Abstract
Early neurogenesis in arthropods has been in the focus of numerous studies, its cellular basis, spatio-temporal dynamics and underlying genetic network being by now comparably well characterized for representatives of chelicerates, myriapods, hexapods and crustaceans. By contrast, neurogenesis during late embryonic and/or post-embryonic development has received less attention, especially in myriapods and chelicerates. Here, we apply (i) immunolabeling, (ii) histology and (iii) scanning electron microscopy to study post-embryonic ventral nerve cord development in Pseudopallene sp., a representative of the sea spiders (Pycnogonida), the presumable sister group of the remaining chelicerates. During early post-embryonic development, large neural stem cells give rise to additional ganglion cell material in segmentally paired invaginations in the ventral ectoderm. These ectodermal cell regions – traditionally designated as ‘ventral organs’ – detach from the surface into the interior and persist as apical cell clusters on the ventral ganglion side. Each cluster is a post-embryonic neurogenic niche that features a tiny central cavity and initially still houses larger neural stem cells. The cluster stays connected to the underlying ganglionic somata cortex via an anterior and a posterior cell stream. Cell proliferation remains restricted to the cluster and streams, and migration of newly produced cells along the streams seems to account for increasing ganglion cell numbers in the cortex. The pycnogonid cluster-stream-systems show striking similarities to the life-long neurogenic system of decapod crustaceans, and due to their close vicinity to glomerulus-like neuropils, we consider their possible involvement in post-embryonic (perhaps even adult) replenishment of olfactory neurons – as in decapods. An instance of a potentially similar post-embryonic/adult neurogenic system in the arthropod outgroup Onychophora is discussed. Additionally, we document two transient posterior ganglia in the ventral nerve cord of Pseudopallene sp. and evaluate this finding in light of the often discussed reduction of a segmented ‘opisthosoma’ during pycnogonid evolution.
Highlights
In the most diverse animal lineage, the Arthropoda, our understanding of the early neurogenic processes that lie at the base of central nervous system formation is founded on numerous studies over several decades
In line with the anamorphic character of post-embryonic development, the ventral nerve cord (VNC) of the hatching post-embryonic stage (PS 1) is still incomplete and new segmental ganglion anlagen are added with ongoing development (Fig. 1B)
A marked developmental gradient is encountered between the anlagen of sub-esophageal ganglion ( = palpal and ovigeral neuromeres) and walking leg ganglia 1 and 2, which have been already formed during embryogenesis [53], and those of walking leg ganglia 3 and 4, which develop mainly during post-embryonic development (Fig. 1B)
Summary
In the most diverse animal lineage, the Arthropoda, our understanding of the early neurogenic processes that lie at the base of central nervous system formation is founded on numerous studies over several decades. Present-day studies on late nervous system development in myriapods and chelicerates with a focus at the cellular level are highly desirable and represent a prerequisite to enable arthropodwide comparison of neurogenic mechanisms across the entire development [53]. Such investigations promise to provide additional arguments in the discussion on the phylogenetic relationships of arthropods and eventually help to unravel the evolution of neurogenic processes within this diverse animal group
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