Abstract
SummaryThe ontogenetic origin of blastocoelar glutamate decarboxylase (GAD)-expressing cells (GADCs) in larvae of the sea urchin Hemicentrotus pulcherrimus was elucidated. Whole-mount in situ hybridisation (WISH) detected transcription of the gene that encodes GAD in H. pulcherrimus (Hp-gad) in unfertilised eggs and all blastomeres in morulae. However, at and after the swimming blastula stage, the transcript accumulation was particularly prominent in clumps of ectodermal cells throughout the embryonic surface. During the gastrula stage, the transcripts also accumulated in the endomesoderm and certain blastocoelar cells. Consistent with the increasing number of Hp-gad transcribing cells, immunoblot analysis indicated that the relative abundance of Hp-Gad increased considerably from the early gastrula stage until the prism stage. The expression pattern of GADCs determined by immunohistochemistry was identical to the pattern of Hp-gad transcript accumulation determined using WISH. In early gastrulae, GADCs formed blastocoelar cell aggregates around the blastopore with primary mesenchyme cells. The increase in the number of blastocoelar GADCs was inversely proportional to the number of ectodermal GADCs ranging from a few percent of total GADCs in early gastrulae to 80% in late prism larvae; this depended on ingression of ectodermal GADCs into the blastocoel. Some of the blastocoelar GADCs were fluorescein-positive in the larvae that developed from the 16-cell stage chimeric embryos; these comprised fluorescein-labeled mesomeres and unlabelled macromeres and micromeres. Our finding indicates that some of the blastocoelar GADCs are derived from the mesomeres and thus they are the new group of mesenchyme cells, the tertiary mesenchyme cells.
Highlights
The adult nervous system (ANS) of sea urchins is unique among the deuterostomes in being pentaradial; it comprises five radial nerves that are connected with axons of the circumoral nerve ring, which itself contains few nerve cell bodies
The increase in the number of blastocoelar GAD-expressing cells (GADCs) was inversely proportional to the number of ectodermal GADCs ranging from a few percent of total GADCs in early gastrulae to 80% in late prism larvae; this depended on ingression of ectodermal GADCs into the blastocoel
Some of the blastocoelar GADCs were fluorescein-positive in the larvae that developed from the 16-cell stage chimeric embryos; these comprised fluorescein-labeled mesomeres and unlabelled macromeres and micromeres
Summary
The adult nervous system (ANS) of sea urchins is unique among the deuterostomes in being pentaradial; it comprises five radial nerves that are connected with axons of the circumoral nerve ring, which itself contains few nerve cell bodies. The serotonergic NS is present at a trace level in the adult rudiment of Holopneustes purpurascens for a short period during the early stage of metamorphosis, but is absent in adults (Katow et al, 2009) This is seen in Hemicentrotus pulcherrimus (Katow et al, 2010). In addition to synaptotagmin-expressing NS, c-amino butyric acid (GABA)-ergic NS is detected distinctively in the primary podia and the epineural fold of the adult rudiment in H. pulcherrimus (H.K., unpublished). This suggests the presence of ontogenetic continuity from the LNS to the ANS in sea urchin, which enables analysis of the ontogeny of GABAergic NS from embryo to adult
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