The ontology of the anatomy and development of the solitary ascidian Ciona: the swimming larva and its metamorphosis

Ciona robusta (Ciona intestinalis type A), a model organism for biological studies, belongs to ascidians, the main class of tunicates, which are the closest relatives of vertebrates. In Ciona, a project on the ontology of both development and anatomy is ongoing for several years. Its goal is to standardize a resource relating each anatomical structure to developmental stages. Today, the ontology is codified until the hatching larva stage. Here, we present its extension throughout the swimming larva stages, the metamorphosis, until the juvenile stages. For standardizing the developmental ontology, we acquired different time-lapse movies, confocal microscope images and histological serial section images for each developmental event from the hatching larva stage (17.5 h post fertilization) to the juvenile stage (7 days post fertilization). Combining these data, we defined 12 new distinct developmental stages (from Stage 26 to Stage 37), in addition to the previously defined 26 stages, referred to embryonic development. The new stages were grouped into four Periods named: Adhesion, Tail Absorption, Body Axis Rotation, and Juvenile. To build the anatomical ontology, 203 anatomical entities were identified, defined according to the literature, and annotated, taking advantage from the high resolution and the complementary information obtained from confocal microscopy and histology. The ontology describes the anatomical entities in hierarchical levels, from the cell level (cell lineage) to the tissue/organ level. Comparing the number of entities during development, we found two rounds on entity increase: in addition to the one occurring after fertilization, there is a second one during the Body Axis Rotation Period, when juvenile structures appear. Vice versa, one-third of anatomical entities associated with the embryo/larval life were significantly reduced at the beginning of metamorphosis. Data was finally integrated within the web-based resource "TunicAnatO", which includes a number of anatomical images and a dictionary with synonyms. This ontology will allow the standardization of data underpinning an accurate annotation of gene expression and the comprehension of mechanisms of differentiation. It will help in understanding the emergence of elaborated structures during both embryogenesis and metamorphosis, shedding light on tissue degeneration and differentiation occurring at metamorphosis.

The ascidian Styela plicata hemocytes as a potential biomarker of marine pollution: In vitro effects of seawater and organic mercury

Toxic metals, such as mercury, contribute substantially to anthropogenic pollution in many estuarine environments. Animals living in those environments, particularly invertebrate filter feeders like tunicates, can be used as bioindicators. In an attempt to identify cellular markers for revealing pollution, this study examined in vitro the effects of different concentrations of methyl mercury on Styela plicata hemocytes. The harvested hemocytes from S. plicata that were exposed to the metal had a significant mortality, cellular count and morphometric alterations. These findings provided evidence of MeHg immunotoxic effects on S. plicata, resulting in hemocyte death and morphological changes induced by cytoskeleton alterations. Thus, a morphometric cellular parameter, such as spreading ability, was used as a complementary method for differentiation between hemocytes treated with a marine solution (as a negative control) and hemocytes incubated with methylmercury and/or Sicilian seawater samples.

Evolutionary diversification of secondary mechanoreceptor cells in tunicata

BACKGROUND

Hair cells are vertebrate secondary sensory cells located in the ear and in the lateral line organ. Until recently, these cells were considered to be mechanoreceptors exclusively found in vertebrates that evolved within this group. Evidence of secondary mechanoreceptors in some tunicates, the proposed sister group of vertebrates, has recently led to the hypothesis that vertebrate and tunicate secondary sensory cells share a common origin. Secondary sensory cells were described in detail in two tunicate groups, ascidians and thaliaceans, in which they constitute an oral sensory structure called the coronal organ. Among thaliaceans, the organ is absent in salps and it has been hypothesised that this condition is due to a different feeding system adopted by this group of animals. No information is available as to whether a comparable structure exists in the third group of tunicates, the appendicularians, although different sensory structures are known to be present in these animals.

RESULTS

We studied the detailed morphology of appendicularian oral mechanoreceptors. Using light and electron microscopy we could demonstrate that the mechanosensory organ called the circumoral ring is composed of secondary sensory cells. We described the ultrastructure of the circumoral organ in two appendicularian species, Oikopleura dioica and Oikopleura albicans, and thus taxonomically completed the data collection of tunicate secondary sensory cells. To understand the evolution of secondary sensory cells in tunicates, we performed a cladistic analysis using morphological data. We constructed a matrix consisting of 19 characters derived from detailed ultrastructural studies in 16 tunicate species and used a cephalochordate and three vertebrate species as outgroups.

CONCLUSIONS

Our study clearly shows that the circumoral ring is the appendicularian homologue of the coronal organ of other tunicate taxa. The cladistic analysis enabled us to reconstruct the features of the putative ancestral hair cell in tunicates, represented by a simple monociliated cell. This cell successively differentiated into the current variety of oral mechanoreceptors in the various tunicate lineages. Finally, we demonstrated that the inferred evolutionary changes coincide with major transitions in the feeding strategies in each respective lineage.

Azumiobodo hoyamushi gen. nov. et sp. nov. (Euglenozoa, Kinetoplastea, Neobodonida): a pathogenic kinetoplastid causing the soft tunic syndrome in ascidian aquaculture

We used morphological and genetic analyses to investigate a pathogenic kinetoplastid isolated from a diseased edible ascidian Halocynthia roretzi with soft tunic syndrome. The morphological characteristics of the kinetoplastid are similar to those in the order Neobodonida in the subclass Metakinetoplastida. However, the presence of unique globular bodies distinguishes this kinetoplastid from the other polykinetoplastic genera (i.e. Cruzella, Dimastigella and Rhynchobodo) in this order. These globular bodies are cytoplasmic inclusions without an outer delimiting membrane and are composed of a homologous granular matrix containing electron-dense bands. A phylogenetic tree based on 18S rRNA gene sequences also indicated that the kinetoplastid belongs to the order Neobodonida, although it forms an independent clade in this order. From these results, we propose a new genus in the order Neobodonida, i.e. Azumiobodo gen. nov., and Azumiobodo hoyamushi as the type species for the genus.

Forming a tough shell via an intracellular matrix and cellular junctions in the tail epidermis of Oikopleura dioica (Chordata: Tunicata: Appendicularia)

A postanal tail is a major synapomorphy of the phylum Chordata, which is composed of three subphyla: Vertebrata, Cephalochordata, and Tunicata (Urochordata). Among tunicates, appendicularians are the only group that retains the tail in the adult, and the adult tail functions in locomotion and feeding in combination with a cellulose-based house structure. Given the phylogenetic position of tunicates, the appendicularian adult tail may possess ancestral features of the chordate tail. We assess the ultrastructural development of the tail epidermis of the appendicularian Oikopleura dioica. The epidermis of the larval tail is enclosed by the larval envelope, which is a thin sheet similar to the outer tunic layer of ascidian larvae. The epidermis of the adult tail seems to bear no tunic-like cellulosic integuments, and the tail fin is a simple folding of the epidermis. Every epidermal cell, except for the triangular cells at the edge of the tail fin, has a conspicuous matrix layer of fibrous content in the apical cytoplasm without enclosing membranes. The epidermis of the larval tail does not have a fibrous matrix layer, suggesting the production of the layer during larval development and metamorphosis. Zonulae adhaerentes firmly bind the epidermal cells of the adult tail to one another, and the dense microfilaments lining the cell borders constitute a mechanical support for the cell membranes. The intracellular matrix, cell junctions, and cytoskeletons probably make the tail epidermis a tough, flexible shell supporting the active beating of the oikopleuran adult tail.

