Figure 1

Expression of Xenopus Six1 appears during transition from Rohon-Beard cells to dorsal root ganglia. (A) Cladogram showing the succession of primary sensory neurons of chordates in the trunk. Intramedullary primary sensory neurons, called Rohon-Beard (RB) cells, dorsal cells, Retzius bipolar cells or DRiii cells (pink), are recognized in various species [5, 6] - including amphioxus [13]; lampreys: Lampetra planeri[14], Lampetra japonica[15]; fish: Lepisosteus osseus[1], Raja (Dipturus) batis[16], Scyliorhinus torazame[17], Danio rerio[18]; and amphibian: Ambystoma punctatum[3], Xenopus laevis[4], Rana pipiens[19], Rana catesbeiana[20], Ceratophrys ornata[21], Eleutherodactylus coqui[22] - but not in amniotic vertebrates [6, 7]. Craniates have extramedullary sensory neurons (blue) that are derived from neural crest cells and form dorsal root ganglia (DRG). (B-D) Distribution of Six1 mRNA and protein in the trunk region of Xenopus embryos (dorsal side: top of transverse sections). Developmental stages are indicated at top line. Arrowheads: Six1-negative RB cells; arrows: Six1-positive RB cells. (B, C) In situ hybridization of six1 (purple) is followed by immunostaining of Isl1/2 (orange), marking RB cells and DRG neurons. (B’, C’) Magnified views of the areas indicated by the rectangles in B and C. (D) Detection of Six1 (green) and Isl1/2 (magenta) by immunofluorescence in sections adjacent or alternate to those in B and C. Dashed lines demarcate the position of the spinal cord. sc, spinal cord; so, somite. Scale bars: 25 μm (B-D) and 10 μm ( B’, C’).