Differentiation of histospecific ultrastructural features in cells of cleavage-arrested early ascidian embryos

Regulators specifying cell fate activate cell cycle regulator genes to determine cell numbers in ascidian larval tissues

In animal development, most cell types stop dividing before terminal differentiation; thus, cell cycle control is tightly linked to cell differentiation programmes. In ascidian embryos, cell lineages do not vary among individuals, and rounds of the cell cycle are determined according to cell lineages. Notochord and muscle cells stop dividing after eight or nine rounds of cell division depending on their lineages. In the present study, we showed that a Cdk inhibitor, Cdkn1.b, is responsible for stopping cell cycle progression in these lineages. Cdkn1.b is also necessary for epidermal cells to stop dividing. In contrast, mesenchymal and endodermal cells continue to divide even after hatching, and Myc is responsible for maintaining cell cycle progression in these tissues. Expression of Cdkn1.b in notochord and muscle is controlled by transcription factors that specify the developmental fate of notochord and muscle. Likewise, expression of Myc in mesenchyme and endoderm is under control of transcription factors that specify the developmental fate of mesenchyme and endoderm. Thus, cell fate specification and cell cycle control are linked by these transcription factors.

Neurogenic and non-neurogenic placodes in ascidians

The late differentiation of the ectodermal layer is analysed in the ascidians Ciona intestinalis and Botryllus schlosseri, by means of light and electron microscopy, in order to verify the possible presence of placodal structures. Cranial placodes, ectodermal regions giving rise to nonepidermal cell types, are classically found exclusively in vertebrates; however, data are accumulating to demonstrate that the nonvertebrate chordates possess both the genetic machinery involved in placode differentiation, and ectodermal structures that are possible homologues of vertebrate placodes. Here, the term "placode" is used in a broad sense and defines thickenings of the ectodermal layer that can exhibit an interruption of the basal lamina where cells delaminate, and so are able to acquire a nonepidermal fate. A number of neurogenic placodes, ones capable of producing neurons, have been recognised; their derivatives have been analysed and their possible homologies with vertebrate placodes are discussed. In particular, the stomodeal placode may be considered a multiple placode, being composed of different sorts of placodes: part of it, which differentiates hair cells, is discussed as homologous to the octavo-lateralis placodes, while the remaining portion, giving rise to the ciliated duct of the neural gland, is considered homologous to the adenohypophyseal placode. The neurohypophyseal placode may include the homologues of the hypothalamus and vertebrate olfactory placode; the rostral placode, producing the sensorial papillae, may possibly be homologous to the placodes of the adhesive gland of vertebrates.

The BMP/CHORDIN antagonism controls sensory pigment cell specification and differentiation in the ascidian embryo

We have investigated the role of the bone morphogenetic protein (BMP) pathway during neural tissue formation in the ascidian embryo. The orthologue of the BMP antagonist, chordin, was isolated from the ascidian Halocynthia roretzi. While both the expression pattern and the phenotype observed by overexpressing chordin or BMPb (the dpp-subclass BMP) do not suggest a role for these factors in neural induction, BMP/CHORDIN antagonism was found to affect neural patterning. Overexpression of BMPb induced ectopic sensory pigment cells in the brain lineages that do not normally form pigment cells and suppressed pressure organ formation within the brain. Reciprocally, overexpressing chordin suppressed pigment cell formation and induced ectopic pressure organ. We show that pigment cell formation occurs in three steps. (1) During cleavage stages ectodermal cells are neuralized by a vegetal signal that can be substituted by bFGF. (2) At the early gastrula stage, BMPb secreted from the lateral nerve cord blastomeres induces those neuralized blastomeres in close proximity to adopt a pigment cell fate. (3) At the tailbud stage, among these pigment cell precursors, BMPb induces the differentiation of specifically the anterior type of pigment cell, the otolith; while posteriorly, CHORDIN suppresses BMP activity and allows ocellus differentiation.

