Centrosome-attracting body: a novel structure closely related to unequal cleavages in the ascidian embryo

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.

Dynamic redistribution of vasa homolog and exclusion of somatic cell determinants during germ cell specification in Ciona intestinalis

Ascidian embryos sequester a specific cytoplasm, called the postplasm, at the posterior pole, where many maternal RNAs and proteins accumulate. Although the postplasm is thought to act as the germ plasm, it is also highly enriched in several factors essential for somatic cell development, and how the postplasm components regulate both germ and somatic cell differentiation remains elusive. Using a vasa homolog, CiVH, and other postplasmic components as markers, we found that the postplasm-containing blastomeres, the B7.6 cells, undergo an asymmetric cell division during gastrulation to produce two distinct daughter cells: B8.11 and B8.12. Most of the postplasmic components segregate only into the B8.11 cells, which never coalesce into the gonad. By contrast, the maternal CiVH RNA and protein are specifically distributed into the B8.12 cells, which divide further and are incorporated into the gonad in juveniles. In the B8.12 cells, CiVH production is upregulated from the maternal RNA source, resulting in the formation of perinuclear CiVH granules, which may be the nuage, a hallmark of germ cells in many animal species. We propose that the redistribution of specific maternal molecules into the B8.12 cells is essential for germ-cell specification in ascidians.

Comparative organization of follicle, accessory cells and spawning anlagen in dynamic semelparous clutch manipulators, the urochordate Oikopleuridae

BACKGROUND INFORMATION

The urochordate appendicularians play a key trophic role in marine ecosystems and are the second largest component of zooplankton after copepods. Part of their success is due to their ability to undergo rapid population blooms in response to changes in primary productivity. Nonetheless, the reproductive biology of this important group remains poorly understood.

RESULTS

In the present study, we investigated the organization of male and female germ and accessory somatic cells in the Oikopleuridae. We found that the structure of the ovary had been previously misconstrued as consisting of germ and accessory 'cells' interspersed together, whereas, in fact, the germline exists as a giant transparent syncytium. Somatic follicle cells, integral to regulation of the temporal progression of gametogenesis, could be classified into three types in females and two in males, and we characterized functional gap junctions between follicle cells and the germline syncytium in both sexes. The number of follicle cells per oocyte produced was much reduced in comparison with many commonly studied model organisms. We further identified a novel anlagen that permits spawning of the animal via rupture of the gonad wall, which is obligatory for the release of oocytes, but optional for the release of sperm that usually occurs via the spermiduct.

CONCLUSIONS

The organization of the female germline in the Oikopleuridae shares some features of meroistic oogenesis with the arthropod Drosophila, but the process of synchronous oogenesis in these semelparous organisms remains quite distinctive with respect to that previously characterized in the animal kingdom and certainly within the chordate phylum.

Embryonic expression profiles and conserved localization mechanisms of pem/postplasmic mRNAs of two species of ascidian, Ciona intestinalis and Ciona savignyi

In many animals, the first cue for development is transcripts and/or proteins that are provided maternally and are localized at specific regions of fertilized eggs and early embryos. The ascidian is known to exhibit a mosaic mode of development, which is largely dependent on localized maternal factors. In early Ciona intestinalis embryos, the posterior-most localization appears to be the major specialized pattern of maternal transcripts. The present study examined the temporal and spatial expression pattern of 40 genes known as pem/postplasmic genes, for which maternal mRNAs are localized at the posterior-most region during early Ciona embryogenesis. Ten of these genes showed redistribution to B8.12-line cells, which are known to give rise to germ cells in ascidians. In addition 23 orthologues were newly identified in a related ascidian species, Ciona savignyi, and 16 of them showed the mRNA localization pattern at the posterior-most region. Furthermore, the localized pattern of exogenous mRNA, which comprised the 3' UTR of C. intestinalis pem/postplasmic genes conjugated with the LacZ ORF, showed the localization at the posterior-most region in C. savignyi embryos. Likewise, the 3' UTR of C. savignyi pem/postplasmic genes conjugated with the LacZ ORF showed localization at the posterior most region in C. intestinalis embryos, suggesting that localization mechanisms are conserved between the two species. The present study therefore provides basic information for future functional analyses of these pem/postplasmic genes and for exploring the mechanisms of localization of mRNAs.

