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Gastrulation involves coordinated movements of cells, facilitating mesoderm and endoderm internalization and proper patterning of tissues across the germ layers. In Xenopus laevis, headmesoderm migrates collectively along the blastocoel roof fibronectin network towards the animal pole. Meanwhile, the trunk mesodermal cells migrate over each other in convergent thickening and convergent extension movements elongating the body axis. The behaviors of cells in these regions are investigated mainly in tissue explants taken from the respective head or trunk mesodermal regions. How cells behave at the transitional zone between these territories is not described in detail. To learn about cell behaviors around this junction, we imaged cell movements in an explant that encompassed the head and trunkmesoderm. We observed that headmesoderm migration on fibronectin employed lamellipodial protrusions at the leading edge and dynamic actin remodeling in the trailing cells. Trunk mesodermal cells underwent mediolateral cell elongation and intercalation to form the notochord. Lateral edges of the notochord were defined before the anterior edge. Our movie reveals distinct mesodermal cell behaviors occurring simultaneously in different regions of gastrulating embryos. This study highlights the power of applying modern microscopy tools to revisit classical experiments, permitting a greater understanding of the cellular dynamics that shape the embryo.
Figure 1. Time-lapse microscopy of headmesoderm migration and notochord segregation and intercalation in Xenopus explant: A) Diagram of the embryonic region used in this experiment and a summary of observed cell behaviors. Early Xenopus laevis embryos were injected with mRNAs encoding LifeAct-GFP and histone H2B-RFP. Dorsal tissues in the anterior region of the post-involuted mesoderm from mid-gastrula embryos (dotted box) were taken for time-lapse microscopy for a total of 8 hours with frame intervals of 4 minutes. Headmesoderm cells exhibited F-actin polarization at the leading edge of the migration front, forming distinctive lamellipodia (top panel). The notochord formed from the trunkmesoderm, first forming lateral borders defined by high intensity F-actin signals, while the anterior border formed later (bottom panel). Cells within the notochord elongated and underwent convergent extension to straighten and extend the notochord. B-I) Selected maximum intensity projections from the movie are shown. The anterior-posterior axis is diagonal from the upper right to the lower left. The starting frame at 0 minutes (min) is shown in panel B, and subsequently every 17th frame is shown in panels C to I. The grey and magenta pseudocolor are the F-actin (LifeAct-GFP) and cell nuclei (H2B-RFP) signals, respectively. The green asterisks in panels B and C indicate the endodermal cells that are pushed out of the focal plane by the advancing mesodermal cells. The yellow and the orange arrowheads point to the lateral (panels D to I) and the anterior (panels F to I) notochordal boundaries, respectively. The blue arrows in panels D to F point to the lamellipodia in the leading-edge migrating head mesodermal cells. The blue arrowheads in panels F and G point to some examples of dynamic F-actin in protrusions in the trailing head mesodermal cells. The red asterisks in panels D to I highlight two cells that moved from their initial positions within the notochordal boundaries into the adjacent somatic region. The yellow asterisks in panels F and G indicate examples of new nuclei that appeared between migrating head mesodermal cells.
Still from Extended data. Time-lapse microscopy of headmesoderm migration and notochord segregation and intercalation in Xenopus explant. (Version 1.0). CaltechDATA. 10.22002/D1.2138
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