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  • 1
    Electronic Resource
    Electronic Resource
    Sexual reproduction in the compound ascidian Diplosoma listerianum (Tunicata). II. Sperm penetration through ovary wall and evidence of internal fertilization (1994)
    Springer
    Marine biology 118 (1994), S. 499-510 
    Details
    ISSN: 1432-1793
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Diplosoma listerianum differs from most ascidians in that, at ovulation, eggs are emitted at the bottom of the ovary and segregated into the tunic, so that fertilization occurs far from seawater. A fertilization canal, a hollow extension of the ovary, conducts sperm towards the egg. In the present paper, ultrastructural evidence is reported on the morphological relationship between the ovary, egg envelopes and oocyte and on the mechanism by which sperm-egg interaction is established. In the ovary, the very complex sperm, equipped with a spiral “dense groove”, undergo metamorphosis as the first step in a sperm reaction and then pass through the ovary epithelium insinuating themselves between the intercellular junctions which appear to be mouldable, although able to maintain the egg-ovary barrier. Sperm then reach the vitelline coat, where a further step in the sperm reaction occurs. Before the egg abandons the ovary, the sperm head is incorporated into the oocyte by a process recalling phagocytosis, with the formation of an engulfing pocket. Sperm-egg contact and incorporation in D. listerianum occur in a way, never previously reported for other ascidians, in which fusion of plasma membranes takes place immediately after sperm-egg contact. Unlike other cytoplasmic components, the dense groove persists until the sperm enters the egg. It gives a corkscrew-like configuration to the sperm head and allows close adhesion to cell membranes, facilitating sperm movement. Expulsion of numerous cortical granules and features of a cortical reaction were observed in the egg penetrated by the sperm. The mode of internal fertilization of this species in comparison with that of other tunicates and phylogenetic aspects are discussed. Ripe colonies of D. listerianum collected in the Lagoon of Venice, Italy in 1986 and 1991 and colonies reared on glass in aquaria were used for our investigations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00350307
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  • 2
    Electronic Resource
    Electronic Resource
    Sexual reproduction in the compound ascidian Diplosoma listerianum (Tunicata). I. Metamorphosis, storage and phagocytosis of sperm in female duct (1994)
    Springer
    Marine biology 118 (1994), S. 489-498 
    Details
    ISSN: 1432-1793
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract In the colonial ascidian Diplosoma listerianum (collected in the Lagoon of Venice in 1986 and 1991), ovulating eggs detach themselves from the ovary wall and segregate in the tunic without exposure to seawater. With the aim of finding evidence of the pathway followed by spermatozoa to approach the oocyte, histological and ultrastructural observations were made. The results showed that the hollow ovary elongates in a “fertilization canal” which flanks the sperm duct and opens externally, next to the anus. Intercellular extended tight junctions isolate the lumen of this canal from the blood. Sperm were found in the fertilization canal, both free and in intracellular vacuoles of wandering cells (phagocytes). These sperm showed differences in comparison to those from the sperm duct, which are possible signs of a sperm reaction. The main modifications were shown by the “dense groove”, a narrow invagination of the plasmalemma bound to the nuclear envelope by dense material. In non-modified sperm, the groove runs spirally all along the head, while in those in the fertilization canal, it coils in the anterior half of the head, pressing back the long mitochondrion and endoplasmic tubules. These modifications were interpreted to be result of the release of the dense groove, considered to be a sort of stretched, contractile spring. The presence of sperm in the fertilization canal and particularly in phagocytes is discussed in relation to the capacity for exogenous sperm storage by zooids and to the necessity of waste clearance in order for successive waves of fresh sperm to be able to move towards fully grown oocytes. The origin of the female fertilization canal, the role of its tight junctions, and the complex sperm morphology are discussed as adaptations to internal fertilization in D. listerianum.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00350306
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  • 3
    Electronic Resource
    Electronic Resource
    Muscle cell differentiation in the ascidian heart
    Amsterdam : Elsevier
    Developmental Biology 68 (1979), S. 371-380 
    Details
    ISSN: 0012-1606
