Cell Division Pattern during Gastrulation of the Ascidian, Halocynthia roretzi. (cell division pattern/gastrulation/neurulation/ascidian embryo)

Ascidian embryonic cells with properties of neural-crest cells and neuromesodermal progenitors of vertebrates

Neural-crest cells and neuromesodermal progenitors (NMPs) are multipotent cells that are important for development of vertebrate embryos. In embryos of ascidians, which are the closest invertebrate relatives of vertebrates, several cells located at the border between the neural plate and the epidermal region have neural-crest-like properties; hence, the last common ancestor of ascidians and vertebrates may have had ancestral cells similar to neural-crest cells. However, these ascidian neural-crest-like cells do not produce cells that are commonly of mesodermal origin. Here we showed that a cell population located in the lateral region of the neural plate has properties resembling those of vertebrate neural-crest cells and NMPs. Among them, cells with Tbx6-related expression contribute to muscle near the tip of the tail region and cells with Sox1/2/3 expression give rise to the nerve cord. These observations and cross-species transcriptome comparisons indicate that these cells have properties similar to those of NMPs. Meanwhile, transcription factor genes Dlx.b, Zic-r.b and Snai, which are reminiscent of a gene circuit in vertebrate neural-crest cells, are involved in activation of Tbx6-related.b. Thus, the last common ancestor of ascidians and vertebrates may have had cells with properties of neural-crest cells and NMPs and such ancestral cells may have produced cells commonly of ectodermal and mesodermal origins.

Collective effects of cell cleavage dynamics

A conserved process of early embryonic development in metazoans is the reductive cell divisions following oocyte fertilization, termed cell cleavages. Cell cleavage cycles usually start synchronously, lengthen differentially between the embryonic cells becoming asynchronous, and cease before major morphogenetic events, such as germ layer formation and gastrulation. Despite exhibiting species-specific characteristics, the regulation of cell cleavage dynamics comes down to common controllers acting mostly at the single cell/nucleus level, such as nucleus-to-cytoplasmic ratio and zygotic genome activation. Remarkably, recent work has linked cell cleavage dynamics to the emergence of collective behavior during embryogenesis, including pattern formation and changes in embryo-scale mechanics, raising the question how single-cell controllers coordinate embryo-scale processes. In this review, we summarize studies across species where an association between cell cleavages and collective behavior was made, discuss the underlying mechanisms, and propose that cell-to-cell variability in cell cleavage dynamics can serve as a mechanism of long-range coordination in developing embryos.

Ascidian gastrulation and blebbing activity of isolated endoderm blastomeres

Gastrulation is the first dynamic cell movement during embryogenesis. Endoderm and mesoderm cells are internalized into embryos during this process. Ascidian embryos provide a simple system for studying gastrulation in chordates. Gastrulation starts in spherical late 64-cell embryos with 10 endoderm blastomeres. The mechanisms of gastrulation in ascidians have been investigated, and a two-step model has been proposed. The first step involves apical constriction of endoderm cells, followed by apicobasal shortening in the second step. In this study, isolated ascidian endoderm progenitor cells displayed dynamic blebbing activity at the gastrula stage, although such a dynamic cell-shape change was not recognized in toto. Blebbing is often observed in migrating animal cells. In ascidians, endoderm cells displayed blebbing activity, while mesoderm and ectoderm cells did not. The timing of blebbing of isolated endoderm cells coincided with that of cell invagination. The constriction rate of apical surfaces correlated with the intensity of blebbing activity in each endoderm-lineage cell. Fibroblast growth factor (FGF) signaling was both necessary and sufficient for inducing blebbing activity, independent of cell fate specification. In contrast, the timing of initiation of blebbing and intensity of blebbing response to FGF signaling were controlled by intrinsic cellular factors. It is likely that the difference in intensity of blebbing activity between the anterior A-line and posterior B-line cells could account for the anteroposterior difference in the steepness of the archenteron wall. Inhibition of zygotic transcription, FGF signaling, and Rho kinase, all of which suppressed blebbing activity, resulted in incomplete apical constriction and failure of the eventual formation of cup-shaped gastrulae. Blebbing activity was involved in the progression and maintenance of apical constriction, but not in apicobasal shortening in whole embryos. Apical constriction is mediated by distinct blebbing-dependent and blebbing-independent mechanisms. Surface tension and consequent membrane contraction may not be the sole mechanical force for apical constriction and formation of cup-shaped gastrulae. The present study reveals the hidden cellular potential of endodermal cells during gastrulation and discusses the possible roles of blebbing in the invagination process.

Imaging developmental cell cycles

The last decade has seen a major expansion in development of live biosensors, the tools needed to genetically encode them into model organisms, and the microscopic techniques used to visualize them. When combined, these offer us powerful tools with which to make fundamental discoveries about complex biological processes. In this review, we summarize the availability of biosensors to visualize an essential cellular process, the cell cycle, and the techniques for single-cell tracking and quantification of these reporters. We also highlight studies investigating the connection of cellular behavior to the cell cycle, particularly through live imaging, and anticipate exciting discoveries with the combination of these technologies in developmental contexts.

Single-cell analysis of cell fate bifurcation in the chordate Ciona

BACKGROUND

Inductive signaling interactions between different cell types are a major mechanism for the further diversification of embryonic cell fates. Most blastomeres in the model chordate Ciona robusta become restricted to a single predominant fate between the 64-cell and mid-gastrula stages. The deeply stereotyped and well-characterized Ciona embryonic cell lineages allow the transcriptomic analysis of newly established cell types very early in their divergence from sibling cell states without the pseudotime inference needed in the analysis of less synchronized cell populations. This is the first ascidian study to use droplet scRNAseq with large numbers of analyzed cells as early as the 64-cell stage when major lineages such as primary notochord first become fate restricted.

RESULTS AND CONCLUSIONS

We identify 59 distinct cell states, including new subregions of the b-line neural lineage and the early induction of the tail tip epidermis. We find that 34 of these cell states are directly or indirectly dependent on MAPK-mediated signaling critical to early Ciona patterning. Most of the MAPK-dependent bifurcations are canalized with the signal-induced cell fate lost upon MAPK inhibition, but the posterior endoderm is unique in being transformed into a novel state expressing some but not all markers of both endoderm and muscle. Divergent gene expression between newly bifurcated sibling cell types is dominated by upregulation in the induced cell type. The Ets family transcription factor Elk1/3/4 is uniquely upregulated in nearly all the putatively direct inductions. Elk1/3/4 upregulation together with Ets transcription factor binding site enrichment analysis enables inferences about which bifurcations are directly versus indirectly controlled by MAPK signaling. We examine notochord induction in detail and find that the transition between a Zic/Ets-mediated regulatory state and a Brachyury/FoxA-mediated regulatory state is unexpectedly late. This supports a "broad-hourglass" model of cell fate specification in which many early tissue-specific genes are induced in parallel to key tissue-specific transcriptional regulators via the same set of transcriptional inputs.

