1,720,979 research outputs found
Hair cells in ascidians and the evolution of lateral line placodes
No full textThe vertebrate hair cells are ciliary highly differentiated mechanoreceptors whose name derives from the peculiar microvilli, called stereovilli, that protrude into the fluid-filled cavities of the inner ear or lateral line organs. They differ from the primary sensory cells found in most invertebrates in that they are axonless, thus being secondary sensory cells that synapse with the dendrites of neurons whose cell bodies are located in the central nervous system (CNS). Although their morphology varies in different vertebrate species, hair cells typically have a single eccentric cilium and a collar of stereovilli graded in length from one side to the other.
Hair cells derive from placodes of the acustico-lateralis system that, together with the other neurogenic placodes, are generally believed to originate ontogenetically from a wide panplacodal field and are usually considered exclusive to craniates. However, recent molecular and morphological data suggest that cell populations with the properties of neurogenic placodes are also present in cephalochordates and tunicates. Extending our study now to eight species belonging to the three orders of ascidians, we find that all possess coronal organs with hair cells. All molecular and morphological data are consistent with the idea that the ancestor of chordates possessed the ability to differentiate hair cells and that the latter derive embryologically from an area having the characteristics of a neurogenic placode. We propose that the ascidian embryonic stomodeum contains a population of cells homologous to the vertebrate acustico-lateralis placodes
Angiogenic-like sprouting mechanism during normal and regenerative development in a colonial ascidian
No full textTunicates are useful models for comparing differing developmental processes such as embryogenesis, asexual reproduction, and regeneration, because they are the closest relatives to vertebrates and are the only chordates to reproduce both sexually and asexually. Among them, the ascidian Botryllus schlosseri forms colonies of numerous individuals embedded in the common tunic, that is the peculiar extracellular matrix (ECM) of tunicates. In B. schlosseri colonies this ECM holds also the colonial circulatory system (CCS): an anastomized network of vessels defined by simple epithelia and connected to the open circulatory system of the zooids. Recently, we have demonstrated that, during asexual propagation, new vessels form by means of a tubular sprouting mechanism, resembling that occurring in other metazoans, particularly during vertebrate angiogenesis. Moreover the immunohistology of CCS structures using antibodies against vertebrate angiogenic factors (VEGF, FGF-2, EGF) and receptors (VEGFR-1, VEGFR-2, EGFR), have shown that CCS sprouting occurs with their participation (1).
In B. schlosseri, the CCS possesses a remarkable regenerative potentiality, as shown by its ability to reform tunic and peripheral vascular network. We have also studied the regeneration of experimentally ablated CCS by analyzing the general dynamics of reorganization of vessels and tunic, their ultrastructure, cell proliferation, and the immunohistology of regenerating structures (2). Results show that the regenerative process of CCS occurs, similarly to normal growth, by the sprouting mechanism, with participation of same angiogenic factors.
Statistical analyses on recent experiments are now indicating a significant increase of tunic and vessels regeneration in groups of colonies injected with angiogenic factors (EGF, VEGF) with respect to injected with PBS. This demonstrates that same factors have an angiogenic impact in B. schlosseri and vertebrates circulatory systems.
Despite the different embryonic origin of their tissues (the former has an ectodermic origin, the latter mesodermic), all these data indicate that an homologous morphodynamic mechanism, controlled by homologous signal pathways, is involved during normal growth and regeneration of both systems. As a conclusion 1) we can hypothesize that tubular sprouting had a parallel evolution in these two structures, 2) our data support the idea that this morphogenetic mechanism was co-opted during the evolution of various developmental processes in several taxa
Botryllus schlosseri: A model ascidian for the study of asexual reproduction
No full textBotryllus schlosseri, a cosmopolitan colonial ascidian reared in the laboratory for more than 50 years, reproduces both sexually and asexually and is used as a model organism for studying a variety of biological problems. Colonies are formed of numerous, genetically identical individuals (zooids) and undergo ciclica generation changes in which the adult zooids die and are replaced by their maturing buds. Because the progression of the colonial life cycle is intimately correlated with blastogenesis, a shared staging method of bud development is required to compare data coming from different laboratories. With the present review, we aim (1) to introduce B. schlosseri as a valuable chordate model to study various biological problems and, especially, sexual and asexual development; (2) to offer a detailed description of bud development up to adulthood and the attainment of sexual maturity; (3) to re-examine Sabbadin’s (1955) staging method and re-propose it as a simple tool for in vivo recognition of the main morphogenetic events and recurrent changes in the blastogenetic cycle, as it refers to the developmental stages of buds and adults
Coronal organ of ascidians and the evolutionary significance of secondary sensory cells in chordates
No full textA new mechanoreceptor organ, the coronal organ, in the oral siphon of some ascidians belonging to the order Pleurogona has recently been described. In contrast to the known mechanoreceptor organs of ascidian atrium that consist of sensory neurons sending their own axons to the cerebral ganglion, coronal sensory cells are secondary mechanoreceptors, i.e., axonless cells forming afferent and efferent synapses with neurites of neurons located in the
ganglion. Moreover, coronal cells exhibit an apical apparatus composed of a cilium accompanied or flanked by rod-like microvilli (stereovilli). Because of the resemblance of these cells to vertebrate hair cells, their ectodermal origin and location in a linear array bordering the bases of the oral tentacles and velum, the coronal organ has been proposed as a homologue to the vertebrate acousticolateralis system. Here we describe the morphology of the coronal organs of six ascidians belonging to the suborders Phlebobranchia and Aplousobranchia (order Enterogona). The sensory cells are ciliated, lack typical stereovilli, and at their bases form synapses with neurites. In two species, the sensory cells are accompanied by large cells involved in synthesis and secretion of protein. We hypothesize that the coronal organ with its
secondary sensory cells represents a plesiomorphic feature of ascidians. We compare the coronal organ with other chordate sensory organs formed of secondary sensory cells, i.e., the ventral lip receptors of appendicularians, the oral secondary sensory cells of cephalochordates, and the acousticolateralis system of vertebrates, and we discuss their homologies at different levels of organization
Stomodeal and neurohypophysial placodes in Ciona intestinalis: Insights into the origin of the pituitary gland
No full textThe ascidian larva has a central nervous system which shares basic characteristics with craniates, such as tripartite organisation and many developmental genes. One difference, at metamorphosis, is that this chordate-like nervous system regresses and the adult’s neural complex, composed of the cerebral ganglion and associated neural gland, forms. It is known that neural complex differentiation involves two ectodermal structures, the neurohypophysial duct, derived from the embryonic neural tube, and the stomodeum, i.e. the rudiment of the oral siphon; nevertheless, their precise role remains to be clarified. We have shown that in Ciona intestinalis, the neural complex primordium is the neurohypophysial duct, which in the early larva is a short tube, blind anteriorly, with its lumen in continuity with that of the central nervous system, i.e. the sensory
vesicle. The tube grows forwards and fuses with the posterior wall of the stomodeum, a dorsal ectodermal invagination of the larva. The duct then loses posterior communication with the sensory vesicle and begins to grow on the roof of the vesicle itself. The neurohypophysial duct differentiates into the neural gland rudiment; its dorsal wall begins to proliferate neuroblasts, which migrate and converge to build up the cerebral ganglion. The most anterior part of the neural gland organizes into the ciliated duct and funnel, whereas the most posterior part elongates and gives rise to the dorsal strand. The hypothesis that the neurohypophysial duct/stomodeum complex possesses cell populations homologous to the craniate olfactory and adenohypophysial placodes and hypothalamus is discussed
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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