Cellular morphogenesis in ascidians: how to shape a simple tadpole

Ascidians are invertebrate chordates that form tadpole larvae with a surprisingly small number of cells. Recently, the emergence of powerful molecular tools to study cell fate determination in ascidians has been complemented by studies, often at cellular resolution, of morphogenetic processes. These studies point to a complex interplay among mechanisms that control cell fate and polarity and those that govern cell shape change and morphogenesis. The relative simplicity and stereotypy of ascidian development suggests that it will be possible to understand, and possibly to mathematically model, this dynamic coupling between cell fate and shape change.

A quantitative approach to the study of cell shapes and interactions during early chordate embryogenesis

BACKGROUND

The prospects of deciphering the genetic program underlying embryonic development were recently boosted by the generation of large sets of precisely organized quantitative molecular data. In contrast, although the precise arrangement, interactions, and shapes of cells are crucial for the fulfilment of this program, their description remains coarse and qualitative. To bridge this gap, we developed a generic software, 3D Virtual Embryo, to quantify the geometry and interactions of cells in interactive three-dimensional embryo models. We applied this approach to early ascidian embryos, chosen because of their simplicity and their phylogenetic proximity to vertebrates.

RESULTS

We generated a collection of 19 interactive ascidian embryos between the 2- and 44-cell stages. We characterized the evolution with time, and in different cell lineages, of the volume of cells and of eight mathematical descriptors of their geometry, and we measured the surface of contact between neighboring blastomeres. These analyses first revealed that early embryonic blastomeres adopt a surprising variety of shapes, which appeared to be under strict and dynamic developmental control. Second, we found novel asymmetric cell divisions in the posterior vegetal lineages, which gave birth to sister cells with different fates. Third, during neural induction, differences in the area of contact between individual competent animal cells and inducing vegetal blastomeres appeared important to select the induced cells.

CONCLUSIONS

In addition to novel insight into both cell-autonomous and inductive processes controlling early ascidian development, we establish a generic conceptual framework for the quantitative analysis of embryo geometry that can be applied to other model organisms.

The aPKC-PAR-6-PAR-3 cell polarity complex localizes to the centrosome attracting body, a macroscopic cortical structure responsible for asymmetric divisions in the early ascidian embryo

Posterior blastomeres of 8-cell stage ascidian embryos undergo a series of asymmetric divisions that generate cells of unequal sizes and segregate muscle from germ cell fates. These divisions are orchestrated by a macroscopic cortical structure, the ;centrosome attracting body' (CAB) which controls spindle positioning and distribution of mRNA determinants. The CAB is composed of a mass of cortical endoplasmic reticulum containing mRNAs (the cER-mRNA domain) and an electron dense matrix, but little is known about its precise structure and functions. We have examined the ascidian homologues of PAR proteins, known to regulate polarity in many cell types. We found that aPKC, PAR-6 and PAR-3 proteins, but not their mRNAs, localize to the CAB during the series of asymmetric divisions. Surface particles rich in aPKC concentrate in the CAB at the level of cortical actin microfilaments and form a localized patch sandwiched between the plasma membrane and the cER-mRNA domain. Localization of aPKC to the CAB is dependent on actin but not microtubules. Both the aPKC layer and cER-mRNA domain adhere to cortical fragments prepared from 8-cell stage embryos. Astral microtubules emanating from the proximal centrosome contact the aPKC-rich cortical domain. Our observations indicate that asymmetric division involves the accumulation of the aPKC-PAR-6-PAR-3 complex at the cortical position beneath the pre-existing cER-mRNA domain.