Conservation of peripheral nervous system formation mechanisms in divergent ascidian embryos

Ascidians with very similar embryos but highly divergent genomes are thought to have undergone extensive developmental system drift. We compared, in four species (Ciona and Phallusia for Phlebobranchia, Molgula and Halocynthia for Stolidobranchia), gene expression and gene regulation for a network of six transcription factors regulating peripheral nervous system (PNS) formation in Ciona. All genes, but one in Molgula, were expressed in the PNS with some differences correlating with phylogenetic distance. Cross-species transgenesis indicated strong levels of conservation, except in Molgula, in gene regulation despite lack of sequence conservation of the enhancers. Developmental system drift in ascidians is thus higher for gene regulation than for gene expression and is impacted not only by phylogenetic distance, but also in a clade-specific manner and unevenly within a network. Finally, considering that Molgula is divergent in our analyses, this suggests deep conservation of developmental mechanisms in ascidians after 390 My of separate evolution.

H3K27me3 suppresses sister-lineage somatic gene expression in late embryonic germline cells of the ascidian, Halocynthia roretzi

Protection of the germline from somatic differentiation programs is crucial for germ cell development. In many animals, whose germline development relies on the maternally inherited germ plasm, such protection in particular at early stages of embryogenesis is achieved by maternally localized global transcriptional repressors, such as PIE-1 of Caenorhabditis elegans, Pgc of Drosophila melanogaster and Pem of ascidians. However, zygotic gene expression starts in later germline cells eventually and mechanisms by which somatic gene expression is selectively kept under repression in the transcriptionally active cells are poorly understood. By using the ascidian species Halocynthia roretzi, we found that H3K27me3, a repressive transcription-related chromatin mark, became enriched in germline cells starting at the 64-cell stage when Pem protein level and its contribution to transcriptional repression decrease. Interestingly, inhibition of H3K27me3 together with Pem knockdown resulted in ectopic expression in germline cells of muscle developmental genes Muscle actin (MA4) and Snail, and of Clone 22 (which is expressed in all somatic but not germline cells), but not of other tissue-specific genes such as the notochord gene Brachyury, the nerve cord marker ETR-1 and a heart precursor gene Mesp, at the 110-cell stage. Importantly, these ectopically expressed genes are normally expressed in the germline sister cells (B7.5), the last somatic lineage separated from the germline. Also, the ectopic expression of MA4 was dependent on a maternally localized muscle determinant Macho-1. Taken together, we propose that H3K27me3 may be responsible for selective transcriptional repression for somatic genes in later germline cells in Halocynthia embryos and that the preferential repression of germline sister-lineage genes may be related to the mechanism of germline segregation in ascidian embryos, where the germline is segregated progressively by successive asymmetric cell divisions during cell cleavage stages. Together with findings from C. elegans and D. melanogaster, our data for this urochordate animal support the proposal for a mechanism, conserved widely throughout the animal kingdom, where germline transcriptional repression is mediated initially by maternally localized factors and subsequently by a chromatin-based mechanism.

Dynein-Mediated Regional Cell Division Reorientation Shapes a Tailbud Embryo

Regulation of cell division orientation controls the spatial distribution of cells during development and is essential for one-directional tissue transformation, such as elongation. However, little is known about whether it plays a role in other types of tissue morphogenesis. Using an ascidian Halocynthia roretzi, we found that differently oriented cell divisions in the epidermis of the future trunk (anterior) and tail (posterior) regions create an hourglass-like epithelial bending between the two regions to shape the tailbud embryo. Our results show that posterior epidermal cells are polarized with dynein protein anteriorly localized, undergo dynein-dependent spindle rotation, and divide along the anteroposterior axis. This cell division facilitates constriction around the embryo's circumference only in the posterior region and epithelial bending formation. Our findings, therefore, provide an important insight into the role of oriented cell division in tissue morphogenesis.

A Nodal/Eph signalling relay drives the transition from apical constriction to apico-basal shortening in ascidian endoderm invagination

Gastrulation is the first major morphogenetic event during animal embryogenesis. Ascidian gastrulation starts with the invagination of 10 endodermal precursor cells between the 64- and late 112-cell stages. This process occurs in the absence of endodermal cell division and in two steps, driven by myosin-dependent contractions of the acto-myosin network. First, endoderm precursors constrict their apex. Second, they shorten apico-basally, while retaining small apical surfaces, thereby causing invagination. The mechanisms that prevent endoderm cell division, trigger the transition between step 1 and step 2, and drive apico-basal shortening have remained elusive. Here, we demonstrate a conserved role for Nodal and Eph signalling during invagination in two distantly related ascidian species, Phallusia mammillata and Ciona intestinalis. Specifically, we show that the transition to step 2 is triggered by Nodal relayed by Eph signalling. Additionally, our results indicate that Eph signalling lengthens the endodermal cell cycle, independently of Nodal. Finally, we find that both Nodal and Eph signals are dispensable for endoderm fate specification. These results illustrate commonalities as well as differences in the action of Nodal during ascidian and vertebrate gastrulation.

Tunicate gastrulation

Tunicates are a diverse group of invertebrate marine chordates that includes the larvaceans, thaliaceans, and ascidians. Because of their unique evolutionary position as the sister group of the vertebrates, tunicates are invaluable as a comparative model and hold the promise of revealing both conserved and derived features of chordate gastrulation. Descriptive studies in a broad range of tunicates have revealed several important unifying traits that make them unique among the chordates, including invariant cell lineages through gastrula stages and an overall morphological simplicity. Gastrulation has only been studied in detail in ascidians such as Ciona and Phallusia, where it involves a simple cup-shaped gastrula driven primarily by endoderm invagination. This appears to differ significantly from vertebrate models, such as Xenopus, in which mesoderm convergent extension and epidermal epiboly are major contributors to involution. These differences may reflect the cellular simplicity of the ascidian embryo.

