Fig. 6

Early neural development during elongation and early mitraria. CLSM images of F-actin and FMRFamide⁺ elements and Differential Interference Contrast (DIC) images showing expression of elav1 and syt1 from a 13 hpf to x 27 hpf. Insets in (c, g, i, k, o–r, u) are close ups of the apical organ in the same view as the larger image. a, b, g, h There are no FMRFamide⁺ cells except for ten refringent globules (yellow arrowheads) in the apical organ and the prototroch either at 13 or 18 hpf. c, d A V-shaped apical organ composed of three elav1⁺ and syt1⁺ cells and seven prototrochal cells (black arrowheads) expressing elav1 are the first neurons to appear. i, j By early mitraria larva (18 hpf) elav1⁺ cells are positioned anterior and lateral to the gut reaching the ventral side of the larva, and k, l syt⁺ cells are also now fully expressed in seven prototrochal cells (black arrowheads). m, n The first FMRFamide⁺ cells are present by 22 hpf in the apical organ and by 24 hpf axons extend from the apical organ to an FMRFamide⁺ prototrochal ring connecting seven FMRFamide⁺ cells in the prototroch (s, t) (see Fig. 7f–g). o–r elav1⁺ and syt1⁺ expression remains very similar from 18 hpf up to 24 hpf. u–x By 27 hpf, more cells express syt1 in the apical organ, but elav1 is now mostly restricted to neurons on the ventral side where neurons of the juvenile will start forming. an: anus; ao: apical organ; cs: chaetal sac; mo: mouth; mt: metatroch; nt: neurotroch; pt: prototroch. Scale bar is 50 µm