Tunic morphology and viral surveillance in diseased Korean ascidians: Soft tunic syndrome in the edible ascidian, Halocynthia roretzi (Drasche), in aquaculture

'Soft tunic syndrome' causes mass mortality in the edible ascidian Halocynthia roretzi in Korean and Japanese aquaculture. In histopathological comparison, there were no specific differences between diseased specimens from Korea and Japan, indicating that soft tunic syndrome occurring in Korea and Japan is the same disease. No bacterial or protozoan cells were microscopically detected in either healthy or diseased tunics suggesting they are not the direct causes of soft tunic syndrome. Attempts were made to isolate virus from affected ascidians taking into account temperature conditions in which soft tunic syndrome is most prevalent in the field. However, no viruses were isolated from diseased or non-diseased specimens using chinook salmon embryo (CHSE-214), flounder fin (FFN) or epithelioma papillosum cyprini (EPC) cell lines.

Muscle differentiation in a colonial ascidian: organisation, gene expression and evolutionary considerations

BACKGROUND

Ascidians are tunicates, the taxon recently proposed as sister group to the vertebrates. They possess a chordate-like swimming larva, which metamorphoses into a sessile adult. Several ascidian species form colonies of clonal individuals by asexual reproduction. During their life cycle, ascidians present three muscle types: striated in larval tail, striated in the heart, and unstriated in the adult body-wall.

RESULTS

In the colonial ascidian Botryllus schlosseri, we investigated organisation, differentiation and gene expression of muscle beginning from early buds to adults and during zooid regression. We characterised transcripts for troponin T (BsTnT-c), adult muscle-type (BsMA2) and cytoplasmic-type (BsCA1) actins, followed by in situ hybridisation (ISH) on sections to establish the spatio-temporal expression of BsTnT-c and BsMA2 during asexual reproduction and in the larva. Moreover, we characterised actin genomic sequences, which by comparison with other metazoans revealed conserved intron patterns.

CONCLUSION

Integration of data from ISH, phalloidin staining and TEM allowed us to follow the phases of differentiation of the three muscle kinds, which differ in expression pattern of the two transcripts. Moreover, phylogenetic analyses provided evidence for the close relationship between tunicate and vertebrate muscle genes. The characteristics and plasticity of muscles in tunicates are discussed.

Morphological characterization of the tunic in the edible ascidian, Halocynthia roretzi (Drasche), with remarks on 'soft tunic syndrome' in aquaculture

'Soft tunic syndrome' is a serious problem in the aquaculture of the edible ascidian, Halocynthia roretzi (Drasche), and often leads to mass mortality. Here, we describe the tunic morphology of intact and diseased ascidians to reveal structural differences between them. Morphologically, diseased tunics are not very different from intact tunics, although the former are thinner and softer than the latter. While several types of cells are distributed in the tunic, the cell types and their cytomorphologies were almost identical in both groups. As bacterial/protozoan cells were not found in either intact or diseased tunics, they are not the direct cause of soft tunic syndrome. The most remarkable difference was in the bundles of tunic fibres that compose the tunic matrix; in intact tunics, the thick bundles interlace to form a firm matrix, whereas in soft tunics, the tunic fibres do not form thick bundles. Furthermore, areas of low fibre density were found in diseased tunics. Therefore, soft tunic syndrome probably causes inhibition of bundle formation and degradation of tunic bundles, creating areas of low fibre density, although the causes remain unknown.

Postembryonic epigenesis of Vasa-positive germ cells from aggregated hemoblasts in the colonial ascidian, Botryllus primigenus

We investigated whether Vasa was a germline-specific marker in the colonial ascidian Botryllus primigenus, and whether it was inducible epigenetically in the adult life span. We cloned a Botryllus Vasa homologue (BpVas). The deduced open reading frame encoded 687 amino acid residues. It was expressed specifically by germline cells such as the loose cell mass, oogonia and juvenile oocytes in the ovary, and the primordial testis (compact cell mass), spermatogonia and juvenile spermatocytes in the testis. The loose cell mass, the most primitive germline cells, showed an ultrastructure of undifferentiated cells known as hemoblasts. The hemoblasts did not contain electron-dense materials or a mitochondrial assembly in the cytoplasm. These organelles appeared later in the oogonia and oocytes. When the loose cell mass and developing germ cells were eliminated by extirpating all zooids and buds from the colonies, BpVas transcripts disappeared completely from the vascularized colonies. After 14 days, when the colonies regenerated by vascular budding, BpVas-positive cells reappeared in some cases, and in 30 day colonies, BpVas-positive germ cells were observed in all the regenerated colonies. These results show that in B. primigenus, germ cells are inducible de novo from the Vasa-negative cells even at postembryonic stages.

Tunic phagocytes are involved in allorejection reaction in the colonial tunicate Aplidium yamazii (Polyclinidae, Ascidiacea)

The colonial ascidian Aplidium yamazii exhibited an allorejection reaction when two allogeneic colonies were brought into contact at their growing edges or at artificial cut surfaces. This species has no vascular network in the tunic, unlike the botryllid ascidians, which have a vascular network throughout the colony's common tunic. In the allorejection reaction induced by contact at the growing edges, some small, hard-packed tunic masses were formed at the contact points. Histological and electron microscopic investigation of these tunic masses revealed that they contained aggregates of tunic cells, with tunic phagocytes being the major cell type present. Some of the tunic phagocytes in these tunic masses appeared to be disintegrating. When allogeneic colonies were placed in contact at their artificial cut surfaces, the colonies partially fused, then separated. In this allorejection reaction, some loosely packed tunic masses remained in the gap between the two withdrawn colonies. These results strongly suggest that the tunic phagocytes are likely to be the major effector cells in the allorejection reaction. We also propose that the tunic phagocytes are not only the effector cells in the allorejection reaction but also bear the sites of allorecognition.

Vertical transmission of photosymbionts in the colonial ascidian Didemnum molle: the larval tunic prevents symbionts from attaching to the anterior part of larvae

Morphological processes in the vertical transmission of photosymbionts were investigated in the Prochloron-bearing ascidian Didemnum molle. Prochloron cells were found exclusively in the common cloacal cavity of the colony, attached mainly to the tunic lining of the cavity wall. Oocytes were found in the abdominal region of each zooid, but no Prochloron cells were associated with this stage. During embryogenesis, embryos moved into the tunic core of the colony and were always separated from Prochloron cells in the cloacal cavity by the tunic matrix, until they hatched out from the tunic core. In swimming larvae, Prochloron cells covered the surface of the posterior half of the larval trunk, whereas a thin larval tunic layer covered the anterior half, where no Prochloron cells were found. The tunic of the posterior half of the larval trunk had many folds that enfolded the Prochloron cells and may be adhesive in order to acquire Prochloron cells from the mother colony. The thin larval tunic layer is probably not adhesive and protects the anterior half of the trunk from interference by Prochloron cells with sensory receptors and adhesive organs.

