Lophotrochozoan neuroanatomy: An analysis of the brain and nervous system of Lineus viridis(Nemertea) using different staining techniques

<p>Abstract</p> <p>Background</p> <p>The now thriving field of neurophylogeny that links the morphology of the nervous system to early evolutionary events relies heavily on detailed descriptions of the neuronal architecture of taxa under scrutiny. While recent accounts on the nervous system of a number of animal clades such as arthropods, annelids, and molluscs are abundant, in depth studies of the neuroanatomy of nemerteans are still wanting. In this study, we used different staining techniques and confocal laser scanning microscopy to reveal the architecture of the nervous system of <it>Lineus viridis </it>with high anatomical resolution.</p> <p>Results</p> <p>In <it>L. viridis</it>, the peripheral nervous system comprises four distinct but interconnected nerve plexus. The central nervous system consists of a pair of medullary cords and a brain. The brain surrounds the proboscis and is subdivided into four voluminous lobes and a ring of commissural tracts. The brain is well developed and contains thousands of neurons. It does not reveal compartmentalized neuropils found in other animal groups with elaborate cerebral ganglia.</p> <p>Conclusions</p> <p>The detailed analysis of the nemertean nervous system presented in this study does not support any hypothesis on the phylogenetic position of Nemertea within Lophotrochozoa. Neuroanatomical characters that are described here are either common in other lophotrochozoan taxa or are seemingly restricted to nemerteans. Since detailed descriptions of the nervous system of adults in other nemertean species have not been available so far, this study may serve as a basis for future studies that might add data to the unsettled question of the nemertean ground pattern and the position of this taxon within the phylogenetic tree.</p

Invertebrate neurophylogeny: suggested terms and definitions for a neuroanatomical glossary

<p>Abstract</p> <p>Background</p> <p>Invertebrate nervous systems are highly disparate between different taxa. This is reflected in the terminology used to describe them, which is very rich and often confusing. Even very general terms such as 'brain', 'nerve', and 'eye' have been used in various ways in the different animal groups, but no consensus on the exact meaning exists. This impedes our understanding of the architecture of the invertebrate nervous system in general and of evolutionary transformations of nervous system characters between different taxa.</p> <p>Results</p> <p>We provide a glossary of invertebrate neuroanatomical terms with a precise and consistent terminology, taxon-independent and free of homology assumptions. This terminology is intended to form a basis for new morphological descriptions. A total of 47 terms are defined. Each entry consists of a definition, discouraged terms, and a background/comment section.</p> <p>Conclusions</p> <p>The use of our revised neuroanatomical terminology in any new descriptions of the anatomy of invertebrate nervous systems will improve the comparability of this organ system and its substructures between the various taxa, and finally even lead to better and more robust homology hypotheses.</p

Can brain structure help to resolve interordinal relationships in insects?

While the monophyly of most insect orders is well supported by morphological data, relationships among orders are still largely undecided. Postulated interordinal relationships are often based on relatively few morphological characters or characters of questionable phylogenetic signifi cance. In studies based on molecular evidence interordinal relationships are usually not signifi cantly supported. Depending on the molecule under scrutiny or on the method of data analysis molecular studies often produced confl icting hypotheses on insect phylogeny. One organ that provides a large amount of independent morphological characters and that has as yet been scarcely utilized by insect phylogenists is the supraoesophageal ganglion or brain. Drawing from the vast literature on insect neuroanatomy, this review explores the value of neuronal characters for deriving relationships among insect orders

<i>Procephalothrix filiformis</i>, confocal laserscanning (cLSM) micrographs of differently immunostained whole mounts of.

