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Retrospective clonal analysis in mice suggested that the vertebrate heart develops from two sources of cells called first and second lineages, respectively. Cells of the first lineage enter the linear heart tube and initiate terminal differentiation earlier than cells of the second lineage. It is thought that both heart lineages arise from a common progenitor cell population prior to the cardiac crescent stage (E7.5 of mouse development). The timing of segregation of different lineages as well as the molecular mechanisms underlying this process is not yet known. Furthermore, gene expression data for those lineages are very limited. Here we provide the first comparative study of cardiac marker gene expression during Xenopus laevis embryogenesis complemented by single cell RT-PCR analysis. In addition we provide fate mapping data of cardiac progenitor cells at different stages of development. Our analysis indicates an early segregation of cardiac lineages and a fairly complex heterogeneity of gene expression in the cardiac progenitor cells. Furthermore, this study sets a reference for all further studies analyzing cardiac development in X. laevis.
Fig. 1. Whole mount in situ hybridization of Xenopus laevis embryos at stage 13. The expression of Isl1, Nkx2.5, GATA6b, Tbx1, and FGF8b is given. Anterior views (upper row) of embryos are shown. Dashed lines indicate the level of sagittal sections shown in the lower row. e, endoderm; ect, ectoderm; m, mesoderm.
Fig. 2. (A) Cardiac expression of Isl1, Nkx2.5, GATA4, GATA5, GATA6b, Xmsr and Flk1 in Xenopus embryos at stages 15/16, 20, 24 and 29 as indicated. (B) Cardiac expression of Tbx20, Tbx1, Tbx5, BMP2, BMP4, Dkk1, and FGF8b at different developmental stages of Xenopus laevis as indicated. For embryos at stages 15/16 and 20 anterior views are presented, whereas embryos at stage 24 and 29 are shown from the ventral side, anterior is to the top. Dotted lines indicate the cement gland.
Fig. 3. (A) Cardiac expression pattern of FoxH1, FoxC1, FoxC2, dHand, eHand, Mef2c and Mef2d during Xenopus laevis. (B) Expression of the cardiac differentiation marker genes DM-GRASP, MHCα, cActin, TnIc, N-Cadherin as well as Wnt11-R and Ror2 during cardiac development. For stages 15/16 and 20 anterior views are shown, stages 24 and 29 are from the ventral side, anterior is to the top. Dotted lines indicate the cement gland.
Fig. 6. (A) Serial transverse sections indicate the expression of Isl1, Nkx2.5, GATA4, GATA5, GATA6b, Xmsr and Flk1 in Xenopus laevis embryos at stage 30/31. (B) Spatial expression of Tbx20, Tbx1, Tbx5, BMP2, BMP4, Dkk1 and FGF8b at stage 30/31. The upper row provides a lateral view. Dashed lines depict the level of sections A, B and C as indicated. Level A corresponds to the anterior region of the heart. Level B is at the level of the closing heart tube. Level C is at the posterior end of the heart where the IFT forms later in development.
Fig. 7. (A) Expression of FoxH1, FoxC1, FoxC2, dHand, eHand, Mef2c and Mef2d at stage 30/31 in Xenopus laevis embryos. (B) Spatial expression pattern of DM-GRASP, Ror2, Wnt11-R, MHCα, cActin, cTnIc and N-Cadherin at stage 30/31. Embryos in the upper raw are shown from the lateral side. The dashed lines indicate the levels A (anterior part of the heart), B (closing linear heart tube) and C (IFT forming tissue) of transverse sections.