Conserved patterns of nuclear compartmentalization are not observed in the chordate Oikopleura

BACKGROUND INFORMATION

Recent results from a limited number of eukaryotic model organisms suggest that major principles governing spatial organization of the genome in functionally distinct nuclear compartments are conserved through evolution.

RESULTS

We examined the in situ spatial organization of major nuclear components and nuclear patterns of gene loci with strictly defined expression patterns in endocycling cells of the transparent urochordate Oikopleura dioica, a complex metazoan with a very compact genome. Endocycling cells with different functions and similar DNA content displayed distinct topologies of nuclear components. However, the generation of the diverse nuclear architectures did not involve specific local organization of active genes or their preferential amplification. Interestingly, endocycling cells lacked nuclear-envelope-associated heterochromatin and prominent splicing-factor domains, which in mammalian cells associate with transcriptionally silent and active loci respectively. In addition, no correlation was found between transcriptional activity of a locus and its association with chromatin domains rich in specific histone modifications.

CONCLUSIONS

Together, these findings and the absence of typical eukaryotic replication patterns reveal a surprisingly limited functional compartmentalization of O. dioica endocycling nuclei. This indicates that robust cell-type-specific gene expression does not necessarily require high levels of spatial genome organization.

Body Muscle-Cell Differentiation from Coelomic Stem Cells in Colonial Tunicates

Body muscle-cell differentiation was ultrastructurally examined in palleal buds of the colonial tunicate Symplegma reptans. Undifferentiated coelomic cells accumulate near the primordial oral siphon and associate with the basal lamina beneath the epidermis. They initially display the characteristics of hemoblast cells that have a large nucleus with a prominent nucleolus and narrow cytoplasm filled with polysomes. However, they soon become unique due to the development of an indented contour of the nucleus. When the basal lamina of the epidermis develops into the fibrous extracellular matrix (ECM), the muscle precursor cell has the deeply-notched nucleus, and thick and thin filaments in the cytoplasm facing the ECM. Collagen fibril-like structures appear in the ECM. Myofilaments are arranged with the ratio of thick to thin filaments being 1:2.5. Dense bodies and plaques become evident before the oral siphon is perforated. These results show that in S. reptans, the sphincter muscle cells arise from undifferentiated hemoblasts, and that their differentiation begins with a morphological change in their nuclei. Epidermal cells and/or the ECM may have an inductive effect on muscle cell differentiation.

The Oikopleura coenocyst, a unique chordate germ cell permitting rapid, extensive modulation of oocyte production

The ability to adjust reproductive output to environmental conditions is important to the fitness of a species. The semelparous, chordate, Oikopleura dioica, is particularly adept in producing a highly variable number of oocytes in its short life cycle. Here we show that this entails an original reproductive strategy in which the entire female germline is contained in a single multinucleate cell, the "coenocyst". After an initial phase of syncytial nuclear proliferation half of the nuclei entered meiosis whereas the other half became highly polyploid. The inner F-actin network, with associated plasma membranes, formed a highly ramified infrastructure in which each meiotic nucleus was contained in a pseudo-compartmentalized pro-oocyte linked to the common cytoplasm via ring canals. At a set developmental time, a subset of the pro-oocytes was selected for synchronous growth and the common coenocyst cytoplasm was equally partitioned by transfer through the ring canals. Examination of related species indicated that the coenocyst arrangement is a conserved feature of Appendicularian oogenesis allowing efficient numerical adjustment of oocyte production. As Appendicularia are the second most abundant class of zooplankton, with a world-wide distribution, the coenocyst is clearly a common and successful reproductive strategy on a global scale.

The cytoskeleton organizes germ nuclei with divergent fates and asynchronous cycles in a common cytoplasm during oogenesis in the chordate Oikopleura

Germline cysts are conserved structures in which cells initiating meiosis are interconnected by ring canals. In many species, the cyst phase is of limited duration, but the chordate, Oikopleura, maintains it throughout prophase I as a unique cell, the coenocyst. We show that despite sharing one common cytoplasm with meiotic and nurse nuclei evenly distributed in a 1:1 ratio, both entry into meiosis and subsequent endocycles of nurse nuclei were asynchronous. Coenocyst cytoskeletal elements played central roles as oogenesis progressed from a syncytial state of indistinguishable germ nuclei, to a final arrangement where the common cytoplasm had been equally partitioned into resolved, mature oocytes. During chromosomal bouquet formation in zygotene, nuclear pore complexes clustered and anchored meiotic nuclei to the coenocyst F-actin network opposite ring canals, polarizing oocytes early in prophase I. F-actin synthesis was required for oocyte growth but movement of cytoplasmic organelles into oocytes did not require cargo transport along colchicine-sensitive microtubules. Instead, microtubules maintained nurse nuclei on the F-actin scaffold and prevented their entry into growing oocytes. Finally, it was possible to both decouple meiotic progression from cellular mechanisms governing oocyte growth, and to advance the timing of oocyte growth in response to external cues.

Localization of symbiotic cyanobacteria in the colonial ascidian Trididemnum miniatum (Didemnidae, Ascidiacea)

Trididemnum miniatum is a colonial ascidian harboring the photosymbiotic prokaryote Prochloron sp. These bacterial cells are located in the tunic of the host animal. The present study revealed, by ultrastructural analysis, that the Prochloron cells were exclusively distributed and proliferated in the tunic. They were shown to be embedded in the tunic matrix and to have no direct contact with ascidian cells. Some tunic cells of the ascidians, however, did phagocytize and digest the symbiont. Round cell masses were sometimes found in the tunic and appeared to consist of disintegrating cyanobacterial cells. The thoracic epidermis of ascidian zooids was often digitated, and the epidermal cells extended microvilli into the tunic. Since there were no Prochloron cells in the alimentary tract of the ascidian zooids, the photosymbionts would not be considered part of the typical diet of the host ascidians. Thin layer chromatography showed that the symbionts possessed both chlorophyll a and b, while a 16S rRNA gene phylogeny supported the identification of the photosymbiont of T. miniatum as Prochloron sp.

Programmed cell death in vegetative development: Apoptosis during the colonial life cycle of the ascidian Botryllus schlosseri

Programmed cell death (PCD) by apoptosis is a physiological mechanism by which cells are eliminated during embryonic and post-embryonic stages of animal life cycle. During asexual reproduction, the zooids of colonial ascidians originate from an assorted cell population instead of a single zygote, so that we assume that regulation of the equilibrium among proliferation, differentiation and cell death may follow different pathways in comparison to the embryonic development. Here we investigate the presence of apoptotic events throughout the blastogenetic life cycle of the colonial ascidian Botryllus schlosseri, by means of terminal deoxynucleotidyl transferase dUTP Nick End Labeling (TUNEL) coupled with histochemical and electron microscopy techniques. The occurrence of low levels of morphogenetic cell death suggests that, in contrast to what happens during sexual development (embryogenesis and metamorphosis), apoptosis does not play a pivotal role during asexual propagation in botryllid ascidian. Nevertheless, PCD emerges as a key force to regulate homeostasis in adult zooids and to shape and modulate the growth of the whole colony.

Coronal organ of ascidians and the evolutionary significance of secondary sensory cells in chordates

A 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.