Asymmetric and Unequal Cell Divisions in Ascidian Embryos

Asymmetric cell division during embryogenesis contributes to cell diversity by generating daughter cells that adopt distinct developmental fates. In this chapter, we summarize current knowledge of three examples of asymmetric cell division occurring in ascidian early embryos: (1) Three successive cell divisions that are asymmetric in terms of cell fate and unequal in cell size in the germline lineage at the embryo posterior pole. A subcellular structure, the centrosome-attracting body (CAB), and maternal PEM mRNAs localized within it control both the positioning of the cell division planes and segregation of the germ cell fates. (2) Asymmetric cell divisions involving endoderm and mesoderm germ layer separation. Asymmetric partitioning of zygotically expressed mRNA for Not, a homeodomain transcription factor, promotes the mesoderm fate and suppresses the endoderm fate. This asymmetric partitioning is mediated by transient nuclear migration toward the mesodermal pole of the mother cell, where the mRNA is delivered. In this case, there is no special regulation of cleavage plane orientation. (3) Asymmetric cell divisions in the marginal region of the vegetal hemisphere. The directed extracellular FGF and ephrin signals polarize the mother cells, inducing distinct fates in a pair of daughter cells (nerve versus notochord and mesenchyme versus muscle). The directions of cell division are regulated and oriented but independently of FGF and ephrin signaling. In these examples, polarization of the mother cells is facilitated by localized maternal factors, by delivery of transcripts from the nucleus to one pole of each cell, and by directed extracellular signals. Two cellular processes-asymmetric fate allocation and orientation of the cell division plane-are coupled by a single factor in the first example, but these processes are regulated independently in the third example. Thus, various modes of asymmetric cell division operate even at the early developmental stages in this single type of organism.

Physical association between a novel plasma-membrane structure and centrosome orients cell division

In the last mitotic division of the epidermal lineage in the ascidian embryo, the cells divide stereotypically along the anterior-posterior axis. During interphase, we found that a unique membrane structure invaginates from the posterior to the centre of the cell, in a microtubule-dependent manner. The invagination projects toward centrioles on the apical side of the nucleus and associates with one of them. Further, a cilium forms on the posterior side of the cell and its basal body remains associated with the invagination. A laser ablation experiment suggests that the invagination is under tensile force and promotes the posterior positioning of the centrosome. Finally, we showed that the orientation of the invaginations is coupled with the polarized dynamics of centrosome movements and the orientation of cell division. Based on these findings, we propose a model whereby this novel membrane structure orchestrates centrosome positioning and thus the orientation of cell division axis.

High-precision morphology: bifocal 4D-microscopy enables the comparison of detailed cell lineages of two chordate species separated for more than 525 million years

BACKGROUND

Understanding the evolution of divergent developmental trajectories requires detailed comparisons of embryologies at appropriate levels. Cell lineages, the accurate visualization of cleavage patterns, tissue fate restrictions, and morphogenetic movements that occur during the development of individual embryos are currently available for few disparate animal taxa, encumbering evolutionarily meaningful comparisons. Tunicates, considered to be close relatives of vertebrates, are marine invertebrates whose fossil record dates back to 525 million years ago. Life-history strategies across this subphylum are radically different, and include biphasic ascidians with free swimming larvae and a sessile adult stage, and the holoplanktonic larvaceans. Despite considerable progress, notably on the molecular level, the exact extent of evolutionary conservation and innovation during embryology remain obscure.

RESULTS

Here, using the innovative technique of bifocal 4D-microscopy, we demonstrate exactly which characteristics in the cell lineages of the ascidian Phallusia mammillata and the larvacean Oikopleura dioica were conserved and which were altered during evolution. Our accurate cell lineage trees in combination with detailed three-dimensional representations clearly identify conserved correspondence in relative cell position, cell identity, and fate restriction in several lines from all prospective larval tissues. At the same time, we precisely pinpoint differences observable at all levels of development. These differences comprise fate restrictions, tissue types, complex morphogenetic movement patterns, numerous cases of heterochronous acceleration in the larvacean embryo, and differences in bilateral symmetry.

CONCLUSIONS

Our results demonstrate in extraordinary detail the multitude of developmental levels amenable to evolutionary innovation, including subtle changes in the timing of fate restrictions as well as dramatic alterations in complex morphogenetic movements. We anticipate that the precise spatial and temporal cell lineage data will moreover serve as a high-precision guide to devise experimental investigations of other levels, such as molecular interactions between cells or changes in gene expression underlying the documented structural evolutionary changes. Finally, the quantitative amount of digital high-precision morphological data will enable and necessitate software-based similarity assessments as the basis of homology hypotheses.

Migratory neuronal progenitors arise from the neural plate borders in tunicates

The neural crest is an evolutionary novelty that fostered the emergence of vertebrate anatomical innovations such as the cranium and jaws. During embryonic development, multipotent neural crest cells are specified at the lateral borders of the neural plate before delaminating, migrating and differentiating into various cell types. In invertebrate chordates (cephalochordates and tunicates), neural plate border cells express conserved factors such as Msx, Snail and Pax3/7 and generate melanin-containing pigment cells, a derivative of the neural crest in vertebrates. However, invertebrate neural plate border cells have not been shown to generate homologues of other neural crest derivatives. Thus, proposed models of neural crest evolution postulate vertebrate-specific elaborations on an ancestral neural plate border program, through acquisition of migratory capabilities and the potential to generate several cell types. Here we show that a particular neuronal cell type in the tadpole larva of the tunicate Ciona intestinalis, the bipolar tail neuron, shares a set of features with neural-crest-derived spinal ganglia neurons in vertebrates. Bipolar tail neuron precursors derive from caudal neural plate border cells, delaminate and migrate along the paraxial mesoderm on either side of the neural tube, eventually differentiating into afferent neurons that form synaptic contacts with both epidermal sensory cells and motor neurons. We propose that the neural plate borders of the chordate ancestor already produced migratory peripheral neurons and pigment cells, and that the neural crest evolved through the acquisition of a multipotent progenitor regulatory state upstream of multiple, pre-existing neural plate border cell differentiation programs.

Transmission and Scanning Electron Microscopy of the Accessory Cells and Chorion During Development of Ciona intestinalis Type B Embryos and the Impact of Their Removal on Cell Morphology

Spawned ascidian oocytes are surrounded by a membrane called the chorion (or vitelline coat) and associated with two populations of maternally-supplied cells. Outside the chorion are follicle cells, which may affect the buoyancy of eggs. Inside the chorion are test cells, which during oogenesis provision the egg and which after fertilisation contribute to the larval tunic. The structure of maternal cells may vary between species. The model ascidian Ciona intestinalis has been recently split into two species, currently named type A and type B. The ultrastructure of extraembryonic cells and structures from type A embryos has been reported. Here we describe the ultrastructure of follicle and test cells from C. intestinalis type B embryos. Test cells are about 5 µm in diameter and line the inside of the chorion of developing embryos in a dense sheet. Follicle cells are large (> 100 µm long) and spike-shaped, with many large vesicles. Terminal electron dense granules are found towards the tips of spikes, adjacent to cytoplasm containing numerous small electron dense bodies connected by filaments. These are probably vesicles containing material for the terminal granules. Removal of maternal structures and cells just after fertilisation, as commonly used in many experiments manipulating C. intestinalis development, has been reported to affect embryonic patterning. We examined the impact of this on embryonic ectoderm cells by scanning electron microscopy. Cells of embryos that developed without maternal structures still developed cilia, but had indistinct cell boundaries and a more flattened appearance than those that developed within the chorion.

