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Embryonic neurogenesis in echinoderms

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The phylogenetic position of echinoderms is well suited to revealing shared features of deuterostomes that distinguish them from other bilaterians. Although echinoderm neurobiology remains understudied, genomic resources, molecular methods, and systems approaches have enabled progress in understanding mechanisms of embryonic neurogenesis. Even though the morphology of echinoderm larvae is diverse, larval nervous systems, which arise during gastrulation, have numerous similarities in their organization. Diverse neural subtypes and specialized sensory neurons have been identified and details of neuroanatomy using neuron‐specific labels provide hypotheses for neural function. The early patterning of ectoderm and specification of axes has been well studied in several species and underlying gene regulatory networks have been established. The cells giving rise to central and peripheral neural components have been identified in urchins and sea stars. Neurogenesis includes typical metazoan features of asymmetric division of neural progenitors and in some cases limited proliferation of neural precursors. Delta/Notch signaling has been identified as having critical roles in regulating neural patterning and differentiation. Several transcription factors functioning in pro‐neural phases of specification, neural differentiation, and sub‐type specification have been identified and structural or functional components of neurons are used as differentiation markers. Several methods for altering expression in embryos have revealed aspects of a regulatory hierarchy of transcription factors in neurogenesis. Interfacing neurogenic gene regulatory networks to the networks regulating ectodermal domains and identifying the spatial and temporal inputs that pattern the larval nervous system is a major challenge that will contribute substantially to our understanding of the evolution of metazoan nervous systems. This article is categorized under: Comparative Development and Evolution > Model Systems Comparative Development and Evolution > Body Plan Evolution Early Embryonic Development > Gastrulation and Neurulation
There are five classes of Echinoderm (Cameron, Garey, & Swalla, ; Smith et al., ), which all feature indirect development through a larval stage. Class Echinoidea (sea urchins) and Holothuroidea (sea cucumbers) are sister taxa, and along with Ophiuroidea (brittle stars) and Asteriodea (sea stars) comprise the grouping known as Eleutherozoa. The remaining class, Crinoidea (feather stars) are the clear outgroup. Although the disparity of the adults of these classes is well known, their larval forms are also morphologically very different. Larvae can be broadly characterized as plutei (i.e., the echinopluteus in the Echinoidea, and ophiopluteus in the Ophiuroidea) or dipleurula‐like (i.e., auricularia in Holothuroidea and Crinoidea, or bipinnaria in Asteriodea). One of the most notable sources of morphological variation between these larval types stem from the presence, or not, of a biomineralized skeleton. Hemichordates when indirectly developing have a tornaria larva, which has many similarities to an echinoderm larva, and is one of the main features used to unite these taxa as Ambulacraria. (a) Deuterostome phylogeny. (b) Echinoderm classes with sketches of representative larval types
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Diagrammatic summary of the developmental sequence of neural specification, asymmetric divisions, and differentiation in echinoderms. Domains with distinct regulatory states are established for the animal pole domain (top row) and other domains of ectoderm (bottom row). Neural progenitors are specified (red nucleus) and neural progenitors initiate an asymmetric division. In the animal pole domain, asymmetric divisions may produce progenitors that also divide, or division give rise to a neuron and an apoptotic cell. In other regions of ectoderm, divisions appear to give rise to only a single neuron. Throughout the specification and differentiation a gene regulatory network controls the expression of a series of regulatory genes that control neurogenesis (single box). Cellular signaling (double box) mediates and regulates specification of ectodermal domains and restricts neurogenesis. Diverse neuronal subtypes, with stereotypic positions and patterns result
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Sites of neurogenesis mapped onto domains of gastrula stage ectoderm. A field of ectoderm surrounding the mouth (ventral ectoderm) circumscribed by a ciliary band is a shared feature of the two larval forms. The concentric domains of FoxQ2 and Six3 expression define the animal pole domain and the ectoderm that has activated BMP signaling (dorsal ectoderm) is identified. Neural progenitors are identified by expression of SoxC or Brn1/2/4. Serotonergic neural progenitors arise in the animal pole domain and peripheral neural progenitors arise outside this domain. In sea urchins, neurons arise in ectoderm that has not been influenced by BMP signaling, whereas in asteroid embryos, neurons appear to arise throughout the ectoderm
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Simplified diagrams of the organization of larval nervous systems in bipinnaria (a, b) and a pluteus (c). The larval nervous systems have a small central ganglionic structure termed the apical organ, which is identified by serotonergic neurons (green), although neurons lacking serotonin are also present. There are extensive peripheral neurons and axonal tracts associated with the ciliary bands and extensive innervation of the esophagus and gut. In late stage sea star larvae, the attachment organ is extensively innervated. Key differences in neural organization relate to the neurites projecting from ciliary band neurons—in asteroids they extend underneath the oral epidermis toward the mouth, whereas in echinoids they extend posteriorly, beneath dorsal epidermis
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