WILSON STEPHEN. Asymmetry in the epithalamus of vertebrates

The epithalamus is a major subdivision of the diencephalon constituted by the habenular nuclei and pineal complex. Structural asymmetries in this region are widespread amongst vertebrates and involve differences in size. neuronal organisation, neurochemistry and connectivity. In species that possess a photoreceptive parapineal organ, this structure projects asymmetrically to the left habenula, and in teleosts it is also situated on the left side of the brain. Asymmetries in size between the left and right sides of the habenula are often associated with asymmetries in neuronal organisation, although these two types of asymmetry follow different evolutionary courses. While the former is more conspicuous in fishes (with the exception of teleosts), asymmetries in neuronal organisation are more robust in amphibia and reptiles. Connectivity of the parapineal organ with the left habenula is not always coupled with asymmetries in habenular size and/or neuronal organisation suggesting that, at least in some species, assignment of parapineal and habenular asymmetries may be independent events. The evolutionary origins of epithalamic structures are uncertain but asymmetry in this region is likely to have existed at the origin of the vertebrate, perhaps even the chordate, lineage. In at least some extant vertebrate species, epithalamic asymmetries are established early in development, suggesting a genetic regulation of asymmetry. In some cases, epigenetic factors such as hormones also influence the development of sexually dimorphic habenular asymmetries. Although the genetic and developmental mechanisms by which neuroanatomical asymmetries are established remain obscure, some clues regarding the mechanisms underlying laterality decisions have recently come from studies in zebrafish. The Nodal signalling pathway regulates laterality by biasing an otherwise stochastic laterality decision to the left side of the epithalamus. This genetic mechanism ensures a consistency of epithalamic laterality within the population. Between species, the laterality of asymmetry is variable and a clear evolutionary picture is missing. We propose that epithalamic structural asymmetries per se and not the laterality of these asymmetries are important for the behaviour of individuals within a species. A consistency of the laterality within a population may play a role in social behaviours between individuals of the species.

Mechanism of neurogenesis during the embryonic development of a tunicate

Ascidian and vertebrate nervous systems share basic characteristics, such as their origin from a neural plate, a tripartite regionalization of the brain, and the expression of similar genes during development. In ascidians, the larval chordate-like nervous system regresses during metamorphosis, and the adult's neural complex, composed of the cerebral ganglion and the associated neural gland is formed. Classically, the homology of the neural gland with the vertebrate hypophysis has long been debated. We show that in the colonial ascidian Botryllus schlosseri, the primordium of the neural complex consists of the ectodermal neurohypophysial duct, which forms from the left side of the anterior end of the embryonal neural tube. The duct contacts and fuses with the ciliated duct rudiment, a pharyngeal dorsal evagination whose cells exhibit ectodermic markers being covered by a tunic. The neurohypophysial duct then differentiates into the neural gland rudiment whereas its ventral wall begins to proliferate pioneer nerve cells which migrate and converge to make up the cerebral ganglion. The most posterior part of the neural gland differentiates into the dorsal organ, homologous to the dorsal strand. Neurogenetic mechanisms in embryogenesis and vegetative reproduction of B. schlosseri are compared, and the possible homology of the neurohypophysial duct with the olfactory/adenohypophysial/hypothalamic placodes of vertebrates is discussed. In particular, the evidence that neurohypophysial duct cells are able to delaminate and migrate as neuronal cells suggests that the common ancestor of all chordates possessed the precursor of vertebrate neural crest/placode cells.

Specification of notochord cells in the ascidian embryo analysed with a specific monoclonal antibody

Among 40 notochord cells of an ascidian tadpole larva, 32 notochord cells originate from the anterior-vegetal blastomeres (the A4.1 pair) of an 8-cell embryo and eight cells originate from the posterior-vegetal blastomeres (the B4.1 pair), but the animal blastomeres (the a4.2 and b4.2 pairs) are not engaged in the formation of the notochord. If four pairs of cells, separated from an 8-cell embryo, were allowed to develop into quarter embryos, expression of the notochord-specific antigen was evident in the A4.1 and B4.1 quarter embryos. Embryos, in which cytokinesis had been permanently blocked at the 8-cell and later stages with cytochalasin B, were found to develop the notochord-specific antigen only in the presumptive notochord cells. These findings suggest the developmental autonomy of presumptive notochord cells in the ascidian embryo.