Cytoplasmic localization and reorganization in ascidian eggs: role of postplasmic/PEM RNAs in axis formation and fate determination

Localization of maternal molecules in eggs and embryos and cytoplasmic movements to relocalize them are fundamental for the orderly cellular and genetic processes during early embryogenesis. Ascidian embryos have been known as 'mosaic eggs' because of their autonomous differentiation abilities based on localized cell fate determinants. This review gives a historical overview of the concept of cytoplasmic localization, and then explains the key features such as ooplasmic movements and cell lineages that are essential to grasp the process of ascidian development mediated by localized determinant activities. These activities are partly executed by localized molecules named postplasmic/PEM RNAs, originating from approximately 50 genes, of which the muscle determinant, macho-1, is an example. The cortical domain containing these RNAs is relocalized to the posterior-vegetal region of the egg by cytoskeletal movements after fertilization, and plays crucial roles in axis formation and cell fate determination. The cortical domain contains endoplasmic reticulum and characteristic granules, and gives rise to a subcellular structure called the centrosome-attracting body (CAB), in which postplasmic/PEM RNAs are highly concentrated. The CAB is responsible for a series of unequal partitionings of the posterior-vegetal cytoplasmic domain and the postplasmic/PEM RNAs at the posterior pole during cleavage. Some components of this domain, which is rich in granules, are eventually inherited by prospective germline cells with particular postplasmic/PEM RNAs such as vasa. The postplasmic/PEM RNAs are classified into two groups according to their final cellular destinations and localization pathways. Localization of these RNAs is regulated by specific nucleotide sequences in the 3' untranslated regions (3'UTRs).

Expression of the Distalless-B gene in Ciona is regulated by a pan-ectodermal enhancer module

The Ci-Dll-B gene is an early regulator of ectodermal development in the ascidian Ciona intestinalis (Imai et al., 2006). Ci-Dll-B is located in a convergently transcribed bigene cluster with a tandem duplicate, Ci-Dll-A. This clustered genomic arrangement is the same as those of the homologous vertebrate Dlx genes, which are also arranged in convergently transcribed bigene clusters. Sequence analysis of the C. intestinalis Dll-A-B cluster reveals a 378bp region upstream of Ci-Dll-B, termed B1, which is highly conserved with the corresponding region from the congener Ciona savignyi. The B1 element is necessary and sufficient to drive expression of a lacZ reporter gene in a pattern mimicking the endogenous expression of Ci-Dll-B at gastrula stages. This expression pattern which is specific to the entire animal hemisphere is activated preferentially in posterior, or b-lineage, cells by a central portion of B1. Expression in anterior, or a-lineage cells, can be activated by this central portion in combination with the distal part of B1. Anterior expression can also be activated by the central part of B1 plus both the proximal part of B1 and non-conserved sequence upstream of B1. Thus, cis-regulation of early Ci-Dll-B expression is activated by a required submodule in the center of B1, driving posterior expression, which works in combination with redundant submodules that respond to differentially localized anterior factors to produce the total animal hemisphere expression pattern. Interestingly, the intergenic region of the cluster, which is important for expression of the Dlx genes in vertebrates, does not have a specific activating function in the reporter genes tested, but acts as an attenuator in combination with upstream sequences.

Coordination of mitosis and morphogenesis: role of a prolonged G2 phase during chordate neurulation

Chordates undergo a characteristic morphogenetic process during neurulation to form a dorsal hollow neural tube. Neurulation begins with the formation of the neural plate and ends when the left epidermis and right epidermis overlying the neural tube fuse to close the neural fold. During these processes, mitosis and the various morphogenetic movements need to be coordinated. In this study, we investigated the epidermal cell cycle in Ciona intestinalis embryos in vivo using a fluorescent ubiquitination-based cell cycle indicator (Fucci). Epidermal cells of Ciona undergo 11 divisions as the embryos progress from fertilization to the tadpole larval stage. We detected a long G2 phase between the tenth and eleventh cell divisions, during which fusion of the left and right epidermis occurred. Characteristic cell shape change and actin filament regulation were observed during the G2 phase. CDC25 is probably a key regulator of the cell cycle progression of epidermal cells. Artificially shortening this G2 phase by overexpressing CDC25 caused precocious cell division before or during neural tube closure, thereby disrupting the characteristic morphogenetic movement. Delaying the precocious cell division by prolonging the S phase with aphidicolin ameliorated the effects of CDC25. These results suggest that the long interphase during the eleventh epidermal cell cycle is required for neurulation.

Sequential activation of apical and basolateral contractility drives ascidian endoderm invagination

BACKGROUND

Epithelial invagination is a fundamental morphogenetic behavior that transforms a flat cell sheet into a pit or groove. Previous studies of invagination have focused on the role of actomyosin-dependent apical contraction; other mechanisms remain largely unexplored.

RESULTS

We combined experimental and computational approaches to identify a two-step mechanism for endoderm invagination during ascidian gastrulation. During Step 1, which immediately precedes invagination, endoderm cells constrict their apices because of Rho/Rho-kinase-dependent apical enrichment of 1P-myosin. Our data suggest that endoderm invagination itself occurs during Step 2, without further apical shrinkage, via a novel mechanism we call collared rounding: Rho/Rho-kinase-independent basolateral enrichment of 1P-myosin drives apico-basal shortening, whereas Rho/Rho-kinase-dependent enrichment of 1P and 2P myosin in circumapical collars is required to prevent apical expansion and for deep invagination. Simulations show that boundary-specific tension values consistent with these distributions of active myosin can explain the cell shape changes observed during invagination both in normal embryos and in embryos treated with pharmacological inhibitors of either Rho-kinase or Myosin II ATPase. Indeed, we find that the balance of strong circumapical and basolateral tension is the only mechanism based on differential cortical tension that can explain ascidian endoderm invagination. Finally, simulations suggest that mesectoderm cells resist endoderm shape changes during both steps, and we confirm this prediction experimentally.

CONCLUSIONS

Our findings suggest that early ascidian gastrulation is driven by the coordinated apposition of circumapical and lateral endoderm contraction, working against a resisting mesectoderm. We propose that similar mechanisms may operate during other invaginations.

