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

BACKGROUND: 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 Lineus viridis with high anatomical resolution. RESULTS: In L. viridis, 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. CONCLUSIONS: 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.

Comparative neuroanatomy suggests repeated reduction of neuroarchitectural complexity in Annelida.

BACKGROUND: Paired mushroom bodies, an unpaired central complex, and bilaterally arranged clusters of olfactory glomeruli are among the most distinctive components of arthropod neuroarchitecture. Mushroom body neuropils, unpaired midline neuropils, and olfactory glomeruli also occur in the brains of some polychaete annelids, showing varying degrees of morphological similarity to their arthropod counterparts. Attempts to elucidate the evolutionary origin of these neuropils and to deduce an ancestral ground pattern of annelid cerebral complexity are impeded by the incomplete knowledge of annelid phylogeny and by a lack of comparative neuroanatomical data for this group. The present account aims to provide new morphological data for a broad range of annelid taxa in order to trace the occurrence and variability of higher brain centers in segmented worms. RESULTS: Immunohistochemically stained preparations provide comparative neuroanatomical data for representatives from 22 annelid species. The most prominent neuropil structures to be encountered in the annelid brain are the paired mushroom bodies that occur in a number of polychaete taxa. Mushroom bodies can in some cases be demonstrated to be closely associated with clusters of spheroid neuropils reminiscent of arthropod olfactory glomeruli. Less distinctive subcompartments of the annelid brain are unpaired midline neuropils that bear a remote resemblance to similar components in the arthropod brain. The occurrence of higher brain centers such as mushroom bodies, olfactory glomeruli, and unpaired midline neuropils seems to be restricted to errant polychaetes. CONCLUSIONS: The implications of an assumed homology between annelid and arthropod mushroom bodies are discussed in light of the 'new animal phylogeny'. It is concluded that the apparent homology of mushroom bodies in distantly related groups has to be interpreted as a plesiomorphy, pointing towards a considerably complex neuroarchitecture inherited from the last common ancestor, Urbilateria. Within the annelid radiation, the lack of mushroom bodies in certain groups is explained by widespread secondary reductions owing to selective pressures unfavorable for the differentiation of elaborate brains. Evolutionary pathways of mushroom body neuropils in errant polychaetes remain enigmatic.

Invertebrate neurophylogeny: suggested terms and definitions for a neuroanatomical glossary.

BACKGROUND: 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. RESULTS: 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. CONCLUSIONS: 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.

150 years beyond Darwin's Origin of species: finding new approaches to reconstruct early animal phylogeny.

The conference 'Celebrating Darwin: From the Origin of Species to Deep Metazoan Phylogeny' was held at the Humboldt University in Berlin, from 3 to 6 March 2009. Specialists from the fields of bioinformatics, molecular biology, developmental biology, comparative morphology and paleontology joined forces to present and discuss novel approaches in reconstructing the still unresolved early branching patterns of the metazoan tree of life.

Arthropod phylogeny: onychophoran brain organization suggests an archaic relationship with a chelicerate stem lineage.

Neuroanatomical studies have demonstrated that the architecture and organization among neuropils are highly conserved within any order of arthropods. The shapes of nerve cells and their neuropilar arrangements provide robust characters for phylogenetic analyses. Such analyses so far have agreed with molecular phylogenies in demonstrating that entomostracans+malacostracans belong to a clade (Tetraconata) that includes the hexapods. However, relationships among what are considered to be paraphyletic groups or among the stem arthropods have not yet been satisfactorily resolved. The present parsimony analyses of independent neuroarchitectural characters from 27 arthropods and lobopods demonstrate relationships that are congruent with phylogenies derived from molecular studies, except for the status of the Onychophora. The present account describes the brain of the onychophoran Euperipatoides rowelli, demonstrating that the structure and arrangements of its neurons, cerebral neuropils and sensory centres are distinct from arrangements in the brains of mandibulates. Neuroanatomical evidence suggests that the organization of the onychophoran brain is similar to that of the brains of chelicerates.

A simple fluorescent double staining method for distinguishing neuronal from non-neuronal cells in the insect central nervous system.

Being able to discriminate between neurons and non-neuronal cells such as glia and tracheal cells has been a major problem in insect neuroscience, because glia-specific antisera are available for only a small number of species such as Drosophila melanogaster and Manduca sexta. Especially developmental or comparative studies often require an estimate of neuron numbers. Since neuronal and glial cell bodies are in many cases indiscernible in situ, a method to distinguish neurons from non-neuronal cells that works in any given species is wanting. Another application is cell culturing. Cultured cells usually change their outward shape dramatically after being isolated so that it is frequently impossible to tell neurons and glia apart. Here, we present a simple method that uses a commercially available antiserum directed against horseradish peroxidase, which specifically stains neurons but no other cell type in every insect species investigated. Counterstaining with DAPI, a fluorescent chromophore that binds to double-stranded DNA in the nuclei of all cells, yields the total number of cells in a given sample. Thus, double labeled cells can be identified as neurons, cells that carry only DAPI staining are non-neuronal.