<p><b>a</b>: Anti-FMRF, z-coded stack. The major lateral medullary cords (<i>mc</i>), some minor lateral nerves (<i>mln</i>) as well as a ventral nerve (<i>vn</i>) are visible. <i>pn</i>: proboscis nerves. <b>b</b>: Anti-FMRF. The neurites (<i>ne</i>) of the intraepidermal plexus are arranged in a ladder-like way. <b>c</b>: Anti-FMRF. On the very tip of the head is a cluster of cells showing immunoreactivity against FMRF (<i>arrow</i>).</p

Staining protocol of the palaeonemertean species investigated.

<p>Staining protocol of the palaeonemertean species investigated.</p

<i>Callinera grandis</i>, schematic drawing of a dorsal view on the central nervous system, based on 3D-reconstruction of 140 aligned 0.5 µm sections.

<p>The central nervous system is composed of neuropil (<i>np</i>, <i>gray</i>), which is surrounded by cell somata (<i>cs</i>, <i>blue</i>). Cephalic nerves (<i>cn</i>) are circularly arranged; the paired proboscidial nerves (<i>pn</i>, <i>yellow</i>) originate from the ventral commissural tract (<i>vct</i>). A dorsal nerve strand (<i>dn</i>) originates from the dorsal commissural tract. The branching esophageal nerves (<i>en</i>) originate from the ventral commissural tract. The lateral medullary cords (<i>mc</i>) originate ventro-caudally in the brain. The letters on the right (<b>a</b>–<b>f</b>) refer to the histological sections in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0066137#pone-0066137-g022" target="_blank">figure <b>22</b></a>.</p

<i>Procephalothrix filiformis</i>, confocal laserscanning (cLSM) micrographs of differently immunostained whole mounts.

<p><b>a</b>: Anti-FMRF. The cephalic cords (<i>cc</i>) originate in the lateral aspects of the brain (<i>br</i>). The lateral medullary cords (<i>mc</i>) extend the full length of the animal. There are two proboscidial nerves (<i>pn</i>) that run opposed to each other, along both sides of the proboscis. The esophageal nerves (<i>en</i>) originate at the ventral nerve (<i>vn</i>) and branch shortly before the mouth opening (<i>mo</i>). <b>b</b>: Anti-FMRF. Only few neuronal cell somata (<i>cs</i>) of the brain (<i>br</i>) are immunoreactive against FMRF. <b>c</b>: Anti-FMRF. Four minor nerves (<i>arrow</i>) unite in the tip of the animals' head (<i>arrowhead</i>). The nerves are interconnected by a circular nerve (<i>asterisk</i>) <b>d</b>: Anti-FMRF. The dorsal nerve (<i>dn</i>) is connected to the medullary cords (<i>mc</i>) in the very posterior part of the animal; bottle shaped sensory cells (<i>sc</i>) are distributed all over the body. <b>e</b>: Anti serotonin. The medullary cords (<i>mc</i>) are connected by serial arranged circular nerves (<i>scn</i>). Note that the proboscis nerves show no immunoreactivity against serotonin. <i>pr</i> proboscis.</p

<i>Tubulanus superbus</i>, light micrographs of Azan stained transverse sections of the brain.

<p><b>a</b>: The cephalic nerves (<i>cn</i>) are circularly arranged around the margins of the head. <b>b</b>: The brain is composed of a central neuropil (<i>np</i>) and surrounding cell somata (<i>cs</i>). An outer neurilemma (<i>on</i>) surrounds the whole brain. The two proboscidial nerves (<i>pn</i>) originate in the ventral commissural tract. <b>c</b>: Posteriorly the brain is divided into a dorsal and ventral lobe (<i>vl</i>). The canal (<i>ca</i>) of the cerebral organ does not pierce the basal lamina of the epidermis. <b>d</b>: The canal ends in a layer of neuronal cell somata (<i>arrow</i>) which have contact to the brain. Two esophageal nerves (<i>en</i>) are present. The dorsal nerve (<i>dn</i>) originates in a dorsal commissural tract. The ventral lobes of the brain are confluent with the lateral medullary cords (<i>mc</i>). <i>mo</i>: mouth opening.</p