Spatiotemporal patterns of neurogenesis in the appendicularian Oikopleura dioica

Incorporation of the thymidine analog bromodeoxyuridine (BrdU) was used to assess cytogenesis in the central nervous system (CNS) of the appendicularian Oikopleura dioica. A series of timed cumulative labelings carried out from 45 minutes (min) to 8 hours (h) after fertilization provided labeling patterns that showed when neurons and support cells residing at specific sites within the 9 h CNS became postmitotic. Throughout the CNS, which includes the cerebral ganglion, caudal ganglion and caudal nerve cord, neurogenesis occurs during an earlier time window than the genesis of support cells. Neurons are first generated at about 45 min to 1 h after fertilization in all 3 CNS regions, starting in the cerebral ganglion. Support cells are generated starting at about 2 h after fertilization. In both the cerebral ganglion and the caudal ganglion, neurons born during different time epochs settle in a specific spatial pattern, following a caudal to rostral gradient in the caudal ganglion and a more complex pattern in the cerebral ganglion. No such regional pattern was seen in the caudal nerve cord, where neurons born during different epochs were evenly distributed along the length of the cord. In the cerebral ganglion a small subpopulation of cells continued to incorporate BrdU from 8 h to at least 15 h and may represent a reserve of stem cells or progenitor cells that generate additional cells seen in the adult. The results show that this simple urochordate exhibits several vertebrate features of CNS cytogenesis, including a different timing of neurogenesis and gliogenesis (support cells being the likely candidates for glial cells in Oikopleura), gradients of neuron position according to birthdate, and a maintenance of neural cell precursors beyond embryonic and larval stages.

Brain induction in ascidian embryos is dependent on juxtaposition of FGF9/16/20-producing and -receiving cells

Coordinated regulation of inductive events, both spatially and temporally, during animal development ensures that tissues are induced at their specific positions within the embryo. The ascidian brain is induced in cells at the anterior edge of the animal hemisphere by fibroblast growth factor (FGF) signals secreted from vegetal cells. To clarify how this process is spatially regulated, we first identified the sources of the FGF signal by examining the expression of brain markers Hr-Otx and Hr-ETR-1 in embryos in which FGF signaling is locally inhibited by injecting individual blastomeres with morpholino oligonucleotide against Hr-FGF9/16/20, which encodes an endogenous brain inducer. The blastomeres identified as the inducing sources are A5.1 and A5.2 at the 16-cell stage and A6.2 and A6.4 at the 24-cell stage, which are juxtaposed with brain precursors at the anterior periphery of the embryo at the respective stages. We also showed that all the cells of the animal hemisphere are capable of expressing Hr-Otx in response to the FGF signal. These results suggest that the position of inducers, rather than competence, plays an important role in determining which animal cells are induced to become brain tissues during ascidian embryogenesis. This situation in brain induction contrasts with that in mesoderm induction, where the positions at which the notochord and mesenchyme are induced are determined mainly by intrinsic competence factors that are inherited by signal-receiving cells.

Ascidian embryonic development: An emerging model system for the study of cell fate specification in chordates

The ascidian tadpole larva represents the basic body plan of all chordates in a relatively small number of cells and tissue types. Although it had been considered that ascidians develop largely in a determinative way, whereas vertebrates develop in an inductive way, recent studies at the molecular and cellular levels have uncovered several similarities in the way developmental fates are specified. In this review, we describe ascidian embryogenesis and its cell lineages, introduce several characteristics of ascidian embryos, describe recent advances in understanding of the mechanisms of cell fate specification, and discuss them in the context of what is known in vertebrates and other organisms.

Tracing cell fate in brain formation during embryogenesis of the ascidian Halocynthia roretzi

Ascidian eggs develop into tadpole larvae. They have a simple central nervous system (CNS) at the dorsal midline. The CNS is formed through neural tube formation at the neurula stage, as in vertebrates. The total number of cells in the CNS is approximately 300. In Halocynthia roretzi, the anterior part of the CNS, which consists of the brain (sensory vesicle) and the visceral ganglion, is exclusively derived from 10 blastomeres at the 110-cell stage. The anterior CNS is relatively complex and shows remarkable left-right asymmetry, with the lumen of the sensory vesicle, the otolith, and the ocellus on the right side, and the presumed hydrostatic pressure organ on the left side. We labeled these 10 precursor blastomeres - six in the animal hemisphere (a-line) and four in the vegetal hemisphere (A-line) - with lineage tracer, and examined the fates in swimming larvae. The clonal organization of the anterior CNS is essentially invariant among individuals, although slight variation in the clonal boundary was observed. There was no extensive mixing between descendants of each precursor. We observed no evidence of cell migration except for two neuronal cells derived from a8.25 blastomeres. The eventual fates of the bilateral blastomeres produced extensive left-right asymmetry. The results suggest that the anterior neural tube rotates in a clockwise direction when viewed from the posterior pole. Staged observation indicated that this rotation takes place during the last 5 h of embryogenesis. We describe detailed positions of descendants of each precursor blastomere. In particular, specific cells of sensory structures were identified by their morphology and staining with specific antibodies and probes. The otolith and ocellus pigment cells were derived from left and right a8.25 blastomeres. Lens cells of the ocellus have a right A8.8 origin, and most of the photoreceptor cells originated from the right A8.7. The presumed pressure organ was formed by descendants of left and right a8.19 and left a8.17 blastomeres. The description of cell lineages of the CNS would facilitate future research to analyze the mechanisms of development of the simple CNS of ascidian tadpole larvae.

Eutely, cell lineage, and fate within the ascidian larval nervous system: determinacy or to be determined?

The larval central nervous system (CNS) of the ascidian Ciona intestinalis (L., 1767) arises from an embryonic neural plate and contains sufficiently few cells, about 330, to enable definitive counts. On the basis of such counts, there is evidence both for cell constancy (eutely) in the larval CNS and for small variations in the overall numbers of cells and among defined cell types within this total. However, evidence for the range of such deviations and the existence of a true phenotypic wild type are lacking. The record of cell lineage, i.e., the mitotic ancestry of each cell, and the fates of some of these cells have recently received increased documentation in both the genus Ciona and Halocynthia roretzi (von Drasche, 1884). Relatively few generations of cells, between 10 and 14, form the entire CNS in C. intestinalis, and cell death does not occur prior to larval hatching. The tiny complement of larval CNS cells can therefore be seen as the product of a small fixed number of determinate cleavages, and variations in cell number as the product of minor deviations in this mitotic ancestry. Within these lineage records, some cell fates have already been identified, but knowledge of most is lacking because the cells lack markers or other identifying features. Nevertheless, this tiny nervous system offers the prospect that all its cells can one day be identified, and their developmental histories and larval functions analyzed, cell by cell.

