engleski

Visualisation of developmental mechanisms in the caudal part of splotch and truncate mouse embryos

Development of the caudal part of the mammalian embryo includes differentiation of mesenchymal cells, named the tail bud, in the posterior end of the tail. The tail bud is continuous with the neural tube, notochord and the tail gut, which led to the conclusion that caudal structures of the embryo emerge from the tail bud (1). As experimental manipulations within mammalian embryos are limited by intrauterine location and small dimensions of the embryo, mouse mutants are invaluable tool for investigation of developmental mechanisms. Development of the caudal part of the truncate and splotch mouse mutants, where tail development is affected, was analyzed. Splotch is a spontaneous Pax3 gene mutant, while truncate is a spontaneous Noto gene mutant. Splotch homozygotes die during embryo development showing neural tube, neural crest and somite malformations (2). Corresponding mutation in human causes Waardenburg syndrome characterized by defects of head and hand development and localized absence of pigmentation. Truncate homozygotes have truncated tail, due to the partially absent notochord (3). Splotch and truncate homozygous embryos and wild type control C57BL/6 embryos aged from 9.0 to 12.5 days were isolated, fixed in a mixture of 1% paraformaldehyde and 1% glutaraldehyde in 0.1 M phosphate buffer and postfixed in 1% osmium tetroxide. They were embedded in Durcopan (Fluka) and serial semithin sections, perpendicular to the longitudinal tail axis, were obtained, stained with toluidine blue and examined by light microscopy. Splotch embryos from 9.0 to 12.5 days showed open neural tube in the embryo tail. In the 9.0 to 11.5 days old embryos development of other axial structures was not influenced by abnormal neural tube. All axial structures were normal and continuous with the tail bud. In contrast, splotch 12.5 days old embryos showed open neural tube accompanied with disturbed development of other axial structures and disturbed morphology of the tail bud. It is suggested that a longer presence of open neural tube influenced development of all structures caudal from the defect. Truncate homozygous embryos exhibited partially disturbed development of the notochord, which was not continuous to the tail bud, but to the ventral neuroepithelium. In addition, discontinuous or, even doubled notochord was found. Lack of the notochord in the caudal part of the tail was accompanied with disturbed development of the neural tube and the somites. Neural tube was smaller in diameter, it lacked lumen and the massive cell death was found in the neuroepithelium. In the parts of the tail without the notochord, the somites were connected in the midline of the tail. It is suggested that disturbed development of the tail bud is a major cause of truncate phenotype. This investigation confirmed continuity of the tail bud and the tail axial structures in the mouse. The observed malformations indicate that the normal sequence of tail development requires the coordinated interactions among the tail axial structures, which can be related to pathogenesis of spina bifida in humans.

neural tube development; embryo; splotch; truncate; mouse

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