Two histospecific enzyme expressions in the same cleavage-arrested one-celled ascidian embryos

Fertilized eggs of the ascidian, Ciona intestinalis, were prevented from undergoing cytokinesis but not nuclear division by treatment with cytochalasin B. After appropriate times, such cleavage-arrested multinucleate zygotes developed acetylcholinesterase of larval tail muscle and an alkaline phosphatase ordinarily localized in the larval endoderm tissues. Separate histochemical reactions on one of a pair of samples taken from the eggs of single animals provided examples (6/34) in which the numbers of cytochalasin-treated embryos displaying the respective reaction product overlapped sufficiently (15-29%) to indicate that some of the zygotes had developed both enzymes in the same uncleaved single cell. With an actual dual-staining technique that can be applied to single cleavage-arrested zygotes, 62% of those developing a strong alkaline phosphatase reaction also had a strong acetylcholinesterase reaction. In other experiments, quantitative measurements of enzyme activity in homogenates of 114 single cleavage-arrested zygotes confirm directly that 18% of the zygotes produce both enzymes. There was no obligatory mutual exclusion of the potential for simultaneous expression of two tissue-specific characteristics that would ordinarily be segregated into different lineages during early cleavages. The cytoplasmic determinants believed responsible for these histotypic expressions can apparently function independently in the same cell.

Determination and regulation in the pigment cell lineage of the ascidian embryo

The brain of the ascidian larva comprises two pigment cells, termed the ocellus melanocyte and the otolith melanocyte. Cell lineage analysis has shown that the two bilateral pigment lineage cells (a-line blastomeres) in the animal hemisphere give rise to these melanocytes in a complementary manner. The results of the present investigation suggest that the specification of the fate of pigment cells proceeds in two distinct steps. First, the determination of pigment lineage cells requires an inductive interaction from the vegetal blastomeres of the A-line. Cell dissociation experiments demonstrated that the inductive interaction is completed by the midgastrula stage. However, the two bilaterally positioned cells destined to become the pigment cells in the first step are still equipotent at this stage in that they can give rise to either the ocellus or otolith. Thus, they constitute what is termed an "equivalence group." In the second step, the individual fates of the two cells that compose the equivalence group are determined. Namely, one cell develops into an ocellus and the other cell develops into an otolith. Photoablation of one of the pigment precursor cells at various stages indicated that the second step of determination occurs at the midtailbud stage. It is suggested that the cue to choose one of the alternative developmental pathways may be positional information that exists along the anteroposterior axis. The second step of determination is thought to be mediated by a hierarchical interaction. In the absence of this interaction, melanocyte specification proceeds along the dominant pathway that results in the differentiation of an ocellus.

Cell differentiation features in embryos resulting from interphylum nuclear transplantation: echinoderm nucleus to ascidian zygote cytoplasm

When an echinoderm nucleus was transplanted into an ascidian zygote cytoplast there was developmental cooperation at the cellular level between nucleus and cytoplasm of these normally nonhybridizable species. A blastula stage nucleus from the sand dollar Echinarachnius parma was injected into an activated but nonnucleate egg fragment of the ascidian Ciona intestinalis. During culture, some of the "hybrid" embryos displayed ultrastructural evidence of cellular differentiation. Two recognizable features were (1) extracellular matrix components, and (2) neural cell characteristics, including elaboration of associated cilia. Nonnucleate zygote fragments alone, and such fragments injected with seawater or punctured by glass needle, did not develop organized subcellular structures. Morphologic expressions resulting from nuclear transplantations between these two phyla (Echinodermata and Chordata) seemingly indicate functional interactions at a gene regulatory level. Creation of such nuclear-cytoplasmic hybrids suggests thereby a means of exploring the nature of the egg cytoplasmic agents in ascidian embryos that appear to determine gene expression related to histospecific differentiation products.

