Convergent evolution of bilaterian nerve cords.

It has been hypothesized that a condensed nervous system with a medial ventral nerve cord is an ancestral character of Bilateria. The presence of similar dorsoventral molecular patterns along the nerve cords of vertebrates, flies, and an annelid has been interpreted as support for this scenario. Whether these similarities are generally found across the diversity of bilaterian neuroanatomies is unclear, and thus the evolutionary history of the nervous system is still contentious. Here we study representatives of Xenacoelomorpha, Rotifera, Nemertea, Brachiopoda, and Annelida to assess the conservation of the dorsoventral nerve cord patterning. None of the studied species show a conserved dorsoventral molecular regionalization of their nerve cords, not even the annelid Owenia fusiformis, whose trunk neuroanatomy parallels that of vertebrates and flies. Our findings restrict the use of molecular patterns to explain nervous system evolution, and suggest that the similarities in dorsoventral patterning and trunk neuroanatomies evolved independently in Bilateria.

[Ectoderm, mesoderm and neuroectoderm are tissue types of importance for understanding and preventing root resorption. Clinical guidelines]. / L'ectoderme, le mésoderme et le neurectoderme sont des types tissulaires importants pour la compréhension et la prévention des résorptions radiculaires. Recommandations cliniques.

INTRODUCTION: This three-part article summarizes ideas already described elsewhere by the author. Part 1. New way of diagnosing the dentition. For diagnostic purposes origin and appearance of the three tissue types - ectoderm, mesoderm (ectomesenchyme) and peripheral nerves - are depicted on orthopantomograms. Same tissue types are marked on the root surface (peri-root sheet). Part 2. Factors provoking root resorption. Resorption can be explained from the composition of the peri-root sheet. Deviations (inborn or acquired) in each of the three tissue layers can provoke inflammation, resulting in resorption. Orthodontic forces resulting in resorption can occur in normal peri-root sheets, but also in peri-root sheets with inborn deviations, important to diagnose. Part 3. How to prevent root resorption - Clinical guidelines. General diseases and different dental morphologies are signs predisposing for root resorption (ectoderm and mesoderm), so are local or general virus attacks (neuroectoderm). Resorption often occurs in dentitions never treated orthodontically. MATERIAL AND METHOD: The author performed a review of the literature in order to present a new diagnostic approach incorporating histological and embryological concepts. RESULTS: The review revealed different etiologies and sites involved in root resorption. Patients presenting variations of the peri-root sheet are most exposed to root resorption. DISCUSSION: At this stage, it is difficult to diagnose these variations. The author offers diagnostic recommendations to be followed prior to orthodontic treatment. Even when no orthodontic treatment is given, root resorption can occur unexpectedly. In these cases, resorption prevention is currently impossible.

An eye on eye development.

The vertebrate eye is composed of both surface ectodermal and neuroectodermal derivatives that evaginate laterally from an epithelial anlage of the forming diencephalon. The retina is composed of a limited number of neuronal and non-neuronal cell types and is seen as a model for the brain with reduced complexity. The eye develops in a stereotypic manner building on evolutionarily conserved molecular networks. Eye formation is initiated at the onset of gastrulation by the determination of the eye field in the anterior neuroectoderm. Homeobox transcription factors, in particular Six3 are crucially involved in the establishment and maintenance of retinal identity. The eye field expands by proliferation as gastrulation proceeds and is initially confined to a single retinal primordium by the differential activity of specifying transcription factors. This central field is subsequently split in response to secreted factors emanating from the ventral midline. Concomitant with medio-lateral patterning at the onset of neurulation, morphogenesis sets in and laterally evaginates the optic vesicle. Strikingly during this process the neuroectoderm in the eye field transiently loses epithelial features and cells migrate individually. In a second morphogenetic event, the vesicle is transformed into the optic cup, concomitant with onset and progression of retinal differentiation. Accompanying optic cup morphogenesis, neural differentiation is initiated from a retinal signalling centre in a stereotypic and species specific manner by secreted signalling factors. Here we will give an overview of key events during vertebrate eye formation and highlight key players in the respective processes.

Sharpening of the anterior neural border in the chick by rostral endoderm signalling.

