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Secondary neurons are arrested in an immature state by formation of epithelial vesicles during neurogenesis of the spider Cupiennius salei
© Stollewerk; licensee BioMed Central Ltd. 2004
Received: 19 October 2004
Accepted: 25 October 2004
Published: 25 October 2004
In the spider Cupiennius salei about 30 groups of neural precursors are generated per hemi-segment during early neurogenesis. Analysis of the ventral neuromeres after invagination of the primary neural precursor groups revealed that secondary neural precursors arise during late embryogenesis that partially do not differentiate until larval stages.
In contrast to the primary groups, the secondary invaginating cells do not detach from each other after invagination but maintain their epithelial character and form so-called epithelial vesicles. As revealed by dye labeling, secondary neural precursors within epithelial vesicles do not show any morphological features of differentiation indicating that the formation of epithelial vesicles after invagination leads to a delay in the differentiation of the corresponding neural precursors. About half of the secondary neural precursor groups do not dissociate from each other during embryogenesis indicating that they provide neural precursors for larval and adult stages.
Secondary neural precursors are arrested in an immature state by formation of epithelial vesicles. This mechanism facilitates the production of larval neural precursors during embryogenesis. I discuss the evolutionary changes that have occured during neural precursor formation in the arthropod group and present a model for the basal mode of neurogenesis.
The arthropods form a diverse group with a correspondingly high variation of neural structures adapted to the specialized behaviour and lifestyles of individual species. This raises the question of how developmental processes have been modified during evolution to generate the wide diversity of nervous systems seen in adult arthropods. Evolutionary modifications that lead to variations in neural structures can occur during different processes of neurogenesis. The establishment of neural networks can be influenced by changes in the generation of neural precursors, modifications of cell fates or elimination of individual neurons as well as changes in axonal guidance. A comparative analysis of neurogenesis in chelicerates and myriapods has revealed that although the developmental program is genetically conserved, there is a major difference in the recruitment of neural precursors as compared to insects and crustaceans [1–5]. Groups of neural precursors invaginate from the ventral neuroectoderm in a regular, strikingly similar pattern in spiders (chelicerates) and myriapods, while in insects and crustaceans single neural precursors are selected. This modification may be the basis for variations in the functions of spider and myriapod neurons, since a comparison of early segmentally repeated neurons that pioneer the major axon tracts in crustaceans and insects has not revealed any similarities in cell body positions or axonal outgrowths to myriapod neurons [6, 7].
In the spider 30 to 32 groups of neural precursors are generated per hemi-segment during neurogenesis. As in Drosophila melanogaster, the neural precursors arise at stereotyped positions that are prefigured by a proneural gene (CsASH 1), while the neurogenic genes Delta and Notch restrict the proportion of cells that adopt the neural fate at each wave of neural precursor formation [1, 2]. In Drosophila melanogaster, the Delta/Notch signalling pathway is used for a decision between two cell fates in the ventral neuroectoderm: delaminating cells become neural precursors, while cells that remain apical give rise to epidermis. This decision does not take place in the central neurogenic regions of the spider . The epidermal cells are derived from lateral regions that overgrow the neuromeres after invagination of the neural precursors.
Since each invagination group consists of five to nine neural precursors, it can be estimated that an embryonic hemineuromere consists of about 220 neurons on average, similar to Drosophila. However, in the adult spider Cupiennius salei the subesophageal ganglion consists of 49,000 neurons  indicating that over 40,000 neurons must be generated during late embryonic and larval stages. In Drosophila melanogaster, 'embryonic' neuroblasts proliferate again and give rise to larval and adult lineages after a phase of cell cycle arrest from late embryogenesis to first larval instar [9–11]. An analysis of the mitotic pattern during neurogenesis has revealed that neuroblasts are missing in the spider . In addition, most of the neural precursors do not divide after invagination. This raises the question of how additional neurons are generated that contribute to the larval and adult CNS of the spider.
In the spider Cupiennius salei the germband develops from aggregations of cells that form the cephalic lobe and the caudal lobe . One to three prosomal segments are generated by a subdivision of the cephalic lobe, while the remaining segments arise sequentially from the caudal lobe, the so-called posterior growth zone [12, 13]. At the beginning of neurogenesis (about 130 hours of development; stages after Seitz ) a longitudinal furrow forms that divides the germband into left and right parts that remain connected only at the cephalic lobe and the posterior growth zone. The two halves of the embryo move laterally until they finally meet at the dorsal midline (ca. 300 hours of developement). This process is called inversion . The formation of neural precursors and the invagination of these cells occurs during inversion .
