Primary motoneurons develop in the absence of floor plate and notochord. Double RNA in situ hybridizations at 18 hr with antisense probes against islet2, a gene expressed in two of the primary motoneurons, CaP and VaP, in blue and col2a1, a gene expressed in floor plate and notochord, in red. Lateral views (A, C, E, G; dorsal to the top and anterior to the left), transverse sections (B, D, F, H). In wild types the notochord (n) and floor plate (f) are present and continuous as revealed by col2a1 expression (A, B). One or two islet2- expressing primary motoneurons are present in the ventral region of each spinal cord segment (black arrowhead). islet2-expressing cells in the dorsal spinal cord are Rohon–Beard sensory neurons. In cyc mutants (C, D) notochord is present but floor plate is absent. The pattern of islet2 expression in the ventral spinal cord is comparable to that seen in wild types, except that the cell bodies are often collapsed at the midline (black arrowhead in D). In flh mutants (E, F) notochord is missing and floor plate is present as discontinuous islands of cells (white arrowheads). One or two islet2-expressing primary motoneurons are present in each segment, most are associated with floor plate, but some are present in segments containing no floor plate (black arrowhead in E). In cyc;flh mutants (G, H) both notochord and floor plate are absent. There are, however, islet2-expressing primary motoneurons present in the ventral spinal cord (black arrowheads). Scale bar, 50 μm.
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