Anteroposterior progression of the islet1 RNA expression pattern in the ventral spinal cord. Panels show islet1 RNA distribution in the ventral spinal cord of a 19-somite-stage embryo at different anteroposterior positions. Anterior is to the left and dorsal is up in all panels. Approximate somite boundaries are indicated by dashed lines. Numbers at the bottom of each panel indicate somite level. Triangles mark boundary between spinal cord and notochord. Scale bar equals 20 mm. (A) Somite level 19 and unsegmented region. High level expression (arrow) at somite 19 boundary. islet1 expression in a nearly contiguous line of cells in unsegmented region (bracket). (B) Somite level 15-16. High level expression in single cells (black arrows) near somite boundaries. Low level expression in midsegment cells (outlined arrows). Dorsal labeling in putative Rohon-Beard sensory neurons. Labeling in segment 16 just dorsal of primary motoneurons represents transient expression in unidentified cells. (C) Somite level 13-14. High level of islet1 expression in single cells at 13 and 15 somite boundaries (black arrows). Low level expression (white arrow) near somite 14 boundary as well as in midsegment cells (outlined arrows). (D) Somite level 10-11. islet1 is expressed in 1 cell at each somite boundary. (E) Somite level 8-9. islet1 is expressed in 2 cells near each somite boundary. (F) Somite level 5-6. islet1 is expressed in approximately 4 cells clustered near the somite boundaries.
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