The functional analysis of Type I postplasmic/PEM mRNAs in embryos of the ascidian Halocynthia roretzi

Maternal factors, such as a muscle determinant macho-1 mRNA that is localized to the posterior-vegetal cortex (PVC) of fertilized ascidian eggs, are crucial for embryonic axis formation and cell fate specification. Maternal mRNAs that show an identical posterior localization pattern to that of macho-1 in eggs and embryos are called Type I postplasmic/PEM mRNAs. We investigated the functions of five of the nine Type I mRNAs so far known in Halocynthia roretzi: Hr-Wnt-5, Hr-GLUT, Hr-PEM3, Hr-PEN1, and Hr-PEN2. Suppression of their functions with specific antisense morpholino oligonucleotides (MOs) had effects on the formation of various tissues: Hr-Wnt-5 on notochord, muscle, and mesenchyme, although zygotic function of Hr-Wnt-5 is responsible for notochord formation; Hr-GLUT on notochord, mesenchyme, and endoderm; and Hr-PEN2 on muscle, mesenchyme, and endoderm. On the other hand, Hr-PEM3 and Hr-PEN1 MOs seemed to have no effect. We conclude that the functions of at least some localized maternal Type I postplasmic/PEM mRNAs are necessary for early embryonic patterning in ascidians.

Specification of embryonic axis and mosaic development in ascidians

Setting up future body axes is the first important event before and at the beginning of embryogenesis. The ascidian embryo is a classic model that has been used to gain insight into developmental processes for over a century. This review summarizes advances made in this decade in our understanding of the developmental processes involved in the specification of the embryonic axes and cell fates during early ascidian embryogenesis. Maternal factors, including mRNAs, are translocated to specific regions of the egg by cytoplasmic and cortical reorganization, so-called ooplasmic segregation, and specify the animal-vegetal axis and the one perpendicular to it, which is defined as the anteroposterior axis in ascidians. Some postplasmic/PEM RNAs that are anchored to cortical endoplasmic reticulum are brought to the future posterior pole of fertilized eggs, and play crucial roles in posterior development. Following specification of the animal-vegetal axis, nuclear localization of beta-catenin takes place in the vegetal blastomeres; this occurrence is important for the acquisition of the vegetal character of the blastomeres in later development. Positioning of these maternal factors lead to subsequent cell interactions and zygotic gene expression responsible for axis establishment and for cell fate specification. We describe how endoderm blastomeres in the vegetal pole region emanate inductive signals mainly attributable to fibroblast growth factor. Marginal blastomeres next to endoderm blastomeres respond differently in ways that are determined by intrinsic competence factors. Expression patterns of developmentally important genes, including key transcription factors of each tissue type, are also summarized.

POPK-1/Sad-1 kinase is required for the proper translocation of maternal mRNAs and putative germ plasm at the posterior pole of the ascidian embryo

Maternal mRNAs localized to specific regions in eggs play important roles in the establishment of embryonic axes and germ layers in various species. Type I postplasmic/PEM mRNAs, which are localized to the posterior-vegetal cortex (PVC) of fertilized ascidian eggs, such as the muscle determinant macho-1 mRNA, play key roles in embryonic development. In the present study, we analyzed the function of the postplasmic/PEM RNA Hr-POPK-1, which encodes a kinase of Halocynthia roretzi. When the function of POPK-1 was suppressed by morpholino antisense oligonucleotides, the resulting malformed larvae did not form muscle or mesenchyme, as in macho-1-deficient embryos. Epistatic analysis indicated that POPK-1 acts upstream of macho-1. When POPK-1 was knocked down, localization of every Type I postplasmic/PEM mRNA examined, including macho-1, was perturbed, showing diffuse early distribution and eventual concentration into a smaller area. This is the probable reason for the macho-1 dysfunction. The postplasmic/PEM mRNAs such as macho-1 and Hr-PEM1 are co-localized with the cortical endoplasmic reticulum (cER) and move with it after fertilization. Eventually they become highly concentrated into a subcellular structure, the centrosome-attracting body (CAB), at the posterior pole of the cleaving embryos. The suppression of POPK-1 function reduced the size of the domain of concentrated cER at the posterior pole, indicating that POPK-1 is involved in the movement of postplasmic/PEM RNAs via relocalization of cER. The CAB also shrank. These results suggest that Hr-POPK-1 plays roles in concentration and positioning of the cER, as well as in the concentration of CAB materials, such as putative germ plasm, in the posterior blastomeres.

