Fig. 6

Co-localized vasa-1, pl10, piwi, nanos-1, and nanos-2 expression and histology suggest presence of i-cells in the nectosomal growth zone. Anterior is up in all figures. a vasa-1 transcript. Transcription was longest detectable along the nectophore ridges (arrowhead). b piwi expression was observed within the protruding nectosomal bulge (arrowhead), young buds, and developing nectophores. c nanos-2 expression in the nectosomal horn and young developing buds. Signal on the nectosomal stem indicates sites of nematogenesis. d pl10 transcript present in the nectosomal horn, youngest buds (1–3), young developing nectophores, and within the protruding nectosomal bulge (arrowheads). pl10 expression within the horn and young buds appeared strongest in deeper layers. e Semi-thin longitudinal section of early nectophore buds and the horn, stained with toluidine blue. Cells with i-cell morphology could be identified in the protruding bulge of the nectosomal stem, the horn and young developing buds (1–2). f Semi-thin longitudinal section in the region of the nectosomal horn showing nematogenesis in the ectoderm of the nectosomal stem subtending the growth zone and interstitial cells in the ectoderm of a developing young nectophore. g Transmission electron micrograph showing interstitial cells in the interstices of the epithelial muscle cells within the ectoderm of a young nectophore. ec epithelial cell, ect ectoderm, end endoderm, gc gastric cavity, h horn of the growth zone, ic interstitial cell, m mesoglea, n nucleus, nb nematoblast, ne nectophore, nst nectosomal stem, pn pneumatophore