Serotonin-immunoreactive neural system and contractile system in the hydroid Cladonema (Cnidaria, Hydrozoa)
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ORIGINAL PAPER
Serotonin-immunoreactive neural system and contractile system in the hydroid Cladonema (Cnidaria, Hydrozoa) T. D. Mayorova • I. A. Kosevich
Received: 25 July 2012 / Accepted: 5 March 2013 Ó Springer-Verlag Berlin Heidelberg 2013
Abstract Serotonin is a widespread neurotransmitter which is present in almost all animal phyla including lower metazoans such as Cnidaria. Serotonin detected in the polyps of several cnidarian species participates in the functioning of a neural system. It was suggested that serotonin coordinates polyp behavior. For example, serotonin may be involved in muscle contraction and/or cnidocyte discharge. However, the role of serotonin in cnidarians is not revealed completely yet. The aim of this study was to investigate the neural system of Cladonema radiatum polyps. We detected the net of serotonin-positive processes within the whole hydranth body using antiserotonin antibodies. The hypostome and tentacles had denser neural net in comparison with the gastric region. Electron microscopy revealed muscle processes throughout the hydranth body. Neural processes with specific vesicles and neurotubules in their cytoplasm were also shown at an ultrastructural level. This work demonstrates the structure of serotonin-positive neural system and smooth muscle layer in C. radiatum hydranths. Keywords Hydrozoa Smooth muscles Neural system Serotonin Cladonema
Introduction Cnidaria is the most ancient and primitive group of animals possessing neural system (Grimmelikhuijzen 1983; Falugi et al. 1994). Despite the fact that cnidarian neural T. D. Mayorova (&) I. A. Kosevich Biological Faculty, Lomonosov Moscow State University, Leninskie Gory 1/12, Moscow, Russia e-mail: [email protected]
organization is quite simple, it is not studied well yet. Moreover, their neural system may include several subpopulations of different neural cells as it was shown in Hydra (Sakaguchi et al. 1996), so the organization of such neural systems in fact turns out to be complex. The main part of the neural system in cnidarian polyps consists of ectoderm neural cells with neurite-like processes. The processes of neural cells form a net embracing all hydranth body. The net usually tends to be denser in the oral and pedal regions of a hydranth as well as in its tentacles (Grimmelikhuijzen 1983; Anderson et al. 2004; KassSimon and Pierobon 2007). A set of different neuroactive substances characterize the subpopulations of neural cells and their processes. At an ultrastructural level, neuroactive substances are found in specific vesicles in the cytoplasm of neural cells and their processes. Electron-translucent content of vesicles is mostly associated with the neurotransmitters, whereas vesicles with electron-dense content may carry neuropeptides (Kass-Simon and Pierobon 2007). The neural processes contact each other and the other cells forming synapse-like structures, where these vesicles are concentrated (Westfall 1973; Westfall and Grimmelikhuijzen 1993; Golz 1994). It is presumed that cnidarian neura
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