Microarray analysis of localization of maternal transcripts in eggs and early embryos of the ascidian, Ciona intestinalis

The establishment of body axes and specification of early embryonic cells depend on maternally supplied transcripts and/or proteins, several of which are localized at specific regions of fertilized eggs and early embryos. The ascidian is known to exhibit a mosaic mode of development, and this mode is largely dependent on localized maternal factors. Using blastomere isolation, microarray and whole-mount in situ hybridization, the present study of Ciona intestinalis demonstrates that maternal transcripts of a total of 17 genes are localized at the posterior-most region of fertilized eggs and early embryos. Ten of them are newly identified in the present study, while the remaining seven genes have already been characterized in previous studies. In addition, maternal transcripts of two genes, in addition to 14 genes encoded by the mitochondrial genome, showed a mitochondria-like distribution. Despite the present comprehensive approach, we could not identify maternal transcripts that are clearly localized to the animal-pole side, the vegetal-pole side, the anterior-side or other specific regions of the early embryo. Therefore, we concluded that the posterior-most localization and mitochondria-like distribution appear to be major specialized patterns of maternal transcripts in early Ciona embryos.

Asymmetric cell division in C. elegans: cortical polarity and spindle positioning

The one-cell Caenorhabditis elegans embryo divides asymmetrically into a larger and smaller blastomere, each with a different fate. How does such asymmetry arise? The sperm-supplied centrosome establishes an axis of polarity in the embryo that is transduced into the establishment of anterior and posterior cortical domains. These cortical domains define the polarity of the embryo, acting upstream of the PAR proteins. The PAR proteins, in turn, determine the subsequent segregation of fate determinants and the plane of cell division. We address how cortical asymmetry could be established, relying on data from C. elegans and other polarized cells, as well as from applicable models. We discuss how cortical polarity influences spindle position to accomplish an asymmetric division, presenting the current models of spindle orientation and anaphase spindle displacement. We focus on asymmetric cell division as a function of the actin and microtubule cytoskeletons, emphasizing the cell biology of polarity.

CiYB1 is a major component of storage mRNPs in ascidian oocytes: implications in translational regulation of localized mRNAs

In ascidian eggs, the existence of several localized maternal cytoplasmic determinants has been proposed and the importance of localized mRNAs for tissue differentiation has been demonstrated. We previously identified the ascidian Y-box proteins (CiYB1, 2 and 3), homologues of which are known to be involved in the storage of maternal mRNA in oocytes of other organisms. In this study, we found that CiYB1 protein is abundant in the gonad, egg, and embryo. Purification of messenger ribonucleoprotein (mRNP) particles from the gonad revealed that CiYB1 was one of their major components. A significant change in the distribution of CiYB1 protein from stored mRNP particles in the gonad to the ribosome fraction in eggs and embryos was observed. This change correlates most likely with the shift of stored maternal mRNAs to polyribosomes. Moreover, we found that CiYB1 colocalized with Cipem and Ci-macho1 mRNAs, which are localized at the posterior end of the embryo at the cleavage stage. Cipem and Ci-macho1 mRNAs were co-immunoprecipitated with CiYB1 in the oocyte and embryo lysates. The formation of a complex between Cipem mRNA and CiYB1 protein resulted in translational repression in the in vitro translation system. Our results indicate that associating with CiYB1 protein contributes to the translational control of the localized mRNA in eggs and embryos.

Maternal mRNAs of PEM and macho 1, the ascidian muscle determinant, associate and move with a rough endoplasmic reticulum network in the egg cortex

Localization of maternal mRNAs in the egg cortex is an essential feature of polarity in embryos of Drosophila, Xenopus and ascidians. In ascidians, maternal mRNAs such as macho 1, a determinant of primary muscle-cell fate, belong to a class of postplasmic RNAs that are located along the animal-vegetal gradient in the egg cortex. Between fertilization and cleavage, these postplasmic RNAs relocate in two main phases. They further concentrate and segregate in small posterior blastomeres into a cortical structure, the centrosome-attracting body (CAB), which is responsible for unequal cleavages.