RGD-containing molecules induce macropinocytosis in ascidian hyaline amoebocytes

Phagocytes of the compound ascidian Botryllus schlosseri are capable of constitutive macropinocytosis (MP) at sites of membrane ruffling along the leading edge. This gives rise to the formation of initially irregular vesicles which then move to the inside of the cells and acquire a more regular morphology. Both phagocyte spreading and MP are enhanced by the recognition of molecules containing the sequence Arg-Gly-Asp (RGD): this suggests that, as in mammals, integrin activation is involved in the induction of both cell spreading and endocytosis. The occurrence of MP is associated with increased oxygen consumption and a rise in the production of superoxide anion, as indicated by nitroblue tetrazolium reduction, and ATP, as indicated by increased cytochrome oxidase activity. On the whole, our results indicate the conservation of common mechanisms of MP induction throughout the Chordate phylum.

Developmental biology meets materials science: Morphogenesis of biomineralized structures

Biomineralization is the process by which metazoa form hard minerals for support, defense, and feeding. The minerals so formed, e.g., teeth, bones, shells, carapaces, and spicules, are of considerable interest to chemists and materials scientists. The cell biology underlying biomineralization is not well understood. The study of the formation of mineralized structures in developing organisms offers opportunities for understanding some intriguing aspects of cell and developmental biology. Five examples of biomineralization are presented: (1) the formation of siliceous spicules and frustules in sponges and diatoms, respectively; (2) the structure of skeletal spicules composed of amorphous calcium carbonate in some tunicates; (3) the secretion of the prism and nacre of some molluscan shells; (4) the development of skeletal spicules of sea urchin embryos; and (5) the formation of enamel of vertebrate teeth. Some speculations on the cellular and molecular mechanisms that support biomineralization, and their evolutionary origins, are discussed.

Cellular aspects of allorecognition in the compound ascidian Botryllus schlosseri

We studied changes in the morphology of morula cells, a common haemocyte type in botryllid ascidians, during both the rejection reaction (occurring between contacting, genetically incompatible colonies) and fusion (occurring between compatible colonies), and in short-term cultures of haemocytes incubated with heterologous or autologous blood plasma. In both the rejection reaction and haemocyte cultures in the presence of heterologous blood plasma, we observed alterations in morula cells, consistent with a degranulation event, and their expression of molecules recognised by anti-IL-1-alpha- and anti-TNF-alpha-antibodies. Anti-cytokine-antibodies markedly reduced the extent of the in vitro cytotoxicity, when haemocytes were exposed to heterologous blood plasma. In addition, the increase in the production of nitrite ions and the decrease of the in vitro cytotoxicity by the nitric oxide synthase inhibitor N(omega)-nitro-L-arginine methyl ester, suggest the role of nitric oxide in cell death. These results provide new clues to understand the process of rejection reaction in botryllid ascidians.

Early stages of test formation in larva of Ascidia malaca (Tunicata, Ascidiacea): ultrastructural and cytochemical investigations

The swimming larvae of ascidians are entirely covered by a hyalin coat called tunic, or test. This covering consists of two cuticular layers, C1 and C2, which surround an inner compartment composed of an amorphous hyalin matrix with numerous fibrils dispersed inside. Data from the literature agree on the key role played by the cells of the larval ectodermic layer in the synthesis and secretion of larval test components. In the present article are reported ultrastructural and cytochemical investigations made during test formation in the swimming larva of Ascidia malaca. Besides confirming the role played by ectodermic cells during the early stages of test formation, the investigations highlight the way in which the fibrillar component of the test is synthetized and secreted. At the ultrastructural level it has been evidenced that the C1 and C2 cuticular layers originate from the tight packing of fibrils. Based on the data reported in the present study, it is hypothesized that while a relevant part of the fibrils, once secreted, remains dispersed inside the matrix of the inner compartment of the test, quite likely in order to increase its consistency, packing of the remaining fibrils leads to the formation of the C1 and C2 cuticular layers. Packing of the fibrils in C1 and C2 could be favoured by their chemically adhesive nature. This hypothesis is strongly supported by the herewith reported results of the cytochemical investigations carried out on the test of the swimming larva of A. malaca. The cytochemical PA-TCH-SP reaction has in fact evidenced that both fibril types, i.e. those dispersed inside the inner compartment and those packed in the C1 and C2 cuticular layers, are constituted by glycoproteins and/or proteoglycans substances whose adhesive properties are well documented in the literature.

Functional role of test cells in swimming larvae of Ascidia malaca: ultrastructural and cytochemical investigations

The functional role played by test cells in larvae of various ascidian species consists in depositing sub-microscopic structures known as ornaments and/or proteoglycan substances on the larval test surface. According to the data reported in the literature, the deposition of ornaments together with proteoglycan substances on the larval test would render the latter hydrophilic and thus allow the larva to swim being immersed in water. Ornament deposition on the larval test does not occur in all the ascidian species. Ultrastructural investigations made on larvae belonging to the Cionidae and Ascididae families, for instance, have failed to evidence the presence of ornaments on the test. For these ascidian families it has been hypothesized that in swimming larvae test cells secrete an amorphous substance that would allow them to adhere to the larval test. In order to ascertain the functional role played by test cells in swimming larvae of the Ascididae family, the presently reported ultrastructural and cytochemical investigations have been made on larvae of Ascidia malaca. Besides suggesting that test cells, tightly adherent to the test surface, present an amoeboidic behaviour, the ultrastructural investigations have evidenced that these cells are still metabolically active. Their cytoplasm, characterized by the presence of a Golgi apparatus actively involved in synthesis, is almost entirely filled with very large granules; some of them gradually empty their contents turning into vacuoles containing scarce residues of electrondense particles. The present ultrastructural observations support the hypothesis that the adhesion of test cells on the larval test could be very likely eased by the secretion of substances synthesized by the Golgi and released through pseudopodes which test cells then wedge into the test. The cytochemical investigations were based on a reaction (fixation in glutaraldehyde-tannic acid) which evidences the presence, at the ultrastructural level, of proteoglycan substances such as glycosaminoglycans (Singley and Solursh, 1980). The reaction has given positive results in test cell granules undergoing emptying, on the outer membrane of the same cells, and on the outer cuticular layer C1 of the larval test. The present investigations, besides confirming the absence of ornament deposition on the test surface by test cells of Ascidia malaca swimming larvae, have evidenced that the secretion products deposited on the larval test surface by test cells consist of glycosaminoglycans, i.e. proteoglycan substances. In agreement with the data reported in the literature, it is hypothesized that the deposition of glycosaminoglycans on the surface of Ascidia malaca larval test makes the larval tunic hydrophilic and thus the larva is able to swim being immersed in water.

Alimentary tract of kowalevskiidae (appendicularia, tunicata) and evolutionary implications

The alimentary tract of Kowalevskia tenuis and K. oceanica, the only species of the appendicularian family Kowalevskiidae, was studied both at the light and electron microscope levels and compared with species belonging to the other two families of the class. Kowalevskids show interesting specializations: 1) the pharynx opens on both sides through two opposing spiracles, modified into long ciliated fissures, and possesses an original filtering system of ciliated combs arranged in two pairs of opposing longitudinal rows; 2) the endostyle is absent, its place being taken by a ciliated groove without any glandular cell; 3) posterior to the esophagus, the globular stomach and rectum form a digestive nucleus comprising a few, large cells including two well-developed, specialized valves, cardiac and pyloric; 4) special apical junctions bearing characteristics of both gap and adherens junctions are diffuse along the gut epithelium; 5) the heart is absent. Our data suggest that Kowalevskiidae underwent a high degree of specialization for food filtering and are more closely related to Fritillariidae, with which they share several characters, rather than Oikopleuridae, the latter probably representing the most primitive family of appendicularians.

