1,720,967 research outputs found
Ovulation and placentation in Botryllus schlosseri(Ascidiacea): An ultrastructural study.
No full textThe mode of ovulation and placentation was studied by light and electron microscopy in the ovoviviparous ascidian Botryllus schlosseri using colonies from the laboratory. The full-grown oocyte is surrounded by the outer and inner follicle cell layers, the acellular vitelline coat (chorion), and the test cells, and it is furnished with its own vesicular oviduct which is interposed between the egg and the atrial epithelium. In contrast to most ascidians, the outer follicle is thick and has an ultrastructure consistent with intense protein synthesis. At ovulation the outer follicle shows signs of involution where it contacts the oviduct. When the oviducal wall breaks and the egg moves through the oviduct, the outer follicle cells are discharged in the mantle to form a sort of corpus luteum. The egg remains hanging in the atrial chamber by means of a cuplike "placenta." The placental tissues are all of maternal origin, being derived from both the atrial and oviducal epithelia together with some of the inner follicle cells. These latter anchor to the oviducal epithelium by means of junctional spots and a filamentous cementing secretion. Our results suggest that the main role of the "placenta" is to attach the embryo to the parent, thus exposing it to the flow of seawater
Tissue repair during egg segregation in tunic of the compound ascidian, Diplosoma listerianum
No full textIn D. listerianum mature eggs are isoltaed in the tunic where embryos develops. The process of egg segregation was studied by light and electron microscopy. Initially the egg is contained in a sac-like pocket of the epidermis which narrows to form an ephemeral collar at its proximal end. The follicle cells discharged from the egg are retracted towards the zooid to block the aperture of the collar. The epidermis breaks close to the egg and the lips of the wound heal rapidly, enclosing the egg in the tunic and later re-establishing the epidermal continuity of the zooid. Both granulocytes form the bloodstream and epitheliocytes engulf tissue debris. Moreover, apocrine secretion of the epidermis participates in tunic restoration. The permeability barrier of the zooid seems to be assured by tight junctions, together with the plug of follicle cells
Ovulation and egg segregation in the tunic of a colonial ascidian, Diplosoma listerianum (Tunicata, Ascidiacea)
Full text linkThe process of egg segregation in the tunic of the ovoviviparous ascidian Diplosoma listerianum was studied by light and electron microscopy. One egg at a time was seen to mature in each zooid. The eggs had large yolk and grew on the ovary wall enveloped in four layers: (1) outer follicle cells (OFC), long and rich in RER (rough endoplasmic reticulum) and with dense granules in the Golgi region; (2) flat inner follicle cells (IFC); (3) a loosely fibrillar vitelline coat (VC); (4) test cells encased on the egg surface. The growing egg protrudes from the ovary wall and presses on the contiguous epidermis. Granulocytes enter the space between the epidermis and the egg and insinuate cytoplasmic protrusions, disrupting the continuity of the OFC layer. At ovulation, OFC and IFC are discharged and form a post-ovulatory follicle (corpus luteum). The epidermis shrinks and closes, possibly by activation of microfilaments, causing the egg to be completely surrounded by the tunic. In the zooid, the wound caused by the passage of the egg is repaired both by contraction of the epidermis and by phagocytic activity. Altered spermatozoans are found in phagocytosing cells in the lumen of the ovary. These are presumably remnants of those which entered to fertilize the egg before segregation
Toxicity of organotin compounds on embryos of a marine invertebrate (Styela plicata; Tunicata)
Full text linkOrganotin compounds are an useful class of tin chemicals, a number of which have found commercial applications and are industrially synthesized in large amounts. Their uncontrolled use may cause profound effects on the fate and long-term environmental impact in natural aquatic environments. Butyltin compounds (TBT and its degradation products, i.e. DBT and MBT) are leached out from antifouling paints; triphenyltin (TPT) and tricyclohexyltin (TCHT) compounds have source in the run off waters of treated plants on which they are use as pesticides. There is much concern about their hurtful effects on marine life, especially fish and molluscs. The lagoon of Venice is a particular coastal area with shallow and incompletely turnovering waters, in which the resident benthonic population suffers the negative impact of organotins for bioaccumulation. In order to clarify the interaction mechanism, we studied the toxic effects of organotins on a benthonic filter-feeding ascidian. We followed the embryonic development of the ascidian Styela plicata with two different approaches: i) in vivo observations on different stages of ascidian development from fertilized egg to larval stages by incubation with 0.1, 1, 10 μM organotins for various exposure times; ii) ultrastructural observations on the early embryonic stages (2-4 cells, morula, gastrula) incubated with 10 μM TBT or TPT for 1 hour. Our results show that organotins significantly affect all stages of ascidian development in a dose- and time-dependent manner and the most