By using high-resolution, fluorescent, in situ hybridization in eggs,zygotes and embryos of Halocynthia roretzi, we showed that macho 1 and HrPEM are localized on a reticulated structure situated within 2 μm of the surface of the unfertilized egg, and within 8 μm of the surface the vegetal region and then posterior region of the zygote. By isolating cortices from eggs and zygotes we demonstrated that this reticulated structure is a network of cortical rough endoplasmic reticulum (cER) that is tethered to the plasma membrane. The postplasmic RNAs macho 1 and HrPEM were located on the cER network and could be detached from it. We also show that macho 1 and HrPEM accumulated in the CAB and the cER network. We propose that these postplasmic RNAs relocalized after fertilization by following the microfilament- and microtubule-driven translocations of the cER network to the poles of the zygote. We also suggest that the RNAs segregate and concentrate in posterior blastomeres through compaction of the cER to form the CAB. A multimedia BioClip `Polarity inside the egg cortex' tells the story and can be downloaded at www.bioclips.com/bioclip.html

Maternal macho-1 is an intrinsic factor that makes cell response to the same FGF signal differ between mesenchyme and notochord induction in ascidian embryos

An extracellular signaling molecule acts on several types of cells, evoking characteristic and different responses depending on intrinsic factors in the signal-receiving cells. In ascidian embryos, notochord and mesenchyme are induced in the anterior and posterior margins, respectively, of the vegetal hemisphere by the same FGF signal emanating from endoderm precursors. The difference in the responsiveness depends on the inheritance of the posterior-vegetal egg cytoplasm. We show that macho-1, first identified as a localized muscle determinant, is also required for mesenchyme induction, and that it plays a role in making the cell response differ between notochord and mesenchyme induction. A zygotic event involving snail expression downstream of maternal macho-1 mediates the suppression of notochord induction in mesenchyme precursors.

Suppression of macho-1-directed muscle fate by FGF and BMP is required for formation of posterior endoderm in ascidian embryos

Specification of germ layers is a crucial event in early embryogenesis. In embryos of the ascidian, Halocynthia roretzi, endoderm cells originate from two distinct lineages in the vegetal hemisphere. Cell dissociation experiments suggest that cell interactions are required for posterior endoderm formation, which has hitherto been thought to be solely regulated by localized egg cytoplasmic factors. Without cell interaction, every descendant of posterior-vegetal blastomeres, including endoderm precursors, assumed muscle fate. Cell interactions are required for suppression of muscle fate and thereby promote endoderm differentiation in the posterior endoderm precursors. The cell interactions take place at the 16- to 32-cell stage. Inhibition of cell signaling by FGF receptor and MEK inhibitor also supported the requirement of cell interactions. Consistently, FGF was a potent signaling molecule, whose signaling is transduced by MEK-MAPK. By contrast, such cell interactions are not required for formation of the anterior endoderm. Our results suggest that another redundant signaling molecule is also involved in the posterior endoderm formation, which is likely to be mediated by BMP. Suppression of the function of macho-1, a muscle determinant in ascidian eggs, by antisense oligonucleotide was enough to allow autonomous endoderm specification. Therefore, the cell interactions induce endoderm formation by suppressing the function of macho-1, which is to promote muscle fate. These findings suggest the presence of novel mechanisms that suppress functions of inappropriately distributed maternal determinants via cell interactions after embryogenesis starts. Such cell interactions would restrict the regions where maternal determinants work, and play a key role in marking precise boundaries between precursor cells of different tissue types.

Localization and expression pattern of type I postplasmic mRNAs in embryos of the ascidian Halocynthia roretzi

The posterior-vegetal cytoplasm (PVC) of fertilized ascidian eggs plays important roles in embryo development. It has been reported that some maternal RNAs are localized to the PVC. We identified four novel type I postplasmic mRNAs that are localized to the PVC through the use of data from a cDNA project of maternal mRNAs in the eggs of Halocynthia roretzi (MAGEST database). The mRNAs are HrGLUT, HrPEN-1, and HrPEM-3, which show similarity to a glucose transporter, a g1-related protein, and Ciona pem-3, respectively; and HrPEN-2, with no similarity. Maternal mRNAs of all four genes were identically localized to the PVC after ooplasmic segregation. During cleavage, they were concentrated in the centrosome-attracting body (CAB) and were then segregated into the small blastomeres located at the posterior pole. This localization pattern is common to all known type I postplasmic mRNAs found so far. HrGLUT, HrPEN-1, and HrPEM-3 were expressed zygotically in various tissues later in embryogenesis: HrGLUT and HrPEM-3 in the mesenchyme and nervous system, and HrPEN-1 in the ectodermal cells.