Development of the central nervous system of the larva of the ascidian, Ciona intestinalis L. II. Neural plate morphogenesis and cell lineages during neurulation

We describe the lineage and morphogenesis of neural plate cells in the ascidian, Ciona intestinalis, from reconstructed cell maps of embryos at 12-min intervals during and after neurulation, between 31 and 61% of embryonic development. Neurulation commences in a posterior to anterior wave following in the wake of the ninth cleavage, when all cells, except possibly four, are in their 10th generation. The neural plate then comprises 76 cells, in up to four posterior rows each of eight vegetal-hemisphere cells, and eight anterior rows each of six animal-hemisphere cells. Two cells are lost from the neural plate to the muscle cell line during neurulation and four cells are gained from ectoderm outside the plate. All cells become wedge-shaped. Simple, stereotyped positional changes transform cells from lateral locations in the plate to posterior locations in the tube; bilateral partners shear their midline positions to form the keel, and ectodermal cells zipper up dorsally to form the capstone, of a tube which is four cells in cross section posteriorly, but more complex anteriorly. Neither cell death nor migration occur during neurulation. Divisions become asynchronous and the cell-cycle extends; 170 10th- to 12th-generation cells exist by the time the neural tube becomes completely internalized. Generally, only one further division is required to complete the lineage analysis, two at the most. Neural plate cell divisions were invariant using our observational methods, and their lineage is compared with that from recent studies of H. Nishida (1987, Dev. Biol. 121, 526-541).

Development of the central nervous system of the larva of the ascidian, Ciona intestinalis L. I. The early lineages of the neural plate

The early lineages of the larval central nervous system (CNS) of the ascidian, Ciona intestinalis, have been traced using scanning electron microscopy (SEM) of embryos fixed at 12-min intervals. The CNS precursors lie superficially, exposed for a long portion (9.3 hr of 42%) of embryonic development, in the neural plate. In the 64-cell stage embryo the neural plate contains 10 cells; in all but the first vegetal division these divide with transverse cleavage planes. Synchrony is progressively lessened, but temporal sequence is always exact. Successive divisions occur initially at 30-min intervals. Our analysis confirms existing lineage descriptions for the neural plate up to the end of gastrulation and advances the lineage record through the crucial and temporally complex ninth cleavage, during which cells divide by the following rules: medial cells in each row divide first; the anterior row of vegetal daughter cells divides before their posterior siblings; the posterior row of animal daughter cells divide before their anterior siblings. All cells attain their 10th generation, but four cannot be followed by SEM. In preparation for neurulation the neural plate then comprises 76 cells, forming up to four rows each of eight vegetal hemisphere cells located on the dorsal surface of the embryo, anterior to the blastopore, and eight rows each of six animal hemisphere cells, located anterior to the rows of eight. The temporal and spatial patterns of early cleavage stages have been confirmed in vivo by observations using Nomarski optics.

Development of myoplasm-enriched ascidian embryos

The fertilized ascidian egg is thought to be comprised of distinct regions of tissue-specific cytoplasmic determinants. This idea was tested by bisecting fertilized eggs into egg fragments and culturing them until the unoperated controls developed into larvae. Fertilized eggs were bisected using a microsurgical method in which part of the uncleaved zygote was extruded through a hole made in the follicular envelope and the cytoplasmic bridge between the two egg regions was severed. One egg fragment contained all of the egg myoplasm (termed myoplasm-enriched or ME fragment), while the other fragment lacked myoplasm. ME fragments consisting of 40-50% of the total egg volume in many cases cleaved normally and developed into larvae. In a few cases, ME larvae initiated metamorphosis and developed into normal juveniles. Triton-extraction of ME embryos and larvae showed that the myoplasm was redistributed into nonmuscle lineage cells at each stage of development. Despite the redistribution of myoplasm into many of the endoderm cells situated in the head region of ME larvae, the expression of the muscle-specific enzyme acetylcholinesterase (AchE) and a muscle-specific antigen (Mu-2) was restricted to the tail muscle cells. The endoderm cells situated in the head region of ME larvae expressed an endoderm-specific enzyme alkaline phosphatase (AP) as in the controls. Furthermore, cleavage-arrested four- and eight-cell ME embryos expressed AchE activity in the expected number of blastomeres. When a greater quantity of myoplasm was redistributed into cells that normally do not express AchE activity by producing 10-30% ME embryos, in a few cases more than the expected number of blastomeres expressed AchE activity. In conclusion, the main finding of the present investigation, based on the development of ME fragments comprising 40-50% of the total egg volume, is that ascidian embryos are capable of regulative development.