sensitive stages are gastrula and neurula. These compounds are able to block the development giving rise to anomalous embryos with irreversible effects. The order of inhibition appears strongly dependent on the organotin liposolubility: TBT>DBT>MBT and TPT>TCHT. The mitosis block of blastomers in the early stages may be related to an inhibition of the microtubule polymerization. Observations with light and electron microscopes reveal globe-shaped blastomers with large intercellular spaces in morula and gastrula stages, suggesting a toxic damage on aggregation of microfilaments. Moreover the occurrence of electron-dense precipitates of organotins in the inner membrane of mitochondria and morphological changes of their cristae suggests an inhibition of mitochondrial oxidative phosphorylation which is important in the gastrula stage. In this stage, these electron-dense aggregates grow from 50-70 nm to 110-170 nm and the alteration of the cristae increases
Gut ultrastructure of the appendicularian Oikopleura dioica (Tunicata)
No full textThe appendicularians, planktonic tunicates, possess a specialized, external filtering system that captures food particles <1 μm in size. In this work the alimentary canal of Oikopleura dioica has been studied by serial sections of whole animals and ultrastructure. The gut includes a dorsal esophagus, a bilobed saccular stomach, and a curved intestine, divided into vertical, mid-, and distal intestine (or rectum). No multicellular glands or cellular proliferative centers were found. Three main cell types were recognized, ciliated microvillar cells, globular cells and gastric band cells, with specializations reflecting different physiological roles in the various regions. Ciliated microvillar cells, the most diffuse, are considered to be involved in food propulsion, fecal pellet formation, absorption, and nutrient storage. Pinocytotic features and vacuoles suggest that absorption of macromolecules and intracellular digestion occur in the globular cells of the stomach and rectum. The large gastric band cells of the left lobe have typical features of intense protein synthesis and probably produce enzymes for extracellular digestion. Diffuse interdigitations of many cells enormously increase the plasmalemma surface and may be involved in liquid/ion exchange. Despite the apparent structural simplicity of the gut epithelium, O. dioica efficiently processes food to fulfill the energy requirements of its exceptionally rapid life-cycle
Differentiation of blood and germ cell lines in ascidians.
No full textLinea sanguigna e linea germinale nelle ascidie hanno origine comune da una cellula staminale del mesenchima embrionale. Con riferimento alla linea sanguigna, tale cellula è denominata emoblasto, linfocita o cellula linfocita-simile per analogia con il linfocita dei vertebrati. Si tratta in realtà di cellula staminale generalizzata, totipotente: da un gruppo di emoblasti, congregati sulla parete di un vaso, si può originare un intero individuo nel processo di 'gemmazione vascolare'.
In corrispondenza di linfonoduli, distribuiti sulla parete della branchia, del tubo digerente e nel mantello, l'emoblasto da origine a diversi tipi cellulari, secondo differenti linee di differenziamento. Nella specie Botryllus schlosseri vengono prese in considerazione una linea fagocitarla ed una linea citotossica. La prima comprende cellule provviste di un corredo di enzimi idrolitici, le quali con la fagocitosi finiscono con l'assumere forma e caratteristiche proprie dei macrofagi dei vertebrati. La fagocitosi è favorita dalla presenza di una opsonina plasmatica, lectina dimerica di tipo S, specifica per beta-galattosidi, ed è inibita da composti stannorganici. La linea cellulare citotossica nei botrilli ha la sua tipica espressione nella cellula morulare, caratterizzata dalla presenza di fenolossidasi e da un substrato fenolico. Nell'area di contatto fra colonie incompatibili, cellule morulari, fuoriuscite con altre cellule dalle ampolle vascolari contrapposte, degranulandosi liberano l'enzima che ossida i fenoli a chinoni citotossici. Questi poi si polimerizzano a melanina che marca in bruno scuro gli ammassi cellulari necrotizzati.
Abbiamo studiato al microscopio elettronico il differenziamento della linea germinale femminile in botrillidi ovovivipari e vivipari, fin dal .costituirsi del blastoma gonadico nelle giovani gemme. Elemento discriminante fra le specie studiate, è nell'oogenesi l'assenza di tuorlo in Botrylloides violaceus, in contrapposizione con la vitellogenesi nell'ovocita di Botrylloides leachi e Botryllus schlosseri. Abbiamo seguito il differenziamento degli involucri ovulari, con particolare riguardo al coinvolgimento delle cellule follicolari esterne nella sintesi del tuorìo, e delle cellule testacee nel modellamento della tunica larvale. Lo sviluppo embrionale entro il corpo materno comporta formazione e differenziamento di strutture atte a trattenerlo e a nutrirlo. Il differenziamento della linea germinale maschile è stato studiato in B. schlosseri nel contesto della graduale sessualizzazione della colonia che comporta anche lo scambio di ovociti fra zooidi di differenti generazioni blastogenetiche. Abbiamo dimostrato sperimentalmente che migrano anche cellule germinali indifferenziate, ma determinate in senso maschile
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
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