The central nervous system of the ascidian larva: mitotic history of cells forming the neural tube in late embryonic Ciona intestinalis

Ascidian larvae develop after an invariant pattern of embryonic cleavage. Fewer than 400 cells constitute the larval central nervous system (CNS), which forms without either extensive migration or cell death. We catalogue the mitotic history of these cells in Ciona intestinalis, using confocal microscopy of whole-mount embryos at stages from neurulation until hatching. The positions of cells contributing to the CNS were reconstructed from confocal image stacks of embryonic nuclei, and maps of successive stages were used to chart the mitotic descent, thereby creating a cell lineage for each cell. The entire CNS is formed from 10th- to 14th-generation cells. Although minor differences exist in cell position, lineage is invariant in cells derived from A-line blastomeres, which form the caudal nerve cord and visceral ganglion. We document the lineage of five pairs of presumed motor neurons within the visceral ganglion: one pair arises from A/A 10.57, and four from progeny of A/A 9.30. The remaining cells of the visceral ganglion are in their 13th and 14th generations at hatching, with most mitotic activity ceasing around 85% of embryonic development. Of the approximately 330 larval cells previously reported in the CNS of Ciona, we document the lineage of 226 that derive predominantly from A-line blastomeres.

Evolutionary origins of the vertebrate heart: Specification of the cardiac lineage in Ciona intestinalis

Here we exploit the extensive cell lineage information and streamlined genome of the ascidian, Ciona intestinalis, to investigate heart development in a basal chordate. Several cardiac genes were analyzed, including the sole Ciona ortholog of the Drosophila tinman gene, and tissue-specific enhancers were isolated for some of the genes. Conserved sequence motifs within these enhancers facilitated the isolation of a heart enhancer for the Ciona Hand-like gene. Altogether, these studies provide a regulatory framework for the differentiation of the cardiac mesoderm, beginning at the 110-cell stage, and extending through the fusion of cardiac progenitors during tail elongation. The cardiac lineage shares a common origin with the germ line, and zygotic transcription is first detected in the heart progenitors only after its separation from the germ line at the 64-cell stage. We propose that germ-line determinants influence the specification of the cardiac mesoderm, both by inhibiting inductive signals required for the development of noncardiac mesoderm lineages, and by providing a localized source of Wnt-5 and other signals required for heart development. We discuss the possibility that the germ line also influences the specification of the vertebrate heart.

Spatio-temporal pattern of MAP kinase activation in embryos of the ascidian Halocynthia roretzi

To understand developmental mechanisms, it is important to know when and where signaling pathways are activated. The spatio-temporal pattern of activation of mitogen-activated protein kinase (MAPK/ERK) was investigated during embryogenesis of the ascidian Halocynthia roretzi, using an antibody specific to the activated form of MAPK. During cleavage stages, activated MAPK was transiently observed in nuclei of the precursor blastomeres of endoderm, notochord, mesenchyme, brain, secondary muscle, trunk lateral cells and trunk ventral cells. These sites of MAPK activation are consistent with results of previous studies that have analyzed the embryonic induction of various tissues, and with results of inhibition of MAPK kinase (MEK) in ascidians. Activation of MAPK in notochord and mesenchyme blastomeres was observed in a short period in a single cell cycle. In contrast, in brain and secondary muscle lineages, MAPK activation spanned two or three cell cycles, and upon each cleavage, MAPK was asymmetrically activated in only one of the two daughter cells that remained brain or secondary muscle lineages. During later stages, MAPK activation was predominantly observed in the central nervous system. A conspicuous feature at this stage was that activation appeared to alternate between positive and negative along the anterior-posterior axis of the neural tube. During the tail elongation stage, MAPK was quiescent.

A novel G protein alpha subunit in embryo of the ascidian, Halocynthia roretzi

A cDNA clone encoding a novel G protein alpha subunit, HrGalpha(n) was isolated from the larvae of ascidian, Halocynthia roretzi. In contrast with overall amino acid identity (63%) with G protein alpha subunit of G(i) or G(o) subclass, HrGalpha(n) has a unique amino acid sequence, which lacks a residue for pertussis toxin substrate, but retains for cholera toxin substrate for ADP-ribosylation. The sequence characteristics and molecular phylogenetic analysis suggest that HrGalpha(n) defines a novel subclass within G(i) class of G protein alpha subunits. The zygotic expression of HrGalpha(n) was first detected at the 64-cell stage and observed in all blastomeres except for B7.4, B7.5 and B7.6 cells till the 110-cell stage. As progress of the developmental stages, the expression of HrGalpha(n) became restricted and was observed in the muscle, mesenchyme and a part of trunk lateral cells in tailbud embryos. With HrGalpha(n)-GFP fusion-gene construct it was showed that the genomic fragment containing 2674 bp upstream of the putative translation start site of HrGalpha(n) contained the regulatory sequence responsible for the expression in the muscle and mesenchyme cells, and that the regulatory sequence functioned also in Ciona intestinalis. Our results suggest a possible involvement of HrGalpha(n) in the signaling system regulates the cell fate during the embryogenesis of the ascidian.

Morphogenetic pattern formation during ascidian notochord formation is regulative and highly robust

The ascidian notochord forms through simultaneous invagination and convergent extension of a monolayer epithelial plate. Here we combine micromanipulation with time lapse and confocal microscopy to examine how notochord-intrinsic morphogenetic behaviors and interactions with surrounding tissues, determine these global patterns of movement. We show that notochord rudiments isolated at the 64-cell stage divide and become motile with normal timing; but, in the absence of interactions with non-notochordal tissues, they neither invaginate nor converge and extend. We find that notochord formation is robust in the sense that no particular neighboring tissue is required for notochord formation. Basal contact with either neural plate or anterior endoderm/lateral mesenchyme or posterior mesoderm are each alone sufficient to ensure that the notochord plate forms and extends a cylindrical rod. Surprisingly, the axis of convergent extension depends on the specific tissues that contact the notochord, as do other patterns of cell shape change, movement and tissue deformation that accompany notochord formation. We characterize one case in detail, namely, embryos lacking neural plates, in which a normal notochord forms but by an entirely different trajectory. Our results show ascidian notochord formation to be regulative in a fashion and to a degree never before appreciated. They suggest this regulative behavior depends on a complex interplay between morphogenetic tendencies intrinsic to the notochord plate and instructive and permissive interactions with surrounding tissues. We discuss mechanisms that could account for these data and what they imply about notochord morphogenesis and its evolution within the chordate phylum.

