Figure 4. Endothelial cells emerge from the dermomyotome at 12 ss.

(A–E) Time-lapse imaging from a dorsal view of Tg(etv2.1:EGFP)^zf372 embryos injected with mOrange:CAAX mRNA and imaged between 10 ss and 15 ss. (A) The expression of Etv2:GFP⁺ cells is visible along the LPM region (arrow) at 10 ss. At this stage, no Etv2:GFP⁺ cells are visible in the somites. (B) Starting at 12 ss, the first Etv2:GFP⁺ SDECs are detected in the lateral lip of the dermomyotome (arrowheads). Simultaneously, the LPM Etv2:GFP⁺ cells start migrating to the midline. (C) Soon after emergence, SDECs change shape and become rounder (arrowheads). (D–E) Etv2:GFP⁺ SDECs bud off from the somite as individual cells (arrowhead). (F) Dorsal view of a 12 ss embryo that was submitted to double fluorescent in situ hybridization for muscle progenitor maker pax3a (green) and endothelial marker etv2 (red). pax3a expression reveals the dermomyotome compartment that contains muscle progenitor cells. An etv2⁺ SDEC (red and arrowhead) is found in the dermomyotome, co-expressing pax3a (green), showing colocalization of an endothelial and muscle progenitor cell marker. We observed 1–2 etv2-positive cells per somite in each of the embryos examined (n=6). (G) Somitic etv2⁺ SDECs (green) do not co-express the muscle differentiation marker myoD (red), suggesting that etv2 expression is restricted to the muscle progenitor region of the somite. Dashed white lines delimitate somite from the LPM (arrow). We observed 1–2 etv2-positive cells per somite in each of the embryos examined (n=6). s, somites; LPM, lateral plate mesoderm; SDECs, somite-derived endothelial cells.

Figure 4—video 1. Time-lapse imaging of Tg(etv2.1:eGFP)^zf372; Tg(phldb1:mCherry) embryo between 12 and 16 ss.

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Lateral view of a transgenic embryo. LPM cells migrate from the left side under the somites. SDECs (Green, arrows) arise from the first and fifth somites (Red) and exit the somites to follow their LPM-derived counterparts to the midline. S, somites; LPM, lateral plate mesoderm; SDECs, somite-derived endothelial cells.

Figure 4—video 2. Time-lapse imaging of Tg(etv2.1:eGFP)^zf372; Tg(phldb1:mCherry) embryo between 12 and 16 ss.

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Lateral view of a transgenic embryo. SDECs (Green, arrows) arise from the somites (S, Magenta). By this time, most of the LPM cells (arrowheads) have ingressed underneath the somites. S, somites; SDECs, somite-derived endothelial cells; LPM, lateral plate mesoderm.