Expression of Brachyury-like T-box transcription factor, Xbra3 in Xenopus embryo

The Xenopus Brachyury-like Xbra3 gene is a novel T-box gene that is closely associated with Xenopus Brachyury. The expression pattern of Xbra3 during development is similar to that of Xbra. During gastrulation Xbra3 is expressed in the marginal zone, with a gradient of increasing expression from ventral to dorsal. In the early neurula stage Xbra3 is expressed in the notochord and posterior mesoderm, but by the tailbud stage its expression is restricted to the forming tailbud and the posterior portion of the notochord.     

Involvement of AP-2rep in morphogenesis of the axial mesoderm in <Emphasis Type="Italic">Xenopus</Emphasis> embryo

We have previously isolated a cDNA clone coding for Xenopus AP-2rep (activator protein-2 repressor), a member of the Krüppel-like factor family, and reported its expression pattern in developing Xenopus embryos. In the present study, the physiological function of AP-2rep in the morphogenetic movements of the dorsal mesoderm and ectoderm was investigated. Embryos injected with either AP-2rep or VP16repC (a dominant-negative mutant) into the dorsal marginal zone at the 4-cell stage exhibited abnormal morphology in dorsal structures. Both AP-2rep and VP16repC also inhibited the elongation of animal cap explants treated with activin without affecting the expression of differentiation markers. Whole-mount in situ hybridization analysis revealed that expression of brachyury and Wnt11 was greatly suppressed by injection of VP16repC or AP-2rep morpholino, but expression was restored by the simultaneous injection of wild-type AP-2rep RNA. Furthermore, the morphogenetic abnormality induced by injection of VP16repC or AP-2rep morpholino was restored by simultaneous injection of brachyury or Wnt11 mRNA. These results show that AP-2rep is involved in the morphogenesis of the mesoderm at the gastrula stage, via the brachyury and/or Wnt pathways. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. This work was partly supported by a grant-in-aid from The Ministry of Education, Science, and Culture of Japan.     

Topographic changes in nascent and early mesoderm in amphioxus embryos studied by DiI labeling and by in situ hybridization for a Brachyury gene

 In amphioxus embryos, the nascent and early mesoderm (including chorda-mesoderm) was visualized by expression of a Brachyury gene (AmBra-2). A band of mesoderm is first detected encircling the earliest (vegetal plate stage) gastrula sub-equatorially. Soon thereafter, the vegetal plate invaginates, resulting in a cap-shaped gastrula with the mesoderm localized at the blastoporal lip and completely encircling the blastopore. As the gastrula stage progresses, DiI (a vital dye) labeling demonstrates that the entire mesoderm is internalized by a slight involution of the epiblast into the hypoblast all around the perimeter of the blastopore. Subsequently, during the early neurula stage, the internalized mesoderm undergoes anterior extension mid-dorsally (as notochord) and dorsolaterally (in paraxial regions where segments will later form). By the late neurula stage, AmBra-2 is no longer transcribed throughout the mesoderm as a whole; instead, expression is detectable only in the posterior mesoderm and in the notochord, but not in paraxial mesoderm where definitive somites have formed. Received: 28 November 1996 / Accepted: 2 January 1997     

Diverse logics and grammar encode notochord enhancers

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Loss of Oct4 expression during the development of murine embryoid bodies

We describe the internal organization of murine embryoid bodies (EBs) in terms of the structures and cell types formed as Oct4 expression becomes progressively lost. This is done by making the EBs from iPS cells carrying a novel Oct4 reporter (Oct4-MerCreMer;mTmG) which is inducible, sensitive, and permanent in all cellular progeny. When these EBs are treated with tamoxifen, the Oct4 expressing cells switch from a red to a green fluorescence color, and this is maintained thereafter by all their progeny. We show that there is no specific pattern in which Oct4 is downregulated, rather it appears to be spatially random. Many of the earliest cells to lose Oct4 expression stain positive for markers of visceral endoderm (DAB2, α-fetoprotein (AFP), HNF4). These are randomly located, although if endoderm differentiation is allowed to commence before EB formation then an external layer is formed. This is true both of EBs made from the reporter iPS cells, or from an embryo-derived mouse ES line (R1 cells). Markers of the early body axis, Brachyury (BRA) and FOXA2, usually showed a concentration of positive cells in one region of the EB, but the morphology is not predictable and there are also scattered cells expressing these markers. These patterns are similar in R1 cells. Use of the Oct4 reporter showed a difference between BRA and FOXA2. BRA, which marks the early mesoderm, node and notochord, arises in Oct4 expressing cells on days 3-4. FOXA2, which marks the floor plate of the neural tube and definitive endoderm, as well as the node and notochord, arises at the same time but mostly in cells that have already lost Oct4 expression. Several clumps of cardiomyocytes are visible by days 7-8 of EB development, both in our iPS cells and in R1 cells. Using the Oct4 reporter we show that the cells forming these clumps lose Oct4 expression between days 3 and 5. Overall, our results indicate that EBs recapitulate normal development quite well in terms of the tempo of events and the appearance of specific markers, but they do not resemble embryos in terms of their morphology.     

Diverse Routes toward Early Somites in the Mouse Embryo

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