The larval ascidian nervous system: the chordate brain from its small beginnings

The body plan of the tadpole larva of ascidians, or sea-squirts, is widely presumed to be close to that of the hypothetical ancestor of all chordate animal groups, including vertebrates. This is nowhere more obvious than in the organization and development of the dorsal tubular nervous system. Several recent developments advocate this model neural system for studies on neurobiology and neurogenesis. These include advances in our understanding of development in ascidian embryos and of differentiation among the cellular progeny of its neural plate; the application of transgenic and mutant approaches to studies on ascidian larval neurones; and the prospect of advances in genomic analyses. In addition to providing ways to study a working chordate brain in miniature, all these offer insights into the ancestral condition of the developing vertebrate brain.

The maternal transcript for truncated voltage-dependent Ca2+ channels in the ascidian embryo: a potential suppressive role in Ca2+ channel expression

Ca2+ entry during electrical activity plays several critical roles in development. However, the mechanisms that regulate Ca2+ influx during early embryogenesis remain unknown. In ascidians, a primitive chordate, development is rapid and blastomeres of the muscle and neuronal lineages are easily identified, providing a simple model for studying the expression of voltage-dependent Ca2) channels (VDCCs) in cell differentiation. Here we isolate an ascidian cDNA, TuCa1, a homologue of the alpha(1)-subunit of L-type class Ca2+ channels. We unexpectedly found another form of Ca2+ channel cDNA (3-domain-type) potentially encoding a truncated type which lacked the first domain and a part of the second domain. An analysis of genomic sequence suggested that 3-domain-type RNA and the full-length type have alternative transcriptional start sites. The temporal pattern of the amount of 3-domain-type RNA was the reverse of that of the full-length type; the 3-domain type was provided maternally and persisted during early embryogenesis, whereas the full-length type was expressed zygotically in neuronal and muscular lineage cells. Switching of the two forms occurred at a critical stage when VDCC currents appeared in neuronal or muscular blastomeres. To examine the functional roles of the 3-domain type, it was coexpressed with the full-length type in Xenopus oocyte. The 3-domain type did not produce a functional VDCC current, whereas it had a remarkable inhibitory effect on the functional expression of the full-length form. In addition, overexpression of the 3-domain type under the control of the muscle-specific actin promoter in ascidian muscle blastomeres led to a significant decrease in endogenous VDCC currents. These findings raise the possibility that the 3-domain type has some regulatory role in tuning current amplitudes of VDCCs during early development.

Structure and developmental expression of the ascidian TRP gene: insights into the evolution of pigment cell-specific gene expression

The tyrosinase family in vertebrates consists of three related melanogenic enzymes: tyrosinase, tyrosinase-related protein-1 (TRP-1), and TRP-2. These proteins control melanin production in pigment cells and play a crucial role in determining vertebrate coloration. We have isolated a gene from the ascidian Halocynthia roretzi which encodes a tyrosinase-related protein (HrTRP) with 45-49% identity with vertebrate TRP-1 and TRP-2. The expression of the HrTRP gene in pigment lineage a8.25 cells starts at the early-mid gastrula stage, which coincides with the stage when these cells are determined as pigment precursor cells; therefore, it provides the earliest pigment lineage-specific marker, which enables us to trace the complete cell lineage leading to two pigment cells in the larval brain. In addition, the expression pattern of the HrTRP gene appears to share similar characteristics with the mouse TRP-2 gene although structurally the HrTRP gene is more closely related to mammalian TRP-1 genes. Based on these observations and on results from molecular phylogenetic and hybridization analyses, we suggest that triplication of the tyrosinase family occurred during the early radiation of chordates. Initially, duplication of an ancestral tyrosinase gene produced a single TRP gene before the urochordate and cephalochordate-vertebrate divergence, and a subsequent duplication of the ancestral TRP gene in the vertebrate lineage gave rise to two TRP genes before the emergence of teleost fishes. Evolution of the melanin synthetic pathway and possible phylogenetic relationships among chordate pigment cells that accommodate the metabolic process are discussed. Dev Dyn 1999;215:225-237.

Cell fate specification by localized cytoplasmic determinants and cell interactions in ascidian embryos

Tadpole larvae of ascidians show the basic body plan of chordates. An ascidian larva consists of only a few types of cells and has a relatively small number of cells. Cell lineages are invariant among individuals and have been described in detail. These advantages facilitate the analysis of how the fate of each blastomere becomes specified during development. Over a century of research on ascidian embryogenesis has uncovered many interesting features concerning cellular mechanisms responsible for the fate specification. During embryogenesis, the developmental fate of a blastomere is specified by one of three different mechanisms: localized maternal cytoplasmic determinants, inductive interactions, or lateral inhibition in an equivalence cell group.

Distinct neuronal lineages of the ascidian embryo revealed by expression of a sodium channel gene

The ascidian larva contains tubular neural tissue, one of the prominent anatomical features of the chordates. The cell-cleavage pattern and cell maps of the nervous system have been described in the ascidian larva in great detail. Cell types in the neural tube, however, have not yet been defined due to the lack of a suitable molecular marker. In the present work, we identified neuronal cells in the caudal neural tube of the Halocynthia embryo by utilizing a voltage-gated Na+ channel gene, TuNa I, as a molecular marker. Microinjection of a lineage tracer revealed that TuNa I-positive neurons in the brain and in the trunk epidermis are derived from the a-line of the eight-cell embryo, which includes cell fates to epidermal and neural tissue. On the other hand, TuNa I-positive cells in the more caudal part of the neural tissue were not stained by microinjection into the a-line. These neurons are derived from the A-line, which contains fates of notochord and muscle, but not of epidermis. Electron microscopic observation confirmed that A-line-derived neurons consist of motor neurons innervating the dorsal and ventral muscle cells. Isolated A-line blastomeres have active membrane excitability distinct from those of the a-line-derived neuronal cells after culture under cleavage arrest, suggesting that the A-line gives rise to a neuronal cell distinct from that of the a-lineage. TuNa I expression in the a-line requires signals from another cell lineage, whereas that in the A-line occurs without tight cell contact. Thus, there are at least two distinct neuronal lineages with distinct cellular behaviors in the ascidian larva: the a-line gives rise to numerous neuronal cells, including sensory cells, controlled by a mechanism similar to vertebrate neural induction, whereas A-line cells give rise to motor neurons and ependymal cells in the caudal neural tube that develop in close association with the notochord or muscle lineage, but not with the epidermal lineage.