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Biology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1016/0012-1606(79)90211-2
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  • 4
    Electronic Resource
    Electronic Resource
    A comparative study on ciliary differentiation in the ascidian branchial stigmata
    Amsterdam : Elsevier
    Cell Biology International Reports 10 (1986), S. 171 
    Details
    ISSN: 0309-1651
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Biology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1016/S0309-1651(86)80025-X
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  • 5
    Electronic Resource
    Electronic Resource
    A comparative study on ciliary differentiation in the ascidian branchial stigmata
    Amsterdam : Elsevier
    Cell Biology International Reports 10 (1986), S. 171 
    Details
    ISSN: 0309-1651
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Biology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1016/S0309-1651(86)80025-X
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  • 6
    Electronic Resource
    Electronic Resource
    Junctional complexes of the branchia and gut of the tunicate, Pyrosoma atlanticum (Pyrosomatida, Thaliacea) (1992)
    Springer
    Cell & tissue research 267 (1992), S. 357-364 
    Details
    ISSN: 1432-0878
    Keywords: Branchial stigmata ; Gut ; Epithelium ; Cell junctions ; Pyrosomatida, Pyrosoma atlanticum (Tunicata)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Summary The branchia of pyrosomes has been found to be like that of ascidians in that it shares with the latter, besides the presence of peribranchial chambers, stigmata bordered by clusters of seven rows of flattened cells, each bearing a single row of long cilia. The intercellular junctions between ciliated cells of the branchial basket and between the cells lining the gut, in pyrosomes, have been studied in thin sections and by freeze-fracture. In both tissues tight and gap junctions are present, but the former are much more extensive in the gut. The gap junctions in the branchial basket exhibit some atypical features. Moreover, although there are extensive zonulae adhaerentes between the ciliated cells of the branchial basket, there are none between the epithelial cells of the intestinal tract. This feature of the branchiae, together with the alignment of their cells into highly ordered rows of seven cells, are similar to those found in some groups of ascidians. The evolutionary relationships between pyrosomes and the aplousobranch ascidians are considered in the light of these results.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00302974
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  • 7
    Electronic Resource
    Electronic Resource
    Involution of the caudal musculature during metamorphosis in the ascidian, Botryllus schlosseri (1974)
    Springer
    Cell & tissue research 153 (1974), S. 293-305 
    Details
    ISSN: 1432-0878
    Keywords: Ascidia (Botryllus schlosseri) ; Metamorphosis ; Muscle tissue involution ; Phagocytes ; Electron microscopy
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Summary The caudal musculature of the free-swimming tadpole of the ascidian, B. schlosseri consists of cylindrical mononucleated cells connected in longitudinal rows flanking the axial notochord. During resorption of the larval tail, which is apparently induced by the contraction of the epidermis, muscle cells are dissociated and pushed into the body cavity where most of them are rapidly engulfed by phagocytes. In the initial stages of tail withdrawal muscle cells display surface alterations due to the disruption of intercellular junctions and disarrangement of myofibrils. Extensive degenerative changes, with shrinkage of mitochondria and disintegration of the contractile material are subsequently observed. Lysosomes and autophagic vacuoles are rarely seen and appear to play a secondary role in the degradation of the muscle cells, which occurs predominantly within the phagocytes. Myofilaments and myofibrils have never been observed within autophagic vacuoles. Clumps of muscle fragments and degenerated phagocytes undergo eventual dissolution in the blood lacunae, concomitantly with the differentiation of the young oozooid.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00229160
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  • 8
    Electronic Resource
    Electronic Resource
    Degenerative regression of the digestive tract in the colonial ascidian Botryllus schlossen (Pallas) (1984)
    Springer
    Cell & tissue research 235 (1984), S. 309-318 
    Details
    ISSN: 1432-0878
    Keywords: Ascidian ; Gut ; Cell involution ; Ultrastructure ; Phagocytes
