Figure 3

Histologic cross sections showing further details of Nar - ap expression. All sections are orthogonal to the anteroposterior (AP) axis; dorsal is to the top. (A-G) 8-chaetiger stage. (H-Q) 7-chaetiger stage. (B), (E), (F) Acetylated α-tubulin immunolabeling; cilia and neuronal cell processes are brown. (A), (B) Just posterior to the antennal ganglia, 10 μm anterior to the brain’s neuropil. Inset image in (B) is 10 μm anterior to, and a blowup of, the Lo region delimited with a dashed box. (C) Cephalic section at the level of the anterior eyes. (D) Dashed ellipses circumscribe areas expected to each contain a nuchal organ. (E) Same AP position as (C). The inset ciliary tuft is 5 μm anterior to, and a blowup of, the boxed area. (F) Same AP position as (D), showing nuchal organ locations in the posterior brain. The three parts of this panel - left, right, and inset ciliary tufts (a blowup of the boxed area) - are consecutive sections 5 μm apart (from anterior to posterior, respectively). (G) Pharyngeal section at the level of the jaws. (H-K) Ventral nerve cords. Double arrowheads point to longitudinal nerve tracts. (L) Posterior section through a sixth parapodium. (M-Q) Pre-chaetigerous segments near the posterior growth zone, arranged from youngest to more mature and showing ectodermal (arrowheads) and mesodermal (brackets) expression domains. adc anterodorsal cortex of the brain, ae anterior eye, ann antennal nerve, cp corpus pedunculus, dc dorsal cirrus, g gut, Lo Langdon’s organ, necl neuropodial chaetal lobe, nn nuchal nerve, np neuropil, plm? presumed parapodial levator muscle, pso palp sense organs, rm nuchal organ retractor muscle, sc nuchal organ sensory cells, sd nuchal organ sensory dendrites, sg/n stomatogastric ganglion/nerve, teLo terminal endings of sensory-cell peripheral processes from Langdon's organ, vc ventral cirrus, vnc ventral nerve cords, vnl ventral notopodial ligule, yo yolk.