Fig. 5. Fine mapping of the developmental fate of cells within and adjacent to the EGO and MGO. (A) Regions I-IV in the posterior midline epiblast of the ES embryo, where cells are tested for contribution to axial mesoderm. Regions I to IV are located anterior to the recently formed primitive streak, with region III and IV located in the domain of the EGO delineated by Hnf3ß and Gsc^lacZ activity (Fig. 1). (B) The contribution to axial mesoderm by cells in Regions I to IV, expressed as the percentage of embryos showing graft-derived cells in this tissue. (C) Regions V-VII in the posterior midline epiblast of the MS embryo, where cells are tested for contribution to axial mesoderm. Regions V and VI are located in the anterior end of the elongating primitive streak within the domain of the MGO, which is defined by the expression of Hnf3ß, Chrd and Gsc^lacZ. Region VII is located in the anterior segment of the primitive streak. (D) The contribution to axial mesoderm by cells in Regions V to VII, expressed as the percentage of embryos showing graft-derived cells in this tissue. (E) Cells that were transplanted to Region III of the early-streak embryo (t=0) colonized the axial tissues of the host embryo after 36 hours of culture (t=36). Histological sections at the level of the forebrain (fb),hindbrain (hb) and trunk (tr) of an embryo showing graft-derived cells in the prechordal mesoderm and the notochord (inset). (F) Cells that were transplanted to Region VI of the MS embryo (t=0) colonized the axial tissues of the host embryo after 36 hours of culture (t=36). Histological sections at the level of the forebrain (fb), hindbrain (hb) and trunk (tr) of an embryo showing graft-derived cells in the prechordal mesoderm (inset a), the notochord of the midbrain (inset b) and the trunk. (G) Cells transplanted to epiblast lateral to Region III of the ES embryo (t=0) colonized the cranial mesenchyme and the heart of the host embryo after 48 hours of culture. (H) Cells transplanted to Region VII of the MS embryo (t=0) colonized the notochord in the trunk of the host embryo after 36 hours of culture (t=36). EGFP-expressing cells show green fluorescence and lacZ-expressing cells are stained blue by X-gal reagent. Scale bars: in E, 50 µm; in G (t=0) and H (t=0),100 µm; in F, 50 µm; in H (t=36), 50 µm.