The nervous systems of basally branching nemertea (palaeonemertea).

In recent years, a lot of studies have been published dealing with the anatomy of the nervous system in different spiralian species. The only nemertean species investigated in this context probably shows derived characters and thus the conditions found there are not useful in inferring the relationship between nemerteans and other spiralian taxa. Ingroup relationships within Nemertea are still unclear, but there is some agreement that the palaeonemerteans form a basal, paraphyletic grade. Thus, palaeonemertean species are likely the most informative when comparing with other invertebrate groups. We therefore analyzed the nervous system of several palaeonemertean species by combining histology and immunostaining. 3D reconstructions based on the aligned slices were performed to get an overall impression of the central nervous system, and immunohistochemistry was chosen to reveal fine structures and to be able to compare the data with recently published results. The insights presented here permit a first attempt to reconstruct the primary organization of the nemertean nervous system. This comparative analysis allows substantiating homology hypotheses for nerves of the peripheral nervous system. This study also provides evidence that the nemertean brain primarily consists of two lobes connected by a strong ventral commissure and one to several dorsal commissures. During nemertean evolution, the brain underwent continuous compartmentalization into a pair of dorsal and ventral lobes interconnected by commissures and lateral tracts. Given that this conclusion can be corroborated by cladistic analyses, nemerteans should share a common ancestor with spiralians that primarily have a simple brain consisting of paired medullary, frontally commissurized and reinforced cords. Such an organization resembles the situation found in presumably basally branching annelids or mollusks.

Neuroarchitecture of the arcuate body in the brain of the spider Cupiennius salei (Araneae, Chelicerata) revealed by allatostatin-, proctolin-, and CCAP-immunocytochemistry and its evolutionary implications.

Here we describe the neuronal organization of the arcuate body in the brain of the wandering spider Cupiennius salei. The internal anatomy of this major brain center is analyzed in detail based on allatostatin-, proctolin-, and crustacean cardioactive peptide (CCAP)-immunohistochemistry. Prominent neuronal features are demonstrated in graphic reconstructions. The stainings revealed that the neuroarchitecture of the arcuate body is characterized by several distinct layers some of which comprise nerve terminals that are organized in columnar, palisade-like arrays. The anatomy of the spider's arcuate body exhibits similarities as well as differences when compared to the central complex in the protocerebrum of the Tetraconata. Arguments for and against a possible homology of the arcuate body of the Chelicerata and the central complex of the Tetraconata and their consequences for the understanding of arthropod brain evolution are discussed.

The organization and evolutionary implications of neuropils and their neurons in the brain of the onychophoran Euperipatoides rowelli.

This account describes the organization of the brain of the adult Euperipatoides rowelli, a member of the Onychophora or "velvet worms." The present account identifies three cerebral divisions, the first of which contains primary olfactory neuropils, visual neuropils, and brain regions that correspond anatomically to the mushroom bodies of annelids, chelicerates, myriapods, and insects. In common with the brains of many chelicerates, the onychophoran brain is supplied by many thousands of uniformly small basophilic perikarya. Other chelicerate-like features include mushroom body lobes that extend across the brain's midline, an unpaired arch-shaped midline neuropil, and visual pathways that supply midline neuropil and that of the mushroom bodies. These and other similarities with chelicerate brains are discussed in the context of arthropod evolution and with reference to recent molecular phylogenies.

Common design in a unique midline neuropil in the brains of arthropods.

Most insects possess an assemblage of midline neuropils in their protocerebrum called the central complex. Recent studies have identified comparable assemblages in the malacostracan protocerebrum. Studies of Drosophila melanogaster locomotory mutants suggest that in insects one role for the central complex might be to orchestrate limb actions. This is anecdotally supported by comparisons amongst insects suggesting that elaboration of central complex architecture correlates with complexity of limb motor repertoires. The present account describes immunocytochemical and neuroanatomical observations that reveal common design principles amongst midline neuropils in four arthropod clades, the hexapods, crustaceans, chilopods, and chelicerates and the absence of midline neuropils in diplopods. The chilopod midline neuropil, which is columnar and stratified and lacks chiasmal axons to the dorsal protocerebrum or connections to discrete satellite regions, may represent the plesiomorphous condition. The complete absence of a midline neuropil in diplopods supports previous neuroanatomical studies suggesting that the 'Myriapoda' are an artificial paraphyletic group. The columnar and layered arcuate midline neuropils of chelicerates are compared with columnar and layered midline neuropils of chilopods. No midline neuropil has been identified in a lophotrochozoan outgroup, the Polychaeta.