Development of a muscle actin specified by maternal and zygotic mRNA in ascidian embryos

In this investigation, we characterize the embryonic and adult actins and describe the embryonic expression of a muscle actin in the ascidian Styela. Two-dimensional polyacrylamide gel electrophoresis showed that embryos, tadpole larvae, and adult organs contain three major and two minor isoforms of actin. Two of the major isoforms, which are present in the mantle, branchial sac, alimentary tract, and gonads of adults and in eggs, embryos, and heads and tails of tadpoles, are likely to be cytoplasmic actins. The third major isoform, which was enriched in the mantle and branchial sac of adults and localized primarily in the tails of tadpoles, is a muscle actin. The muscle actin isoform was not detected in eggs and early embryos. Radioactivity incorporation studies showed that the cytoplasmic actins were synthesized throughout early development, but muscle actin synthesis was first detected between the 16- and 64-cell stages, 2-3 hr after fertilization. Two lines of evidence indicate that embryonic muscle actin synthesis is directed in part by maternal mRNA. First, poly(A)+ RNA isolated from unfertilized eggs directed the synthesis of muscle actin in an mRNA-dependent reticulocyte lysate. Second, muscle actin was synthesized in anucleate egg fragments. Arguments are also presented that muscle actin synthesis is not directed exclusively by maternal mRNA. It is concluded that embryonic and adult Styela exhibit actin heterogeneity, that one of the actin isoforms is a muscle actin, and that the muscle actin is synthesized during embryogenesis under the direction of maternal and zygotic mRNA.

Expression of epidermis-specific antigens during embryogenesis of the ascidian, Halocynthia roretzi

We have produced two monoclonal antibodies (Epi-1 and Epi-2) which specifically recognize epidermal cells and their derivative, the larval tunic, of developing embryos of the ascidian Halocynthia roretzi. The antigens, examined by indirect immunofluorescence staining, first appear at the early tailbud stage and are present until at least the swimming larval stage. There were distinct and separate puromycin and actinomycin D sensitivity periods for each antigen. Aphidicolin, a specific inhibitor of DNA synthesis, prevented the appearance of each antigen when embryos were exposed to the drug continuously from cleavage stages. These results suggest that the antigens are synthesized during embryogenesis by developing epidermal cells and that several rounds of DNA replication are required for the antigen expression. Early cleavage stage embryos, including fertilized but unsegmented eggs, in which cytokinesis had been blocked with cytochalasin B expressed the antigens, and blastomeres exhibiting the antigens were always of the epidermis lineage. In partial embryos produced by four separated blastomere pairs of the 8-cell embryos, the expression of antigens was seen only in those developed from the animal blastomere pairs, which are progenitors of epidermal cells. These observations indicate that differentiation of epidermal cells in ascidian embryos takes place in a typical "mosaic" fashion.

Alkaline phosphatase expression in ascidian egg fragments and andromerogons

Alkaline phosphatase (AP) activity is expressed in the endodermal cell lineage of ascidian embryos beginning at gastrulation. AP expression is resistant to levels of actinomycin D which completely suppress the appearance of other tissue-specific enzyme and morphological markers including acetylcholinesterase (AchE), a larval muscle enzyme whose expression requires embryonic transcription. The resistance of AP expression to actinomycin D has led to the proposal that AP may be expressed independent of embryonic transcription by the translational activation of maternal AP mRNA. To test this hypothesis, AP expression was examined in fragments of unfertilized and fertilized Styela plicata eggs by histochemical methods. As expected, nucleate fragments from fertilized eggs developed into larvae which exhibited AP activity in their endodermal cells and AchE activity in their muscle cells. In contrast, anucleate fragments from fertilized eggs, cultured until controls reached the larval stage, did not develop AP or AchE activity. The lack of AP activity was unrelated to the absence of cleavage or to the ooplasmic composition of the anucleate fragments. Anucleate fragments from unfertilized eggs were also AP negative, unless they were inseminated, after which they often developed to the larval stage as andromerogons and exhibited AP activity in their endodermal cells. The development of endodermal AP in andromerogons suggests that the factors responsible for AP expression are not localized in or attached to the maternal nucleus. In summary, the results suggest that AP expression requires nuclear events and is not determined exclusively by maternal cytoplasmic factors such as preformed mRNA.