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Fig. 3. Atr2 expression is elevated in the developing notochord, apical ectodermal ridge and neurons. A time course of Atr2 expression, from E8.0 through E11.5, is illustrated using the PT026 Atr2-ßgal fusion protein. (A) At E8.0, Atr2 is expressed throughout the embryo and is elevated in the anterior midline (region between arrows; front view, anterior up; this embryo has been flattened). (B) At E8.25, Atr2 shows a more pronounced elevation in the anterior midline (arrows). (b) A transverse section through the anterior of the embryo in B shows elevated expression in midline cells (red arrow). (b') A more posterior section from the same embryo shows no elevated expression in the posterior midline (red arrow). (C) At E8.5, Atr2 is elevated throughout the anteroposterior extent of the notochord and is downregulated in the heart. (c,c') Transverse sections confirm elevated expression in anterior and posterior regions of the notochord (red arrows). (D) At E8.75, expression is increased in the ventral brain. (d) A section through the hindbrain reveals continued expression in the notochord (red arrow), and the beginning of elevation in the ventral brain (black arrow). (d') A more posterior section shows uniform expression in the spinal cord, and upregulation in the notochord (red arrow). (E) At E9.5, additional sites of elevation besides the notochord (black arrow) appear, including the apical ectodermal ridge, the isthmus and the ventral diencephalon (red arrows). (F) At E10.5, sites of elevation include the notochord (black arrow), the AER, and spinal and brain neurons (red arrows). (G) A wild-type embryo at E9.75, showing that the pattern of Atr2 transcripts detected with a 5' RNA probe is similar to the fusion protein at E10.5. (H) A wild-type forelimb bud at E9.5 shows concentrated expression of Atr2 mRNA in the AER (left). A section through the forelimb bud of an E9.5 Atr2^PT026/+ embryo shows ßgal expression in the AER (right). (I) Formation of the AER occurs normally in Atr2^PT026/+ embryos at E9.0. (J) The AER is defective in Atr2^PT026/Atr2^om embryos at E9.0. (K) A transverse section through the caudal hindbrain at E9.5 shows elevated expression in ventrolateral neurons (arrow). (L) A horizontal section through the optic region at E11.5 shows expression in telencephalic and diencephalic neurons. (M) A transverse section at sacral level at E11.5 shows high levels of expression in spinal cord neurons.

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