Fig. 3. Expression of Flk-1 and CD41 in the gastrulating murine embryo. (A) E7.0 (mid-streak) embryo reveals Flk-1-expressing putative mesoderm cells migrating from the level of the embryo proper to the more proximal region of the egg cylinder (white box indicates region depicted in B and C). (B) Flk-1 staining only. (C) Emergence of CD41^dim expression in the E7.0 yolk sac (arrowheads). (D) E7.25 (late-streak stage) embryo expresses both Flk-1 (E) and CD41^dim (F) cells with the CD41^dim cells forming a band of cells in the most proximal portion of the yolk sac while Flk-1⁺ cells extend toward the distal leading edge of the yolk sac as it approaches the embryo proper. (G) A composite higher magnification image of the proximal yolk sac (in orthogonal view) of an E7.25 embryo. The unlabeled visceral endoderm (endo) overlies Flk-1⁺ (H) and CD41^dim (I) expressing cells. The arrowheads (G-I) reflect cells in transition from Flk-1⁺ CD41^lo/- (green) to Flk-1⁺ CD41^dim (cyan) to Flk-1^dim CD41^dim (blue). (J) E7.5 (neural plate stage) embryos reveal a wider distribution of Flk-1-expressing cells with appearance of numerous capillary-like structures. The band of CD41^dim-expressing cells has expanded but remains centered at the proximal end of the yolk sac. Arrows indicate contaminating maternal red blood cells expressing high levels of the erythroid marker, TER119 (small bright red cells also present in D and Fig. 4B). (K) Reconfiguration of the neural plate stage embryo into a planar fashion by dividing the embryo from the most proximal to distal, results in a clear view of the continuity of the CD41^dim-expressing cells for the full circumference of the yolk sac. (L) Flk-1 expression alone in the proximal region of the yolk sac. Some of the cells are Flk-1^bright and many are Flk-1^dim and these cells correspond to the same cells (M) that are CD41^dim; the band of CD41-expressing cells is 5-10 cells in width and 1 cell thick. Flk-1 highly expressing cells overlie the band of Flk-1^dimCD41^dim cells but an orthogonal view (N) reveals that the Flk-1^bright cells (asterisks) are interposed between the band of CD41^dim cells and the outer endoderm layer (endo). Scale bar: 100 µm (A-F,J-M) and 33 µm (G-I,N).

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