Fig. 4

Expression of Acr-Pax2/5/8 in early larvae of the polyplacophoran Acanthochitona crinita. Dorsal (d)–ventral (v), apical (a)-abapical (aa), and left (l)-right (r) axes indicate the orientation. Blastopores/mouth labeled by asterisks and the location of the apical organ are encircled. a-b Consecutive optical sections through a gastrula showing Acr-Pax2/5/8-expressing cells (arrowheads; 3 h post fertilization (hpf)). c Confocal reflection scan highlighting same Acr-Pax2/5/8-expressing cells (arrowheads) in apical ectoderm as shown in a-b. d Three serotonin-like immunoreactive neurons of the apical organ in a 12 hpf early trochophore. e-h Optical sections from dorsal to ventral through an early trochophore (12 hpf) showing Acr-Pax2/5/8-expressing domains 1–4 (termed “group 1-4” in the following) in the episphere and single Acr-Pax2/5/8-expressing cells in the hyposphere (arrowheads in (f-g)). e Upper inset: Several Acr-Pax2/5/8-expressing cells (1) are located in the ectoderm of the dorsal episphere adjacent to the prototroch (pt). Lower inset: This very early trochophore (10 hpf) exhibits faint FMRFamide-like immunoreactive signal (red staining) in the region of the apical organ and in two groups of four FMRFamide-like immunoreactive ectodermal cells each in the episphere close to the prototroch. f Two other bilateral groups of Acr-Pax2/5/8-expressing cells (2) are located anterior to group 1 and additional Acr-Pax2/5/8-expressing cells lie in the region of the nascent shell fields in the hyposphere (arrowhead). g Two Acr-Pax2/5/8-expressing ectodermal cells (3) are located bilaterally in the episphere adjacent to the trochoblasts. h A group of 5–10 ectodermal Acr-Pax2/5/8-expressing cells (4) lies in the central ventral episphere. i-k Same specimen as shown in e-h from the left side (i) to the mid-sagittal plane (k). l Apical view of same specimen as shown in e-k with Acr-Pax2/5/8 expression domains 1, 2, and 4. Abbreviations: bc, blastocoel; f, foot; pt, prototroch. Scale bars: 20 μm