Expression of CD34 mRNA in the developing mouse yolk sac and allantois. (A) 4-5–somite stage: many of the stained cells are elongated and surround unstained tissue, suggesting that they are endothelial cells lying around the periphery of the blood islands. (B) 12-somite stage: endothelial staining is weaker but the rounded haematopoietic cells retain CD34 expression. (C) Day 9, 20-somite stage: the only CD34 expression in the yolk sac is in the vitelline artery at the point that it emerges from the embryo. (D) Vitelline artery at the 20-somite stage at a higher magnification: the stained endothelial cells are discontinuous. (E) Day 10 embryo (around 30 somites), showing the points of emergence of the umbilical (u) and vitelline (v) vessels from the embryo. (F ) Day 10 umbilical vessels dissected away from the embryo, showing foci of CD34⁺ staining; the allantoic capillaries within the chorioallantoic placenta are strongly stained. (G and H) Day 10 vitelline artery dissected away from the embryo, showing foci of CD34⁺ cells at the proximal end (left) only. Scale bars = 100 μm