Novel, secondary sensory cell organ in ascidians: in search of the ancestor of the vertebrate lateral line

A new mechanoreceptor organ, the "coronal organ," located in the oral siphon, is described by light and electron microscopy in the colonial ascidians Botryllus schlosseri and Botrylloides violaceus. It is composed of a line of sensory cells (hair cells), accompanied by supporting cells, that runs continuously along the margin of the velum and tentacles of the siphon. These hair cells resemble those of the vertebrate lateral line or, in general, the acoustico-lateralis system, because they bear a single cilium, located centrally or eccentrically to a hair bundle of numerous stereovilli. In contrast to other sensory cells of ascidians, the coronal hair cells are secondary sensory cells, since they lack axonal processes directed towards the cerebral ganglion. Moreover, at their base they form synapses with nerve fibers, most of which exhibit acetylcholinesterase activity. The absence of axonal extensions was confirmed by experiments with lipophilic dyes. Different kinds of synapses were recognized: usually, each hair cell forms a few afferent synapses with dendrites of neurons located in the ganglion; efferent synapses, both axo-somatic (between an axon coming from the ganglion and the hair cell) and axo-dendritic (between an axon coming from the ganglion and an afferent fiber) were occasionally found. The presence of secondary sensory cells in ascidians is discussed in relation to the evolution of sensory cells and placodes in vertebrates. It is proposed that the coronal organ in urochordates is homologous to the vertebrate acoustico-lateralis system.

Adrenocorticotropin-like immunoreactivity in the granules of neural complex cells of the ascidian Halocynthia roretzi

Immunohistochemical studies on the neural complex (neural gland, dorsal strand, and cerebral ganglion) of an ascidian, Halocynthia roretzi, were performed by using an antiserum against porcine ACTH. The antiserum recognized a considerable number of the cells scattered along the tubular structure of the dorsal strand and a few cells in the cerebral ganglion. Immunoelectron microscopic studies revealed that the ACTH-like substance resided within secretory granules with diameter of 300-500 nm. Furthermore, those ACTH-immunoreactive cells were demonstrated to be different from PRL-immunoreactive cells, the presence of which had previously been reported.

Cell reorganisation during epithelial fusion and perforation: the case of ascidian branchial fissures

In this study, we have analysed ultrastructurally the mechanism of epithelial fusion and perforation during the development of branchial fissures in the larva and bud of the colonial urochordate Botryllus schlosseri. Perforation of membranes represents an important process during embryogenesis, occurring to create communication between two separate compartments. For example, all chordate embryos share the formation of pharyngeal plates, which are constituted of apposed endodermal and ectodermal epithelia, which have the capacity to fuse and perforate. Although the process of perforation is extremely common, its cellular mechanism remains little understood in detail, because of the complexity of the structures involved. In B. schlosseri, two epithelial monolayers, the peribranchial and the branchial ones, with no interposed mesenchymal cells, participate in pharyngeal perforation. Blood flows in the interspace between the two cellular leaflets. Apico-lateral zonulae occludentes seal the cells of each epithelium, so that the blood compartment is separated from the environment of the peribranchial and branchial chambers; here, sea water will flow when the zooid siphons open. Stigmata primordia appear as contiguous thickened discs of palisading cells of branchial and peribranchial epithelia. The peribranchial component invaginates to contact the branchial one. Here, the basal laminae intermingle, compact, and are degraded, while the intercellular space between the two epithelia is reduced to achieve the same width as that found between the lateral membranes of adjacent cells. Cells involved in this fusion rapidly change their polarity: they acquire a new epithelial axis, because part of the adhering basal membrane becomes a new lateral surface, whereas the original lateral membranes become new apical surfaces. Before disassembling the old tight junctions and establishing communication between branchial and peribranchial chambers, cells of the stigmata rudiments form new tight junctions organised as distinct entities, so that the structural continuum of the epithelial layers is maintained throughout the time of fusion and perforation.

Development of the motor nervous system in ascidians

The motor nervous system of adult ascidians consists of neurons forming the cerebral ganglion from which axons run out directly to the effectors, i.e., muscular and ciliary cells. In this study, we analyzed the development of the motor fibers, correlating this with organ differentiation during asexual reproduction in Botryllus schlosseri. We used a staining method for acetylcholinesterase, whose reaction product is visible with both light and electron microscopy and which labels entire nerves, including their thin terminals, making them identifiable between tissues. While the cerebral ganglion is forming, the axons elongate and follow stereotypical pathways to reach the smooth muscle cells of the body, the striated muscle of the heart, and the ciliated cells of the branchial stigmata and the gut. A strict temporal relation links the development of the local neural network with its target organ, which is approached by nerves before the effector cells are fully differentiated. This process occurs for oral and cloacal siphons, branchial basket, gut, and heart. Axons grow through the extracellular matrix and arrive at their targets from different directions. In some cases, the blood sinuses constitute the favorite roads for growing axons, which seem to be guided by a mechanism involving contact guidance or stereotropism. The pattern of innervation undergoes dynamic rearrangements and a marked process of elimination of axons, when the last stages of blastogenesis occur. The final pattern of motor innervation seems to be regulated by axon withdrawal, rather than apoptosis of motor neurons.

Ultrastructural and cytochemical investigations on the formation of chorion in oocyte of Ascidia malaca

In the present work are reported investigations on the formation and chemical nature of the chorion of Ascidia malaca oocytes. The ultrastructural observations have shown that both follicle cells and test cells play a key role on chorion formation. At the beginning of vitellogenesis (stage I), chorion is formed by a single fibrogranular layer. During vitellogenesis (stage II), the chorion becomes, at first, bi-layered and successively presents three layers. The outer layer (ofl) is composed of a thin and irregular network of short fibrils. The central layer (cdl) is also made by a fibrillar component which is very compact to form an electron-dense layer. The inner layer (ifl) has a larger size compared to that of outer and central layer and it is composed by a matrix containing little and scattered globular electron-dense granules and several fibrils oriented in all directions. At the end of vitellogenesis beneath the three-layered chorion a large perivitelline space will be formed. In this space are found the test cells. Cytochemical investigations performed at light microscopy (toluidine blue) and electron microscopy (PA-TCH-SP) have shown in Ascidia malaca chorion the presence of substances with a polysaccharidic nature that are probably conjugated as glycoprotein and/or proteoglycans.

Cellulose in the house of the appendicularian Oikopleura rufescens

By electron diffraction analysis, highly crystalline cellulose I beta was found in the house (a special structure in which the tunicate lives) of the appendicularian Oikopleura rufescens. Cellulose microfibrils 20 nm in width were observed in a random array or highly organized with rectangular spacing of 2 to 10 microns in the house. The bundled cellulose microfibrils formed in the inlet filters, which are highly ordered meshwork structures. This paper provides the first account of the existence of cellulose in the house of an appendicularian. Our findings showed that the house and tunic are homologous tissues among the tunicates, and that the common ancestor of the tunicates (ascidians, thaliaceans, and appendicularians) already possessed cellulose-biosynthetic ability.

