The Complex Simplicity of the Brittle Star Nervous System

Brittle stars (Ophiuroidea, Echinodermata) have been increasingly used in studies of animal behavior, locomotion, regeneration, physiology, and bioluminescence. The success of these studies directly depends on good working knowledge of ophiuroid nervous system. Here, we describe the arm nervous system at different levels of organization: microanatomy of the radial nerve cord and peripheral nerves, neural ultrastructure, and localization of different cell types using specific antibody markers. We standardize the nomenclature of nerves and ganglia and provide an anatomically accurate digital 3D model of the arm nervous system as a reference for future studies. Our results helped identify several general features characteristic to the adult echinoderm nervous system, including the extensive anatomical interconnections between the ectoneural and hyponeural components and neuroepithelial organization of the central nervous system with its supporting scaffold formed by radial glial cells. In addition, we provide further support to the notion that the echinoderm radial glia is a complex and diverse cell population. We also tested the suitability of a range of specific cell-type markers for studies of the brittle star nervous system and established that the radial glial cells are reliably labeled by the ERG1 antibodies, whereas the best neuronal markers are acetylated tubulin, ELAV and synaptotagmin B. The transcription factor Brn1/2/4, a marker of neuronal progenitors, is expressed not only in neurons, but also in a subpopulation of radial glia. For the first time, we describe putative ophiuroid proprioceptors associated with the hyponeural part of the central nervous system.