Secondary invagination sites form after invagination of the primary neural precursors
Secondary invagination sites persist as epithelial vesicles
Dissociation of epithelial vesicles is not associated with cell divisions
achaete-scute homologues and neurogenic genes are re-expressed during formation of the secondary precursors
Two achaete-scute homologues have been identified in the spider . CsASH1 is expressed like a proneural gene in the neurogenic regions prior to formation of the primary invagination sites and is necessary for the generation of neural precursors. CsASH2, in contrast, shows a pan-neural mode of expression: it is exclusively expressed in all invaginating neural precursors. Simlar to Drosophila melanogaster, the neurogenic genes Notch and Delta restrict the proportion of cells that adopt a neural fate at each wave of neural precursor formation .
Formation of epithelial vesicles – a conserved character in arthropod neurogenesis?
Analysis of the ventral neuromeres of spider embryos after invagination of the primary neural precursor groups revealed that secondary neural precursors arise during late embryogenesis that partially do not differentiate until larval stages. In contrast to the primary groups, the secondary invaginating cells do not detach from each other after invagination but maintain their epithelial character. In common with epithelial cells, they show a pronounced apico-basal polarity. The apical surface is covered with microvilli, while the lateral surfaces adhere to those of neighbouring cells of a group via specialized cell junctions, i.e. zonulae adhaerentes.
Although the formation of epithelial cell groups has not been observed in the ventral neuromeres of other arthropods, epithelial vesicles have been described during development of the stomatogastric nervous system and the brain in Drosophila melanogaster. After invagination of the individual neuroblasts that pioneer the frontal connective and recurrent nerve , three groups of cells invaginate from the stomatogastric nervous system primordium . They loose contact with the surrounding stomodeal epithelium and form elongated, hollow epithelial vesicles, similar to the secondary neural precursors of the spider. Finally, they dissociate into apolar cells and are incorporated into different stomatogastric ganglia [15, 16]. In a similar way, the vesicle forming the optic lobe invaginates from the posterior head region of Drosophila melanogaster embryos. In contrast to the stomatogastric vesicles, this cell group remains epithelial throughout embryogenesis and larval life .
It has been shown in Drosophila melanogaster that the Delta-Notch signaling pathway is involved in maintaining the epithelial character of the optic lobe and stomatogastric nervous system (SNS) precursors . In Notch mutant Drosophila melanogaster embryos, cells with the identity of SNS and optic lobe precursors develop at approximately normal numbers, but they do not form epithelial vesicles. Instead, these cells appear as solid, irregular clusters of apolar cells [15–17]. In the spider, the function of CsNotch during development of the secondary neural precursors could not be analysed, because injection of ds CsNotch RNA leads to a premature differentiation of neural precursors due to an ealier function of CsNotch in lateral inhibition . However, the up-regulation of CsNotch in the secondary invagination sites suggests a role in formation of the epithelial vesicles (see Fig. 7E).
Similar to Notch, the proneural genes achaete, scute and lethal of scute are continuously expressed in the SNS of Drosophila melanogaster . Loss of proneural gene function leads to the absence of a subpopulation of SNS precursors and subsequently to an irregular invagination of the SNS placode. Furthermore, proneural genes seem to promote the dissociation of SNS precursors from the epithelial vesicles, since loss of proneural gene function results in a delay of this process. Similar to Drosophila melanogaster, both achaete-scute homologues of the spider are expressed in the epithelial vesicles that are formed by the secondary neural precursors. However, in contrast to its function in the recruitment of the primary neural precurors, the expression pattern of the spider proneural gene CsASH1 does not suggest a role in the establishment of the secondary neural fate. CsASH1 transcripts can only be detected in subsets of neural precursors after generation of the secondary invagination sites. A similar expression pattern can be observed for CsASH2, although the transcripts in the primary neural precursors are down-regulated later than the CsASH1 transcripts. The function of these two genes during generation of the secondary invagination sites and the formation of the epithelial vesicles could not be analysed. Due to their ealier role in the recruitment and differentiation of the primary precurors, injection of ds RNA of either gene leads to severe morphological defects in the ventral neuroectoderm .
The formation of epithelial vesicles leads to a delay in neural differentiation
As revealed by DiI-labeling, secondary neural precursors within epithelial vesicles do not show any morphological features of differentiation. Obviously, the formation of epithelial vesicles after invagination leads to a delay in the differentiation of the corresponding neural precursors. Although the epithelial vesicles are formed at about the same time, they dissociate from each other subsequently. About half of them are still visible at the end of embryogenesis indicating that they provide neural precursors for larval stages.
