Fig. 5

tbx6 expression is maintained in undifferentiated derivatives of the ventrolateral mesendodermal precursors. (A and A′) Lateral (A, dorsal is right) and dorsal (A′) views of tbx6 expression in late gastrulae (95% epiboly, 9.5 h). During gastrulation, tbx6 continues to be expressed at the margin of the blastoderm and is maintained in recently involuted cells as they move rostrally and converge dorsally toward the developing chordamesoderm (*). (B) A transverse section through the axis (at the level indicated by the bar in A′) of a late gastrula (95% epiboly) embryo indicates that tbx6 is specifically expressed in involuting hypoblast cells. The presumptive notochord (*, dorsal) is to the right. (C) A dorsal view of the midaxial region of a two-somite (11 h) embryo in which tbx6-expressing cells (blue staining) are identified by in situ hybridization and No Tail protein-expressing cells (brown) are identified by immunohistochemistry. The domain of tbx6 expression lies immediately adjacent to the axial mesoderm identified by its expression of the No Tail protein. (D) A ventral view of a 15-h embryo shows that tbx6 is expressed in the region of the early developing tail as an arc of cells whose apex is located at the posterior tip of the tail and whose wings extend anteriorly, bordering the axial mesoderm. A lateral view of a similar embryo (Fig. 6G) shows that tbx6 expression is confined to the ventrolateral aspects of the developing tail at this stage. (E and F) Lateral views of two approximately 24-h embryos show the changing expression of tbx6 in the developing tail and in the blood precursors. The embryo in D is slightly younger than the embryo in E. As tail development proceeds, tbx6 expression becomes progressively restricted to smaller domains at the posterior tip of the tail. In the 24-h embryo, tbx6 is also expressed in blood precursor cells that originate in the intermediate cell mass (arrowhead) and that move rostrally along the ventral midline (arrow) prior to entering the circulation. (G) A dorsal view (anterior to the top) of the midtrunk region of a 26-h embryo shows that blood precursor cells maintain expression of tbx6 as they move laterally over the yolk cell to enter the circulation. Expression of tbx6 among circulating blood cells has not been detected.