Neural tube is partially dorsalized by overexpression of HrPax-37: the ascidian homologue of Pax-3 and Pax-7

The origin and elaboration of the central nervous system played an important role in chordate and vertebrate history. All chordates possess a dorsal tubular central nervous system, but elaboration of dorsoventral and segmental pattern is far more pronounced in cephalochordates and vertebrates than in the more basal urochordates. Analysis of the urochordates, therefore, should allow deduction of the neural organization and neuronal patterning mechanisms that predated overt dorsoventral and segmental complexity. Here we report functional studies of the ascidian Pax gene (HrPax-37). The spatiotemporal expression pattern of HrPax-37 has suggested involvement in two distinct developmental processes: specification of dorsal cell fates of ectoderm during neurulation, and regional differentiation of the neural tube in later stages. Here we show that HrPax-37 is descendent from the precursor of the Pax-3 and Pax-7 genes implicated in specification of dorsal fate in the vertebrate neural tube. We also demonstrate that injection of HrPax-37 RNA into fertilized eggs causes ectopic expression of the dorsal neural marker tyrosinase gene in neurulae, confirming a regulatory role in dorsal patterning of the neural tube comparable to its vertebrate homologues. These results suggest that dorsal specification in the neural tube by Pax-3/7 subfamily genes was established in the ancestors of extant chordates during emergence of the dorsal tubular nervous system.

Ascidian tyrosinase gene: its unique structure and expression in the developing brain

Tadpole larvae of ascidians have two sensory pigment cells in the brain. One is the otolith cell that functions as a gravity receptor, the other pigment cell is part of a primitive photosensory structure termed the ocellus. These sensory cells, like vertebrate pigment cells, contain membrane-bounded melanin granules and are considered to reflect a crucial position in the evolutionary process of this cell type. To investigate the molecular changes accompanying the evolution of pigment cells, we have isolated from Halocynthia roretzi a gene encoding tyrosinase, a key enzyme in melanin biosynthesis. The cDNA has an open reading frame (ORF) of 596 amino acids, which is 36-39% identical in amino acid sequence to vertebrate tyrosinases. In addition, the sequence analysis of both cDNA and genomic clones reveals an unusual organization of the tyrosinase gene, an extraordinary 3' untranslated region of the transcripts with significant homology to the coding sequence, and a single short intron in the sequence encoding a cytoplasmic domain. Expression of the gene is detected first in two pigment precursor cells positioned in the neural plate of early neurulae, and later in two melanin-containing pigment cells within the brain of late tailbud embryos. Its expression pattern correlates well with the appearance of tyrosinase enzyme activity in the developing brain. These results provide the first description of pigment cell differentiation at the molecular level in the ascidian embryo, and also will contribute to a better understanding of the evolution of chordate pigment cells.

Neural expression of a sodium channel gene requires cell-specific interactions

In the protochordate Halocynthia roretzi, voltage-activated sodium current undergoes a change in kinetics within 48 hr of fertilization. Molecular cloning and microinjection of antisense DNA into single cells suggest that the kinetic changes are due to the increased expression of a putative neural-specific sodium channel gene, TuNa I. TuNa I gene transcription is first induced in late stage gastrulae, preceding the appearance of the rapidly inactivating sodium current unique to neural cells. In cleavage-arrested and intact embryos, cell interactions between specific animal and vegetal blastomeres are required for induction of TuNa I gene expression. Our results implicate cell contact, prior to neurulation, as a mechanism for selectively activating the TuNa I gene expressed in cells of the neural lineage.

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.

The ascidian embryo as a prototype of vertebrate neurogenesis

Ascidian tadpole larvae, composed of only about 2500 cells, have a primitive nervous system which is derived from the neural plate. The stereotyped cell cleavage pattern and well characterized cell lineage in these animals allow the isolation and culture of identified blastomeres in variable combinations. Ascidian embryos express cell-type-specific markers corresponding to their cell fates, even when cultured under cleavage-arrest by cytochalasin B. This system provides us with a unique opportunity to study the roles of cell lineage and cell contact in early neuronal differentiation in the absence of events associated with complex morphogenesis. In addition, the isolated, cleavage-arrested blastomeres are ideally suited to electrical recording, permitting the use of ionic channels as specific markers for differentiation. In the cleavage-arrested embryos, suppression of one type of K+ channel, and induction of two types of Na+ channels, occur following cell contact with the vegetal blastomere. The combination of molecular and electrophysiological analyses on this simple animal system may provide insights into the nature of the cell interactions important in early neurogenesis, both in ascidians and in vertebrates.

Developmental potential for tissue differentiation of fully dissociated cells of the ascidian embryo

Initially, each tissue-progenitor blastomere of embryos of the ascidian Halocynthia was identified and isolated manually at the 110-cell (late-blastula) stage, the time at which most of the blastomeres have assumed a particular fate, such that each gives rise to a single type of tissue. The isolates were allowed to develop as partial embryos, then tissue differentiation was examined by monitoring the expression of specific molecular markers for differentiation of epidermis, endoderm, muscle and notochord. Essentially, all of the precursor blastomeres of these four kinds of tissue expressed the appropriate features of tissue differentiation in isolation, indicating that determination is already complete in most of the blastomeres by the 110-cell stage. Next, in order to evaluate the absolute capacity of cells for autonomous development, embryos were maintained continuously in a dissociated state from the first cleavage to the 110-cell stage, then the cells were allowed to develop into partial embryos. Tissue differentiation in the partial embryos was examined. The results showed the striking autonomy of the processes of segregation of developmental potential, as well as the autonomy of the processes of expression of differentiated phenotypes, namely those of epidermis and endoderm. Autonomous muscle differentiation was also observed; however, excess formation of "muscle" partial embryos occurred. The hypothesis that fate determination is mediated by localized maternal information in the egg cytoplasm is supported by the evidence of development of these tissues. By contrast, no evidence of notochord differentiation was observed in the partial embryos.

Structure of the caudal neural tube in an ascidian larva: vestiges of its possible evolutionary origin from a ciliated band

Ultrastructural analysis and differential immunocytochemical staining with two antitubulin monoclonal antibodies were used to reexamine the organization and development of the neural tube in the larva of an ascidian, Ciona intestinalis, in appraisal of a theory that the dorsal tubular nervous system of the chordates evolved from two halves of a ciliated band in an auricularia-like larva of the kind found in echinoderms and hemichordates. One of the antibodies stained cilia in the nervous system and elsewhere; the other reacted primarily with neuronal axons. The caudal neural tube consists of four rows of large ciliated ependymal-glial cells enclosing an axial neural canal into which their single cilia extend. Two ventrolateral nerve tracts, containing axons, arise in the posterior brain region and extend along the length of the caudal tube, partially surrounded by the ependymal cells. The nonnervous, ciliated, ependymal neural tube of the ascidian larva with its two associated nerve tracts survives as a primitive early condition that could result from a ciliated band transformation. Tissues in the distal-most part of the ascidian larval tail have cell lineage origins that indicate an evolutionary history different from those in the proximal majority of the tail. The ependymal cells in this presumed later addition to the tail are not ciliated, although all of the others in the caudal ependymal tube appear to be.