The anterior border of the neural plate, presumed to contain the prospective peripheral portion (roof) of the prospective telencephalon, emerges within a vaguely defined proneural ectodermal region. Fate maps carried out at HH4 in the chick reveal that this region still produces indistinctly neural, placodal and non-neural derivatives; it does not express neural markers. We examined how the definitive anterior border domain of the rostral forebrain becomes established and comes to display a neural molecular profile, whereas local non-neural derivatives become separated. The process, interpreted as a border sharpening mechanism via intercalatory cell movements, was studied using fate mapping, time-lapse microscopy and in situ hybridization. Separation of neural and non-neural domains proceeds along stages HH4-HH4+, is well advanced at HH5, and is accompanied by a novel dorsoventral intercalation, oriented orthogonal to the border, that distributes transitional cells into molecularly distinct neural and non-neural fields. Meanwhile, neuroectodermal Sox2 expression spreads peripherally from the neighbourhood of the node, reaching the nascent anterior border domain at HH5. We also show that concurrent signals from the endodermal layer are necessary to position and sharpen the neural border, and suggest that FGF8 might be a component of this signalling.

Specification and regionalisation of the neural plate border.

During early vertebrate development, the embryonic ectoderm becomes subdivided into neural, neural plate border (border) and epidermal regions. The nervous system is derived from the neural and border domains which, respectively, give rise to the central and peripheral nervous systems. To better understand the functional nervous system we need to know how individual neurons are specified and connected. Our understanding of the early development of the peripheral nervous system has been lagging compared to knowledge regarding central nervous system and epidermal cell lineage decision. Recent advances have shown when and how the specification of border cells is initiated. One important insight is that border specification is already initiated at blastula stages, and can be molecularly and temporally distinguished from rostrocaudal regionalisation of the border. From findings in several species, it is clear that Wnt, Bone Morphogenetic Protein and Fibroblast Growth Factor signals play important roles during the specification and regionalisation of the border. In this review, we highlight the individual roles of these signals and compare models of border specification, including a new model that describes how temporal coordination and epistatic interactions of extracellular signals result in the specification and regionalisation of border cells.

Correct anteroposterior patterning of the zebrafish neurectoderm in the absence of the early dorsal organizer.

BACKGROUND: The embryonic organizer (i.e., Spemann organizer) has a pivotal role in the establishment of the dorsoventral (DV) axis through the coordination of BMP signaling. However, as impaired organizer function also results in anterior and posterior truncations, it is of interest to determine if proper anteroposterior (AP) pattern can be obtained even in the absence of early organizer signaling. RESULTS: Using the ventralized, maternal effect ichabod (ich) mutant, and by inhibiting BMP signaling in ich embryos, we provide conclusive evidence that AP patterning is independent of the organizer in zebrafish, and is governed by TGFß, FGF, and Wnt signals emanating from the germ-ring. The expression patterns of neurectodermal markers in embryos with impaired BMP signaling show that the directionality of such signals is oriented along the animal-vegetal axis, which is essentially concordant with the AP axis. In addition, we find that in embryos inhibited in both Wnt and BMP signaling, the AP pattern of such markers is unchanged from that of the normal untreated embryo. These embryos develop radially organized trunk and head tissues, with an outer neurectodermal layer containing diffusely positioned neuronal precursors. Such organization is reflective of the presumed eumetazoan ancestor and might provide clues for the evolution of centralization in the nervous system. CONCLUSIONS: Using a zebrafish mutant deficient in the induction of the embryonic organizer, we demonstrate that the AP patterning of the neuroectoderm during gastrulation is independent of DV patterning. Our results provide further support for Nieuwkoop's "two step model" of embryonic induction. We also show that the zebrafish embryo can form a radial diffuse neural sheath in the absence of both BMP signaling and the early organizer.

Genomics and developmental approaches to an ascidian adenohypophysis primordium.