Specification of developmental fates in ascidian embryos: molecular approach to maternal determinants and signaling molecules

Tadpole larvae of ascidians represent the basic body plan of chordates with a relatively small number and few types of cells. Because of their simplicity, ascidians have been intensively studied. More than a century of research on ascidian embryogenesis has uncovered many cellular and molecular mechanisms responsible for cell fate specification in the early embryo. This review describes recent advances in our understanding of the molecular mechanisms of fate specification mainly uncovered in model ascidian species--Halocynthia roretzi, Ciona intestinalis, and Ciona savignyi. One category of developmentally important molecules represents maternal localized mRNAs that are involved in cell-autonomous processes. In the second category, signaling molecules and downstream transcription factors are involved in inductive cell interactions. Together with genome-wide information, there is a renewed interest in studying ascidian embryos as a fascinating model system for understanding how single-celled eggs develop a highly organized chordate body plan.

Gene expression profiles in Ciona intestinalis cleavage-stage embryos

A total of 1612 expression sequence tags derived from Ciona intestinalis cleavage-stage embryos were examined to explore detailed gene expression profiles. The 3' sequences indicate that the 1612 clones can be categorized into 1066 independent clusters. DDBJ database search suggested that 496 of them showed significant matches to reported proteins with distinct functions. Among them 69 are associated with cell-cell communications and 41 with transcription factors. In situ hybridization of all 1066 clusters showed that 84 clusters exhibited blastomere-specific pattern of expression, and many of these genes seem to encode for novel proteins. One of the interesting findings is that most of them were expressed in the precursor cells of multiple tissues. Among them 28 genes were expressed in the marginal zone of the 32-cell embryo. The blastomeres in this region are thought to receive an inductive signal from the vegetal blastomeres. Many of the blastomere-specific genes did not show similarity to known proteins. The present analysis therefore provides new information for further analyses on the cell fate specification in the Ciona embryo.

Repression of zygotic gene expression in the putative germline cells in ascidian embryos

In germline cells of early C. elegans and Drosophila embryos, repression of zygotic gene expression appears to be essential to maintain the germ cell fate. In these animals, specific residues in the carboxy-terminal domain (CTD) of RNA polymerase II large subunit (RNAP II LS) are dephosphorylated in the germline cells, whereas they are phosphorylated in the somatic cells. We investigated, in early embryos of the ascidian Halocynthia roretzi, the expression patterns of three genes that are essentially expressed in the entire embryo after the 32-cell stage. We found that the expression of these genes was inactive in the putative germline cells during the cleavage stages. Once cells were separated from the germline lineage by cleavages, the expression of the genes was initiated in the cells. These results suggest that repression of transcription in germline cells may also be common in chordate embryos. We then examined the phosphorylation state of the CTD of RNAP II using a phosphoepitope-specific antibody. At cleavage stages after the 32-cell stage, CTD was phosphorylated in every blastomere, including the germline cells. Therefore, in the ascidian, the inactivation of zygotic transcription is not correlated with dephosphorylation of the CTD. These observations indicate that zygotic transcription is inactivated in ascidian germline cells, but the mechanism of the repression may differ from that in C. elegans and Drosophila.