A morphological study of nonrandom senescence in a colonial urochordate

Botryllus schlosseri is a clonally modular ascidian, in which individuals (zooids) have a finite life span that is intimately associated with a weekly budding process called blastogenesis. Every blastogenic cycle concludes with a synchronized phase of regression called takeover, during which all zooids in a colony die, primarily by apoptosis, and are replaced by a new generation of asexually derived zooids. We have previously documented that, in addition to this cyclical death phase, entire colonies undergo senescence during which all asexually derived individuals in a colony, buds and zooids, die in concert. In addition, when a specific parent colony (genet) is experimentally separated into a number of clonal replicates (ramets), ramets frequently undergo senescence simultaneously, indicating that mortality can manifest itself in nonrandom fashion. Here, we document a morphological portrait of senescence in laboratory-maintained colonies from Monterey Bay, California, that exhibit nonrandom mortality. Nonrandom senescence proceeded according to a series of characteristic changes within the colony over a period of about one week. These changes included systemic constriction and congestion of the vasculature accompanied by massive accumulation of pigment cells in the zooid body wall (mantle), blood vessels, and ampullae; gradual shrinkage of individual zooids; loss of colonial architecture, and ultimately death. At the ultrastructural level, individual cells exhibited changes typical of ischemic cell death, culminating in necrotic cell lysis rather than apoptosis. Collectively, these observations indicate that senescence is accompanied by unique morphological changes that occur systemically, and which are distinct from those occurring during takeover. We discuss our findings in relation to current experimental models of aging and the possible role of a humoral factor in bringing about the onset of senescence.

Ultrastructural studies on the centrosome-attracting body: electron-dense matrix and its role in unequal cleavages in ascidian embryos

In ascidian embryos, three successive unequal cleavages occur at the posterior pole, generating a specific cleavage pattern. A recently reported novel structure designated the centrosome-attracting body (CAB) has been suggested to play essential roles in the unequal cleavages attracting centrosomes and the nucleus towards the posterior pole. To examine the morphological features of the CAB, the ultrastructure of the CAB of two ascidian species, Halocynthia roretzi and Ciona intestinalis was observed by transmission electron microscopy. Detailed observations clarified that the electron-dense matrix (EDM) was a CAB-specific component that was commonly observed in the CAB of both species but was not found in other areas of the embryo. Further observations of the CAB in various staged embryos revealed that the ultrastructure was quite stable, with no difference between points of a cell cycle or between each stage from the 8- to 64-cell stage when unequal cleavage occurred. Observations of extracted embryos implied that the EDM was the extraction-resistant component of the CAB and was tightly anchored to the plasma membrane. It has been proposed that the EDM functions as a physical attachment site at the cell cortex for microtubules emanating from centrosomes and provides a scaffold for the centrosome-attracting machinery. Interestingly, the ultrastructure of the CAB resembled germ plasm reported in other animals, raising the possibility that the CAB-containing posterior-most blastomeres are germline precursors.

Cell adhesion in the process of asexual reproduction of tunicates

Cell adhesion during budding of tunicates is reviewed from the viewpoints of histology, cytology, biochemistry, and molecular biology. Two kinds of multipotent cells play important roles in bud formation and development: epithelial cells, such as the atrial epithelium of botryllids and polystyelids, and mesenchymal cells, referred to as haemoblasts. Haemoblasts are able to aggregate to form a solid mass of cells, which soon becomes a hollow vesicle. The vesicular epithelium has junctional complexes that contain adherens junctions, and, sometimes, tight junctions; both occur apicolaterally on the plasma membrane. The hollow vesicle develops into the heart, the pyloric gland and duct, the gonad, including germ cells, and even the multipotent epithelium of buds. Cell culture studies suggest that multipotent epithelial cells may be interchangeable with haemoblasts. Several kinds of calcium-dependent, galactose-binding tunicate lectins (TC-14s) have been isolated and sequenced, and have been found to facilitate both in vivo and in vitro cell aggregation and migration. Tunicate homologs of cadherin and integrin genes have recently been isolated from Botryllus and Polyandrocarpa, respectively. Their unique molecular characteristics are discussed in the context of roles that they play in cell adhesion in the process of tunicate budding.

Cytoskeleton alterations by tributyltin (TBT) in tunicate phagocytes

The effects of tributyltin chloride (TBT) on cytoskeletal components, as possible cell targets of toxicity, were examined on cultured hemocytes of the colonial ascidian Botryllus schlosseri by means of indirect immunofluorescence. The immunotoxic effect of 10 microM TBT (sublethal concentration) consists of (1) inhibition of yeast phagocytosis, Ca2+ ATPase activity, and respiratory burst; (2) increase in intracellular Ca2+ concentration; and (3) alterations in cell morphology. After 60 min, TBT-exposed amebocytes become spherical, withdrawing their long pseudopodia, and lose motility. Their microfilaments assemble in clusters around the peripheric cytoplasm, indicating massive disassembly, with the exception of unaltered adhesion plaques. Analogously, their microtubules reveal extensive disaggregation, being scattered in the cytoplasm and not recognizable as single filaments, whereas the microtubule organizing center (MTOC) is still visible. Treatment together with 20 micrograms/ml calmodulin (CaM) can partially restore the cytoskeleton architecture. These results suggest a relationship between TBT and Ca2+ homeostasis in ascidian hemocytes. By interfering with Ca2+ ATPase activity through CaM inhibition, either directly or indirectly, TBT induces an excess of intracellular Ca2+ accumulation, which first causes internal disorganization of cytoskeletal proteins and consequently inhibition of phagocytosis, beginning from chemotaxis and particle adhesion.

Prolactin-like immunoreactivity in the granules of neural complex cells in the ascidian Halocynthia roretzi

Electron-microscopic studies of the neural complex (neural gland, dorsal strand, and cerebral ganglion) of an ascidian, Halocynthia roretzi, were performed, paying particular attention to the secretory systems. We found that cells scattered along the dorsal strand and neural cells in the cerebral ganglion contained electron-dense secretory granules of variable size. Immunoelectron-microscopic studies with an antiserum to bullfrog prolactin revealed that the secretory granules (100-250 nm in diameter) of some granulated cells contained a prolactin-like substance. Cells belonging to the neural gland and dorsal strand neither contained electron-dense granules nor showed immunoreactivity. The possibility that cells in the cerebral ganglion and those along the dorsal strand are phylogenetic progenitors of vertebrate adenohypophyseal cells is discussed.

Ascidian interspecific fertilization. III. Ultrastructural investigations of sperm-egg interaction

Transmission electron microscope investigations on ascidian interspecific fertilization have been carried out on some crosses: Ascidia malaca female x Ascidiella aspersa male, Ascidia malaca female x Phallusia mammillata male, Ascidiella aspersa female x Ascidia malaca male. The morphological aspects of heterospecific sperm-egg interaction were compared with those of homospecific fertilization to complete the previous light and scanning electron microscope observations in this field. Sperm behaviour in homo- and heterospecific crosses mainly differs in the power of sperm passageway through the egg envelopes. Foreign spermatozoa, firstly interacting with the follicle cells (FCs), remain on the FC surface, leaving the interfollicular clefts empty. In homospecific fertilized eggs the initial events, also observed in hybrid crosses, are: attraction, entrapment and sperm mitochondrion translocation. But in homospecific crosses the FCs, by means of projections on their surface, facilitate the sperm path between the clefts for reaching the vitelline coat; the clefts are full of spermatozoa. These observations suggest a role of initial reproductive barrier for the FCs.