Ascidians, which are the closest phylogenetic relatives to vertebrates, lack a distinct pituitary gland, which is the major endocrine gland in vertebrates. Nevertheless, for the past 130 years, it has been debated that the ascidian neural complex (NC) is homologous to the pituitary. Of the three major components of the NC, the neural gland (NG) has mainly been thought to be the ascidian counterpart of the pituitary. Recently, however, the ciliated funnel, and not the NG, was postulated to be the adenohypophysis (AH) primordium because it is likely derived from oral ectoderm, and because the expression of several placodal genes is comparable to their expression in vertebrates. An extensive in silico survey of the Ciona intestinalis genome sequence revealed that genes encoding pituitary hormones are absent in ascidians. Under the circumstances, this thesis attempts to find a path that shows that the AH primordium is recognizable in the ascidian by revisiting molecular and developmental data from recent public resources on C. intestinalis, and through the use of advanced bio-imaging techniques. A putative Ciona genetic pathway, which was constructed by referring to data from mammals, shows that only a patchwork of the genetic network exists to achieve terminal differentiation of the AH endocrine cells in the Ciona genome. Re-annotation on glycoprotein hormone related proteins, a GPA2/ARP and two GPB5/BRP ones previously reported, reveals that the GPA2 locus contains two splicing variants, and one variant likely formed a three-dimensional conformation similar to that of human GPA2. No clone of the GPB5/BRP1 locus has been isolated, and another candidate, BRP2, is unlikely to be a GPB5. Next, I argued a possibility that endocrine activities of Ciona species could be specialized in association with its short generation time, and I suggest that not only Ciona species but also other ascidians should be studied in order to understand ascidian endocrinology. Confocal images of the stages of tailbud development reconfirmed the presence of an oral ectoderm placode, and I propose to update the stomodeum development by adding descriptions of a folded structure of the stomodeum and deeply positioned opening of the sensory vesicle. Finally, YFP expression driven by Ci-Six3 promoter demonstrated a boundary between the pharyngeal endoderm and other ectodermal and neuroectodermal tissues around the ciliated funnel. These updates on the ascidian model, which complement other lower chordates and vertebrates, shed light on the evolutionary origin of the pituitary primordium.

Notch signaling downstream of foxD5 promotes neural ectodermal transcription factors that inhibit neural differentiation.

We investigated the role of the Notch signaling pathway in regulating several transcription factors that stabilize a neural fate and expand the neural plate. Increased Notch signaling in a neural lineage via a constitutively activated form (NICD) up-regulated geminin and zic2 in a cell-autonomous manner, and expanded the neural plate domains of sox11, sox2, and sox3. Loss- and gain-of-function assays show that foxD5 acts upstream of notch1 gene expression. Decreasing Notch signaling with an anti-morphic form of a Notch ligand (X-Delta-1(STU)) showed that the foxD5-mediated expansion of the sox gene neural plate domains requires Notch signaling. However, geminin and zic2 appear to be dually regulated by foxD5 and Notch1 signaling. These studies demonstrate that: (1) Notch signaling acts downstream of foxD5 to promote the expression of a subset of neural ectodermal transcription factors; and (2) Notch signaling and the foxD5 transcriptional pathway together maintain the neural plate in an undifferentiated state. Developmental Dynamics 238:1358-1365, 2009. (c) 2009 Wiley-Liss, Inc.

A functional interaction between Irx and Meis patterns the anterior hindbrain and activates krox20 expression in rhombomere 3.

Patterning of the vertebrate hindbrain involves a segmentation process leading to the formation of seven rhombomeres along the antero-posterior axis. While recent studies have shed light on the mechanisms underlying progressive subdivision of the posterior hindbrain into individual rhombomeres, the early events involved in anterior hindbrain patterning are still largely unknown. In this paper we demonstrate that two zebrafish Iroquois transcription factors, Irx7 and Irx1b, are required for the proper formation and specification of rhombomeres 1 to 4 and, in particular, for krox20 activation in r3. We also show that Irx7 functionally interacts with Meis factors to activate the expression of anterior hindbrain markers, such as hoxb1a, hoxa2 and krox20, ectopically in the anterior neural plate. Then, focusing on krox20 expression, we show that the effect of Irx7 and Meis1.1 is mediated by element C, a conserved cis-regulatory element involved in krox20 activation in the hindbrain. Together, our data point to an essential function of Iroquois transcription factors in krox20 activation and, more generally, in anterior hindbrain specification.