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Summary Degenerative changes in the digestive tract of zooids of Botryllus schlosseri were studied by light and electron microscopy. Three main processes occurred in the tissues: contraction, involution and phagocytosis. The contraction of epidermis and peribranchial epithelium in which cytoplasmic microfilaments probably participate, seemed to have a special role in compressing the underlying organs. During contraction most of the body cavities collapsed, the branchial walls disintegrated and the fragments were rapidly taken up by large phagocytes. The gut epithelium retained its apparent continuity longer, though isolated phagocytes infiltrated it to eliminate single cells. Cell degeneration came about chiefly either through swelling and lysis of cells or through loss of water and condensation of cytoplasm and nucleus. The fate of all regressed tissues was to be engulfed and digested by wandering phagocytes. However, it was also observed that numerous cells of different epithelia could act as fixed phagocytes by engulfing cell debris and entire cells into heterophagic vacuoles.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00217855
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  • 9
    Electronic Resource
    Electronic Resource
    Unusual features of intercellular junctions in the larvacean Oikopleura dioica (1990)
    Springer
    Cell & tissue research 260 (1990), S. 299-305 
    Details
    ISSN: 1432-0878
    Keywords: Intercellular junctions ; Alimentary tract ; Freeze fracture ; Oikopleura dioica (Copelata)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Summary In the pelagic larvacean Oikopleura dioica, the epithelium lining the alimentary tract consists of ciliated and unciliated cell types. The ciliated cells also exhibit an apical border of long microvilli. Between the microvilli, the cellular membrane often projects deeply down into the cytoplasm; the membranes of these invaginations and those of apicolateral interdigitations may be associated with one another by tight junctions. Some of these junctions may be autocellular. The tight junctions are seen by freeze-fracture to be very simple in construction, composed of a single row of intramembranous particles, which may be fused into a P-face ridge. There is a dense cytoplasmic fuzz associated with these tight junctions which may extend into adjoining zonula adhaerens-like regions. The invaginations of the apical membranes are, in addition, associated by gap junctions which may also be autocellular. More conventional homocellular and heterocellular tight and gap junctions occur along the lateral borders of ciliated cells and between ciliated and unciliated cells. These gap junctions possess a reduced intercellular cleft and typical P-face connexons arranged in macular plaques, with complementary E-face pits. Both cell types exhibit extensive stacks of basal and lateral interdigitations. The tight junctions found here are unusual in that they are associated with a dense cytoplasmic fuzz which is normally more characteristic of zonulae adhaerentes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00318632
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  • 10
    Electronic Resource
    Electronic Resource
    Unusual structures in the spermatozoa of the ascidians Lissoclinum perforatum and Diplosoma listerianum (Didemnidae) (1985)
    Springer
    Cell & tissue research 241 (1985), S. 513-521 
    Details
    ISSN: 1432-0878
    Keywords: Spermiogenesis ; Spermatozoa ; Mitochondrion ; Nucleus ; Ascidians
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Summary Sperm and spermiogenesis of L. perforatum and D. listerianum have been investigated by light- and electron microscopy. The study revealed basic ascidian organization of sperm cells with the nucleus flanked by a unique mitochondrion, but with specializations not previously reported: a dense groove on the head surface, extended nucleus-mitochondrion association, and well developed endoplasmic derivatives. The groove is a long furrow of the plasmalemma stretching the nucleus towards the cell periphery. At least two parallel, 20 nm high, ridges emerge from the bottom of the groove. At the opposite side of the groove the long mitochondrion embraces the nucleus by extending its outer membrane over the nuclear envelope; a restricted cytoplasmic space is always found between the two organelles. Filamentous inclusions organized in two strands are present in the mitochondrial matrix. Endoplasmic derivatives appear as a single, extended cistern in L. perforatum, and series of long tubules in D. listerianum. They appear to originate in the spermatid from the Golgi complex and endoplasmic reticulum. The spermatozoon lacks an evident acrosomal complex, but the possible homology of small apical densities with an acrosomal complex is considered. It is suggested that the functional role of these specialized structures is related to the special mode of internal fertilization of these species.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00214570
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