Profiles of maternally expressed genes in fertilized eggs of Ciona intestinalis

A set of 1,378 expressed sequence tags (ESTs), both the 5'-most and 3'-most ends, derived from Ciona intestinalis fertilized eggs was categorized into 1,003 independent clusters. When compared with sequences in databases, 452 of the clusters showed significant matches with reported proteins, while 190 showed matches with putative proteins for which there is not enough information to categorize their function, and 361 had no significant similarities to known proteins. Sequence similarity analyses of the 452 clusters in relation to the biological function as well as the structure of the message population at this stage demonstrated that 362 of them have functions that many kinds of cells use, 65 are associated with cell-cell communication, including a candidate cDNA for sonic hedgehog, and 25 are transcription factors. Sequence prevalence distribution analysis demonstrated that the great majority (78%) of the mRNAs are rare mRNAs or are represented by a single clone/cluster. All of the 1,003 clusters were subjected to whole-mount in situ hybridization to analyze the distribution of the maternal mRNAs in fertilized eggs, and a total of 329 genes showed localized distribution of the mRNAs: 16 showed cortical localization, 12 showed mitochondrial-like distribution, 99 crescent-like distribution, 63 partial localization, and 139 weak localization. When the distribution pattern of all the maternally expressed mRNAs was examined in the 8-cell stage embryos, it became evident that 248 genes which have localized mRNA patterns at the fertilized egg stage lose their localized distribution by the 8-cell stage. In contrast, 13 genes newly gain a localized pattern by the 8-cell stage. In addition, a total of 39 genes showed distinct in situ signals in the nucleus of blastomeres of the 8-cell stage embryo, suggesting early zygotic expression of these genes by this stage. These results suggest that complicated cytoplasmic movements are associated with the characteristic distribution of maternal mRNAs, which in turn support proper embryonic axis formation and establishment of the genetic network for embryonic cell specification.

Determinants of cell and positional fate in ascidian embryos

Ascidians have played a major role in studies to understand the function of cytoplasmic determinants in animal development. Special qualities, including eggs with colored cytoplasmic regions, an invariant cleavage pattern and cell lineage, embryos with low cell numbers, larvae with typical chordate features and only six different tissues, rapid development, and a small genome, combine to make these animals a unique system for studying cytoplasmic determinants. There is evidence for determinants that specify the cleavage pattern; the differentiation of epidermal, endodermal, and muscle cells; and cell movements associated with gastrulation. The muscle determinants appear to be modified in concert with tail and muscle regression in species that have evolved an anural, or tailless, larva. Several lines of evidence suggest that determinants may be localized maternal mRNAs, which encode transcription factors or signal transduction components responsible for initiating differential gene activity. Different approaches and strategies are being used to isolate and characterize the function of these localized maternal mRNAs.

Ca(2+) in specification of vegetal cell fate in early sea urchin embryos

In sea urchin embryos, the first specification of cell fate occurs at the fourth cleavage, when small cells (the micromeres) are formed at the vegetal pole. The fate of other blastomeres is dependent on the receipt of cell signals originating from the micromeres. The micromeres are fated to become skeletogenic cells and show the ability to induce the endoderm (the archenteron) in the neighbouring cells during the 16- to 60-cell stage. Several molecules involved in signaling pathways, i.e. Notch for mesoderm specification, bone morphogenic protein (BMP) for ectoderm specification and beta-catenin for endoderm specification, are spatially and temporally expressed during development. In the micromeres, beta-catenin increases and subsequently localizes to the nuclei under the regulation of TCF, a nuclear binding partner of beta-catenin, until the 60-cell stage. However, the mechanisms activating these signaling substances are still unclear. In this article, I demonstrate some specific properties of the membrane and cytoplasm of micromeres including new findings on intracellular Ca(2+) concentration, and propose a mechanism by which the functional micromeres are autonoumously formed. The possible roles of these in the specification of vegetal cell fate in early development are discussed.

Interdependence of filamentous actin and microtubules for asymmetric cell division

Asymmetric cell divisions are crucial to the generation of cell fate diversity. They contribute to unequal distribution of cellular factors to the daughter cells. Asymmetric divisions are characterized by a 90 degrees rotation of the mitotic spindle. There is increasing evidence that a tight cooperation between cortical, filamentous actin and astral microtubules is indispensable for successful spindle rotation. Over the past years, the dynactin complex has emerged as a key candidate to mediate actin/microtubule interaction at the cortex. This review discusses our current understanding of how spindle rotation is accomplished by the interplay of filamentous actin and microtubules in a variety of experimental systems.

When cells tell their neighbors which direction to divide

Many cells must divide in specific orientations, yet for only a handful of cases do we have some understanding of how cells choose division orientations. We know of only a few cases where division orientations are controlled by specific cell-cell interactions. These cases are of interest, because they tell us something new and seemingly fundamental about how cells can function during development. Here, the evidence that interactions control division orientation in some cells of the early C. elegans embryo is presented, and what is known about how contact can regulate division orientation is discussed. Whether contact-mediated division orientation is a peculiarity of C. elegans or whether it may be more widespread is addressed.

