Nerve Ring of the Hypostome in Hydra: Is It an Origin of the Central Nervous System of Bilaterian Animals?

Abstract

A hypothesis, ‘the nerve ring in hydra shares a common origin with the central nervous system in bilaterian animals’, is discussed in this review. The nerve ring of hydra is a ring of neurons whose neurites make a bundle running circumferentially around the hypostome just above the tentacle zone. This nervous structure has unique features in the hydra nervous system. It shows a tight association of neurons in contrast to the diffuse nerve net seen in other regions. It shows static developmental characters in contrast to the dynamic features of hydra nerve net present in other regions. Moreover, its structure and location are similar to the central nervous system (CNS) of other animals without a complex CNS such as nematodes and starfishes. Functions of the hydra nerve ring are also studied to test the hypothesis. The identified function is a crumpling of the tentacles, corresponding to the function of the inner nerve ring of hydrozoan jellyfish. The jellyfish nerve ring is considered to be a primitive central nervous system of radiates. Considering all the information available, the hypothesis is highly possible.

Coordinated modulation of cellular signaling through ligand-gated ion channels in Hydra vulgaris (Cnidaria, Hydrozoa)

“…Current knowledge indicates that in Hydra species the nerve net is not diffused homogenously throughout the body, but shows a far greater structural and functional complexity (Koizumi, 2007). An immunocytochemical study performed with anti-RFamide antibodies (Koizumi et al, 1992) demonstrated the occurrence of a nerve ring at the base of the hypostome in Hydra oligactis, formed by neurites of ganglion cells running circumferentially in a bundle, providing the first anatomical evidence of the existence of a ring structure previously inferred by electrophysiological studies (Kass-Simon, 1972).…”

“…An immunocytochemical study performed with anti-RFamide antibodies (Koizumi et al, 1992) demonstrated the occurrence of a nerve ring at the base of the hypostome in Hydra oligactis, formed by neurites of ganglion cells running circumferentially in a bundle, providing the first anatomical evidence of the existence of a ring structure previously inferred by electrophysiological studies (Kass-Simon, 1972). Labeling with different monoclonal antibodies or antibodies raised against different Hydra neuropeptides identified distinct subsets of neurons with specific regional distributions, dependent on axial position (Koizumi andBode, 1986, 1991;Koizumi et al, 2004). A remarkable feature of the net is, in fact, its plasticity: neurons are continually migrating to be eliminated at the apical and basal ends of the polyp together with the adjacent epithelia (Dunne et al, 1985;Bode et al, 1986), thus implying that extensive synapse remodeling must occur in physiological conditions.…”

Revisiting the age, evolutionary history and species level diversity of the genus Hydra (Cnidaria: Hydrozoa)

“…As a consequence, the pace and timeframe of crucial evolutionary processes and novelties, like the emergence of numerous orphan genes specific for Hydra or some of its species (Khalturin et al, 2008(Khalturin et al, , 2009, cannot be assessed. Thus, as Hydra species differ in morphology, development, physiology, and ecology (Campbell, 1983;Hemmrich et al, 2007;Koizumi, 2007), the lack of a solid species level taxonomy hampers research on species specific differences in gene expression and development (Khalturin et al, 2008;Thomsen and Bosch, 2006).…”

Fast Neurotransmission Related Genes Are Expressed in Non Nervous Endoderm in the Sea Anemone Nematostella vectensis

“…[9]. In addition, certain cnidarians have centralized nerve structures, the nerve rings, which is present in anthozoans [10], [11] and medusozoans [11]–[14] including hydrozoans [15].…”