In vitro reactions of coelomocytes against sheep red blood cells in the solitary ascidian Halocynthia roretzi

Coelomocytes (blood cells) of the solitary ascidian Halocynthia roretzi were placed in culture to which sheep red blood cells (SRBCs) were introduced. Their reactions against SRBCs were observed by phase-contrast, time-lapse video and scanning electron microscopy. Three cell types, the phago-amoebocyte, the vacuolated cell and the fusogenic phagocyte, reacted to the SRBCs by phagocytosis. The vacuolated cell occasionally discharged the material of their vacuoles at the time of or shortly after ingesting the SRBC. The lymphoid cell captured SRBCs, but did not ingest them. Three other cell types, the fibroblastic cell, the giant cell and the multipolar cell, showed no reaction to the SRBC. The SRBCs captured by the lymphoid cells were transferred either to phago-amoebocytes or fusogenic phagocytes, probably to be ingested. Such collaboration between multifunctional cell is considered to characterize the cellular defense mechanism of H. roretzi.

Serial repetition of cilia pairs along the tail surface of an ascidian larva

Regularly spaced cilia pairs were found in two rows immediately opposite to each other mid-dorsally and mid-ventrally along the larval tail surface of the ascidian protochordate Ciona intestinalis. There were approximately ten such equidistantly placed dorsal-ventral sets embedded in the matrix of the extracellular larval test which forms the flattened vertical tail fin. These immotile cilia originate from pairs of cell bodies in mid-dorsal and mid-ventral peripheral nerves running beneath the tail epidermis. The cilia and neural cell bodies were visualized by immunocytochemical staining with anti-tubulin antibodies; their nature was confirmed by ultrastructural examination. This pattern of cilia and neural cell body placement is conceivably related to the segmentation found in vertebrates.

Evolutionary modifications of morphogenetic mechanisms and alternate life history strategies in ascidians

Comparative embryological studies using anural and urodele ascidians provide an experimental system to study interactions between developmental and evolutionary mechanisms that produce alternate life history strategies. In this paper, cellular features of anural morphogenesis in Molgula pacifica are compared to morphogenesis in species that develop tailed (urodele) larvae and other anural molgulid species. The results of these studies are discussed with regard to possible mechanisms responsible for the evolution of anural morphogenesis and the ecological consequences of anural development. Early developmental processes including ooplasmic segregation, cleavage patterns, and the site and timing of gastrulation were similar in M. pacifica compared to urodele embryos and embryos produced by other anural species. The limited extent of invagination caused by large, yolky cells that restricted vegetal pole cell movements in M. pacifica gastrulae contrasted with the extensive movements of vegetal pole cells that accompanied invagination in M. provisionalis embryos and the embryos of four urodele species. The modified mode of gastrulation exhibited by M. pacifica embryos is likely due to the relatively high yolk content of their eggs. The developmental fates of muscle and epidermal progenitor cells in M. pacifica embryos were altered compared to urodele embryos. Ultrastructural studies and acetylcholinesterase histochemistry experiments indicate that muscle progenitor cells have lost their potential to develop muscle cell features. This loss in myogenic potential suggests that muscle progenitor cells were re-programmed to die. However, this possibility was not supported by the results of ultrastructural studies. A second possibility is discussed in that muscle progenitor cells may have been re-specified to differentiate into adult cells after metamorphosis. Evidence is presented suggesting that the timing mechanism responsible for controlling the onset of metamorphosis, first evident by the outgrowth of epidermal ampullae, was modified in M. pacifica. This paper concludes with a discussion of how anural morphogenesis altered the ancestoral urodele life cycle and the possible ecological benefits of these evolutionary alterations.

Mechanisms of gastrulation and tail formation in ascidians

Ascidian embryos are useful for examining how events that occur during fertilization and cleavage affect gastrulation because they gastrulate early in development, during the seventh cleavage. In ascidians, both dorsal-ventral and anterior-posterior axes are determined before first cleavage. The dorsal-ventral axis is fixed along the animal-vegetal axis of the fertilized egg following the first phase of ooplasmic segregation, perhaps due to determinants moved to the vegetal pole in concert with the myoplasm and plasma membrane components. The first ooplasmic movements appear to be driven by the actin network in the cortical myoplasm. The anterior-posterior axis becomes apparent after the second phase of ooplasmic segregation, when the cortical myoplasm becomes detached from the egg membrane, and moves to the posterior pole of the embryo. This movement is dependent on microtubules and has been attributed to the formation and movement of the sperm aster. A major component of the cortical myoplasm, p58, is co-localized along the microtubules emanating from the sperm aster. Gastrulation begins during the seventh cleavage with the invagination of the large endodermal cells at the vegetal pole of the embryo. The neural plate appears as a thickening of the epidermis on the dorsal side of the larva during the ninth cleavage; then the neural folds are formed, join, and close, elaborating the neural tube. Following neurulation, the tail is elongated as the neural tube and notochord cells intercalate at the midline of the embryo. Investigations using anural (tailless) ascidian larvae suggest that some of the processes underlying elongation can be restored by the zygotic genome. Although ascidian larvae contain fewer cells and cell types than vertebrate embryos, ascidian gastrulation and morphogenesis appear to employ similar mechanisms to those in vertebrate embryos. The extent of our current knowledge about the mechanisms involved in gastrulation and tail formation is summarized, and further experiments are suggested to explore the molecular mechanisms underlying these processes.

Budding-specific lectin induced in epithelial cells is an extracellular matrix component for stem cell aggregation in tunicates

We have examined immunocytochemically the expression, localization and in vivo function of a calcium-dependent and galactose-binding 14 × 10(3) Mr lectin purified from the budding tunicate, Polyandrocarpa misakiensis. Lectin granules first appeared in the inner epithelium of a double-walled bud vesicle. Soon after the bud entered the developmental phase, the granules were secreted into the mesenchymal space, where the lectin-positive extracellular matrix (ECM) developed. The lectin was also produced and secreted by granular leucocytes during budding. Hemoblasts, pluripotent stem cells in the blood, were often found in association with the ECM and they aggregated with epithelial cells to form organ rudiments. The lectin showed a high binding affinity for hemoblast precursors. The blockage of epithelial transformation of stem cells by galactose in in vivo bioassy was ineffective in the presence of the lectin. Polyclonal anti-lectin antibody prevented the hemoblasts spreading on the ECM and moving toward the epithelium, but it did not block the cell-cell adhesion of hemoblasts. By three days of bud development, lectin granules and ECM have almost disappeared from the developing bud together with a cessation of hemoblast aggregation. These results show that Polyandrocarpa lectin is a component of the ECM induced specifically in budding and suggest strongly that it plays a role in bud morphogenesis by directing the migration of pluripotent stem cells to the epithelium.