In insects a distinct mechanism has evolved for generating larval neural precursors during embryonic life. After a phase of cell cycle arrest from late embryogenesis to first larval instar, 'embryonic' neuroblasts proliferate again. [9, 10]. Both in Drosophila melanogaster and in Manduca sexta, the larval progeny of these neuroblasts accumulate in groups of cells that are separated by glial cell processes and do not finish their differentiation until the onset of metamorphosis [10, 19]. It has been shown that the secreted glycoprotein anachronism (ana) regulates release of central brain neuroblasts from cell cycle arrest . Ana is expressed in glial cells that ensheath central brain and optic lobe neuroblasts. In ana mutant larvae, neuroblasts proliferate earlier than in normal development which in turn leads to a premature differentation of neurons in certain brain regions. This heterochronic defect has an impact on the axonal pattern: the ana mutant phenotype ranges from subtle missrouting of fiber tracts to massive disorganization that affects the entire optic lobe . These data show that factors regulating the differentiation state of neural precursors can have an important influence on the organization of neural networks.
The distinct morphology of the sheath cells in the spider neuromeres, i.e. their translucent cytoplasm, the absence of microvilli and the extension of cell processes that enwrap the neural precursors suggests that these are glial cells. Further analysis will show if these cells express genes that can influence the epithelial organization, i.e. the differentiation state of the secondary neural precursors, comparable to the glial cells of Drosophila melanogaster.
Formation of epithelial vesicles – a basal mode of neurogenesis?
A recent study on neurogenesis in the onychophoran Euperipatoides kanangrensis shows that, rather than forming individual invagination groups, the whole medial regions of the hemi-segments invaginate into the embryo . The invaginated cells remain attached to each other forming transitory epithelial vesicles. Although the phylogenetic position of Onychophora is still being debated, they are generally placed basally in the arthropodan clade [22–27]. Since onychophorans have retained many pleisiomorphic features, it can be assumed that they reflect a basal mode of CNS development [28–30]. This leads to the following model of changes in neural precursor formation during arthropod evolution: the basal mode of neurogenesis is the invagination of one large cluster of neural precursors from the central region of each hemi-neuromere. These clusters form transitory epithelial vesicles in the ventral neuromeres . An advanced mode of neurogenesis is seen in chelicerates and myriapods: groups of cells that arise in several waves at stereotyped positions invaginate form the ventral neuroectoderm [1, 3, 4]. Interestingly, both chelicerate and myripod neurogenesis reflects some ancestral features. In the spider, epithelial vesicles are formed by secondary invaginating cell groups, while in myriapods the whole central regions of the hemi-neuromeres sink into the embryo after invagination of individual groups of neural precursors [3, 32]. An even complexer mode of neurogenesis is seen in insects and crustaceans: individual neuroblasts are singled out from the ventral neuroectoderm that divide in sterotyped patterns to give rise to ganglion mother cells and finally neurons [33–42].
To summarize, the model suggests that the invagination of large groups of neuroepithelial cells that form transient epithelial vesicles represents the basal mode of neurogenesis. Subsequently, more parameters have been introduced to the process of neurogenesis during arthropod evolution, i.e. sequential invagination/delamination of neural precursors and connection between neural precursor formation and cell proliferation. It can be assumed that these additional parameters have contributed to the diversity of neural precursor populations. This diversity might have been used as an evolutionary tool to develop neural networks that are adapted to the specialized behaviour and morphologies of the individual arthropod groups.
Materials and Methods
Cupiennius salei stocks
Fertilized females of the Central American wandering spider Cupiennius salei Keyserling (Chelicerata, Arachnida, Araneae, Ctenidae) were obtained from Ernst-August Seyfarth, Frankfurt, Germany. Embryos were collected as described before .
Histology and stainings
Whole-mount in situ hybridisations were performed as described . Phalloidin-rhodamine staining of spider embryos was performed as has been described for flies . Anti-Phospho-Histone 3 immunocytochemestry has been performed as described .
After chemically removing the chorion, embryos were fixed in 4 % formaldehyde in PBS and 1 vol heptane. The vitelline membrane was removed with needles and the embryos stained with phalloidin-FITC. Flat preparations of these embryos were attached to a coverslip with a double-sticky tape and covered with PBS. 1,1'-dioctadecyl 3,3,3',3'-tetramethyl indocarbocyanine perchlorate (DiI) was dissolved in ethanol and applied with glass needles. A small droplet of DiI was injected into single or several cells of an invagination group using a 63 × water-immersion lens and a FITC filter on a Zeiss fixed stage microscope and a micromanipulator.
I thank Diethard Tautz for continued support, critical discussion and helpful comments. Many thanks to Pat Simpson, Volker Hartenstein and Diethard Tautz for critical reading of the manuscript. I am grateful to José A. Campos-Ortega for critical comments on the project and for providing access to the electron microscope and the histological equipment. I thank Michael Bate for critical comments on the project and access to the injection facility. Thanks to Ernst-August Seyfarth for providing the spiders. This work was supported by a grant from the Deutsche Forschungsgemeinschaft.
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