the tail bud cells pluripotentcy.

Noto; tail bud; embryo; stem cells

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