Gap-junctional permeability in early and cleavage-arrested ascidian embryos

Using the whole-cell voltage clamp technique, we have studied junctional conductance (Gj), and Lucifer Yellow (LY) coupling in 2-cell and 32-cell ascidian embryos. Gj ranges from 17.5 to 35.3 nS in the 2-cell embryo where there is no passage of LY, and from 3.5 to 12.2 nS in the later embryo where LY dye spread is extensive. In both cases, Gj is independent of the transjunctional potential (Vj). Manually apposed 2-cell or 32-cell embryos established a junctional conductance of up to 10 nS within 30 min of contact. Furthermore, since we did not observe any significant number of cytoplasmic bridges at the EM and Gj is sensitive to octanol, it is probable that blastomeres in the 2-cell and 32-cell embryos are in communication by gap junctions. In order to compare Gj in the two stages and to circumvent problems of cell size, movement and spatial location, we used cytochalasin B to arrest cleavage. Gj in cleavage-arrested 2-cell embryos ranged from 25.0 to 38.0 nS and remained constant over a period of 2.5 h. LY injected into a blastomere of these arrested embryos did not spread to the neighbour cell until they attained the developmental age of a 32- to 64-cell control embryo. Our experiments indicate a change in selectivity of gap junctions at the 32-cell stage that is not reflected by a macroscopic change in ionic permeability.

The myoplasm of ascidian eggs: a localized cytoskeletal domain with multiple roles in embryonic development

The myoplasm of ascidian eggs is a localized cytoplasmic region containing a unique cytoskeletal domain. During ooplasmic segregation, the myoplasm moves first to the vegetal pole and then to the future posterior region of the fertilized egg, where it subsequently enters the muscle cell lineage during cleavage. In the vegetal pole region, the myoplasm defines a developmental center which later controls gastrulation and embryonic axis formation. In the posterior region, the myoplasm defines another developmental center, which specifies muscle cell development. Evidence is described suggesting that the integrity of the myoplasmic cytoskeletal domain is required for normal embryonic functions of the myoplasm.

Fine structural observations of the granulocytes involved in the tunic inflammatory-like reaction of Ciona intestinalis (Tunicata)

Erythrocytes injected into the tunic of Ciona intestinalis induced an inflammatory-like reaction which can cause lysis of the neighboring tissue. Electron microscopy observations of the granulocytes of the injured tunic show lysosomic figures and degranulations features suggesting granulocyte involvement in the destructive process.

Effects of cytoskeletal inhibitors on ooplasmic segregation and microtubule organization during fertilization and early development in the ascidian Molgula occidentalis

The effects of microtubule and microfilament inhibitors on ooplasmic segregation and microtubule organization were examined during fertilization, parthenogenetic activation, and early development in the ascidian Molgula occidentalis. At fertilization the egg cortex contracts as the first phase movement and shortly after mitochondria migrate as the myoplasmic crescent develops in the second phase. The microtubule inhibitors colcemid and nocodazole inhibit the second phase, but not the first phase, of ooplasmic segregation. The microfilament inhibitor cytochalasin E has the reciprocal effect of inhibiting the first, but not the second, phase. It appears that sperm may initially bind at any site on the egg surface and that the contractile activities at the first phase and during polar body formation occur independent of the microtubule system. Since the second phase migration occurs as the sperm astral microtubules assemble and since microtubule, but not microfilament, inhibitors arrest this aspect of ooplasmic segregation, microtubules appear necessary for mitochondrial migration. These results demonstrate that the two phases of ascidian ooplasmic segregation are mediated by different systems, the first by microfilaments and the second by microtubules. The microtubule and microfilament systems appear to operate independent of one another and their combined actions result in the completion of ooplasmic segregation. A model is proposed in which the cortical contraction following fertilization is important not only as the motive force for the first phase movement but also as a method to unite the myoplasm with the entering sperm which can initially bind anywhere on the egg surface. The association between myoplasmic components and the growing sperm aster would ensure that the migration and the spatial distribution of myoplasm in the second phase results in the formation of the myoplasmic crescent.

Microtubules in ascidian eggs during meiosis, fertilization, and mitosis

The sequential changes in the distribution of microtubules during germinal vesicle breakdown (GVBD), fertilization, and mitosis were investigated with antitubulin indirect immunofluorescence microscopy in several species of ascidian eggs (Molgula occidentalis, Ciona savignyi, and Halocynthia roretzi). These alterations in microtubule patterns were also correlated with observed cytoplasmic movements. A cytoplasmic latticework of microtubules was observed throughout meiosis. The unfertilized egg of M. occidentalis had a small meiotic spindle with wide poles; the poles became focused after egg activation. The other two species had more typical meiotic spindles before fertilization. At fertilization, a sperm aster first appeared near the cortex close to the vegetal pole. It enlarged into an unusual asymmetric aster associated with the egg cortex. The sperm aster rapidly grew after the formation of the second polar body, and it was displaced as far as the equatorial region, corresponding to the site of the myoplasmic crescent, the posterior half of the egg. The female pronucleus migrated to the male pronucleus at the center of the sperm aster. The microtubule latticework and the sperm aster disappeared towards the end of first interphase with only a small bipolar structure remaining until first mitosis. At mitosis the asters enlarged tremendously, while the mitotic spindle remained remarkably small. The two daughter nuclei remained near the site of cleavage even after division was complete. These results document the changes in microtubule patterns during maturation in Ascidian oocytes, demonstrate that the sperm contributes the active centrosome at fertilization, and reveal the presence of a mitotic apparatus at first division which has an unusually small spindle and huge asters.

Tight and gap junctions in the intestinal tract of tunicates (Urochordata): a freeze-fracture study

The intestinal tracts from seven different species of tunicates, some solitary, some colonial, were studied fine-structurally by freeze-fracture. These urochordates occupy an intermediate position phylogenetically between the vertebrates and the invertebrates. The various regions of their gut were isolated for examination and the junctional characteristics of each part investigated. All the species examined exhibited unequivocal vertebrate-like belts of tight-junctional networks at the luminal border of their intestinal cells. No septate junctions were observed. The tight junctions varied in the number of their component strands and the depth to which they extended basally, some becoming loose and fragmented towards that border. The junctions consisted of ridges or rows of intramembranous particles (IMPs) on the P face, with complementary, but offset, E face grooves into which IMPs sometimes fractured. Tracer studies show that punctate appositions, the thin-section correlate of these ridge/groove systems, are sites beyond which exogenous molecules do not penetrate. These junctions are therefore likely to represent permeability barriers as in the gut tract of higher chordates. Associated with these occluding zonular junctions are intermediate junctions, which exhibit no identifiable freeze-fracture profile, and macular gap junctions, characterized by a reduced intercellular cleft in thin section and by clustered arrays of P face particles in freeze-fractured replicas; these display complementary aggregates of E face pits. The diameters of these maculae are rarely very large, but in certain species (for example, Ciona), they are unusually small. In some tissues, notably those of Diplosoma and Botryllus, they are all of rather similar size, but very numerous. In yet others, such as Molgula, they are polygonal with angular outlines, as might be indicative of the uncoupled state. In many attributes, these various junctions are more similar to those found in the tissues of vertebrates, than to those in the invertebrates, which the adult zooid forms of these lowly chordates resemble anatomically.