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J Morphol
2018 May 01;2795:589-597. doi: 10.1002/jmor.20794.
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SALMFamide2 and serotonin immunoreactivity in the nervous system of some acoels (Xenacoelomorpha).
Dittmann IL
,
Zauchner T
,
Nevard LM
,
Telford MJ
,
Egger B
.
Abstract
Acoel worms are simple, often microscopic animals with direct development, a multiciliated epidermis, a statocyst, and a digestive parenchyma instead of a gut epithelium. Morphological characters of acoels have been notoriously difficult to interpret due to their relative scarcity. The nervous system is one of the most accessible and widely used comparative features in acoels, which have a so-called commissural brain without capsule and several major longitudinal neurite bundles. Here, we use the selective binding properties of a neuropeptide antibody raised in echinoderms (SALMFamide2, or S2), and a commercial antibody against serotonin (5-HT) to provide additional characters of the acoel nervous system. We have prepared whole-mount immunofluorescent stainings of three acoel species: Symsagittifera psammophila (Convolutidae), Aphanostoma pisae, and the model acoel Isodiametra pulchra (both Isodiametridae). The commissural brain of all three acoels is delimited anteriorly by the ventral anterior commissure, and posteriorly by the dorsal posterior commissure. The dorsal anterior commissure is situated between the ventral anterior commissure and the dorsal posterior commissure, while the statocyst lies between dorsal anterior and dorsal posterior commissure. S2 and serotonin do not co-localise, and they follow similar patterns to each other within an animal. In particular, S2, but not 5-HT, stains a prominent commissure posterior to the main (dorsal) posterior commissure. We have for the first time observed a closed posterior loop of the main neurite bundles in S. psammophila for both the amidergic and the serotonergic nervous system. In I. pulchra, the lateral neurite bundles also form a posterior loop in our serotonergic nervous system stainings.
Figure 1. Live images of the three acoel species used in this study: (a) Symsagittifera psammophila, (b) Aphanostoma pisae, (c) Isodiametra pulchra. Note that in (a) a juvenile is shown, while stainings have been performed with adults in all cases
Figure 2.
Symsagittifera psammophila stained with S2 antibody (a–e, magenta) and 5‐HT antibody (e, yellow). (a) Projection of whole‐mount. (b) Projection of the anterior body‐part. (c) Projection of the posterior body‐part. (d) Confocal projection with temporal colour coding; colour‐code: pink (dorsal) to orange/red (ventral). (e) Overlay projection of the anterior body‐part of a double‐labelled animal. (a–c, e) Same individual. Abbreviations: 1 dorsal neurite bundle; 2 lateral neurite bundle; 3 ventral neurite bundle; ac dorsal anterior commissure; cc collum commissure; fr frontal ring; m mouth; pc dorsal posterior commissure; plo posterior loop. Asterisk marks position of statocyst
Figure 3. Confocal projection of Isodiametra pulchra stained with S2 antibody (a,b,e, magenta) and 5‐HT (c–e, yellow). (a) Confocal projections of a whole‐mount and (b) of the anterior body part with temportal colour coding, colour‐code: grey (dorsal) to red (ventral). Green arrowhead points to delicate anterior process of neurites. (c) Confocal projections of a whole‐mount and (d) of the anterior body‐part. (e) Overlay projection of the anterior body part of a double‐labelled animal. Green arrowheads point to delicate anterior process of neurites. (f‐g) Schematic drawings of the nervous system stained with S2 (f) and 5‐HT (g) antibodies. (h) Detailed view of bipolar and multipolar (red and white arrowheads, respectively) neurons of the colour‐coded (blue dorsal, red ventral) S2amidergic nervous system. (a,b), (c,d), (e,h) same individuals. Abbreviations: 1 dorsal neurite bundle; 2 lateral neurite bundle; 3 ventral neurite bundle; 4 medio‐ventral neurite bundle; ac: dorsal anterior commissure; al anterior lobe; cop male copulatory organ; fr frontal ring; m mouth; pc dorsal posterior commissure; pl posterior lobe. Asterisks mark position of statocyst
Figure 4. Confocal projection of a whole‐mount Aphanostoma pisae stained with S2 antibody with temporal colour coding. Colour‐code: pink (ventral) to red (dorsal), arrowheads point to putative neuronal cell bodies of bipolar (white) or multipolar (red) neurons. Abbreviations: 1 dorsal neurite bundle; 2 lateral neurite bundle; 3 ventral neurite bundle; ac dorsal anterior commissure; cop male copulatory organ; fr frontal ring; m mouth; pc dorsal posterior commissure. Asterisk marks the position of the statocyst
Figure 5. Schematic drawings of the nervous systems stained with S2 (magenta) and 5‐HT (yellow) antibodies. (a) Symsagittifera psammophila. (b) Isodiametra pulchra. (c) Aphanostoma pisae 5‐HT (yellow) after Zauchner et al. (2015). Abbreviations: 1 dorsal neurite bundle; 2 lateral neurite bundle; 3 ventral neurite bundle; 4 medio‐ventral neurite bundle; ac dorsal anterior commissure; al anterior lobe; cc collum commissure; fr frontal ring; pc dorsal posterior commissure; pl posterior lobe. Asterisks mark position of statocyst
Figure 6.
Symsagittifera psammophila stained with 5‐HT antibody. Projections of a whole‐mount (a), of the anterior body‐part (b) and of the posterior body‐part (c). (a–c) Same individual. Abbreviations: 1 dorsal neurite bundle; 2 lateral neurite bundle; 3 ventral neurite bundle; ac dorsal anterior commissure; fr frontal ring; pc dorsal posterior commissure; plo posterior loop. Asterisks mark position of statocyst
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