Two pathways of maternal RNA localization at the posterior-vegetal cytoplasm in early ascidian embryos

A cDNA library prepared from fertilized eggs of the ascidian Halocynthia roretzi was screened for prelocalized mRNAs in the early embryo by means of whole-mount in situ hybridization using a digoxigenin-labeled antisense RNA of each clone. Random mass screening of 150 cDNAs in a fertilized egg yielded six different clones which showed mRNA localization in the posterior-vegetal cytoplasm of the 8-cell embryo. An in situ hybridization study of the detailed spatial distribution of each mRNA in embryos of various stages revealed that there are, in contrast to the identical localization in embryos after the 16-cell stage, two distinct patterns of RNA distribution at earlier stages. One is colocalization with the myoplasm from the prefertilization stage to the 8-cell stage (type I postplasmic RNAs). The other is delayed accumulation of RNA at the posterior-vegetal cytoplasm after fertilization (type II postplasmic RNAs). We found that both types of RNAs associate with the cytoskeleton, but that they show different sensitivities to inhibitors of the cytoskeleton; translocation of the type I RNAs is dependent upon microfilaments during the first phase of ooplasmic segregation and dependent upon microtubules during the second phase of segregation, whereas translocation of the type II RNAs is dependent upon microfilaments throughout ooplasmic segregation. These results show that there are two pathways for the localization of the RNAs at the posterior-vegetal cytoplasm in the 8-cell embryo of the ascidian H. roretzi.

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.

Developmental gene activities in ascidian embryos

The fertilized egg of ascidians develops quickly into a tadpole-type larva consisting of several distinct types of tissues including epidermis, central nervous system, endoderm, mesenchyme, notochord, and muscle. This architecture of the ascidian larva represents the most simplified chordate body plan. Taking advantage of simple, well-defined cell lineages, the expression of developmental genes is analyzed at single-cell level. Advances in the methodology promote the ascidian embryo as a useful system for studying transcriptional control involved in the specification of embryonic cells and pattern formation of the embryo.

Phases of cytoplasmic and cortical reorganizations of the ascidian zygote between fertilization and first division

Many eggs undergo reorganizations that localize determinants specifying the developmental axes and the differentiation of various cell types. In ascidians, fertilization triggers spectacular reorganizations that result in the formation and localization of distinct cytoplasmic domains that are inherited by early blastomeres that develop autonomously. By applying various imaging techniques to the transparent eggs of Phallusia mammillata, we now define 9 events and phases in the reorganization of the surface, cortex and the cytoplasm between fertilization and first cleavage. We show that two of the domains that preexist in the egg (the ER-rich cortical domain and the mitochondria-rich subcortical myoplasm) are localized successively by a microfilament-driven cortical contraction, a microtubule-driven migration and rotation of the sperm aster with respect to the cortex, and finally, a novel microfilament-dependant relaxation of the vegetal cortex. The phases of reorganization we have observed can best be explained in terms of cell cycle-regulated phases of coupling, uncoupling and recoupling of the motions of cortical and subcortical layers (ER-rich cortical domain and mitochondria-rich domain) with respect to the surface of the zygote. At the end of the meiotic cell cycle we can distinguish up to 5 cortical and cytoplasmic domains (including two novel ones; the vegetal body and a yolk-rich domain) layered against the vegetal cortex. We have also analyzed how the myoplasm is partitioned into distinct blastomeres at the 32-cell stage and the effects on development of the ablation of precisely located small fragments. On the basis of our observations and of the ablation/ transplantation experiments done in the zygotes of Phallusia and several other ascidians, we suggest that the determinants for unequal cleavage, gastrulation and for the differentiation of muscle and endoderm cells may reside in 4 distinct cortical and cytoplasmic domains localized in the egg between fertilization and cleavage.

The centrosome-attracting body, microtubule system, and posterior egg cytoplasm are involved in positioning of cleavage planes in the ascidian embryo

Many kinds of animal embryos exhibit stereotyped cleavage patterns during early embryogenesis. In the ascidian Halocynthia roretzi, cleavage patterns are invariant but they are complicated by successive unequal cleavages that occur in the posterior region. Here we report the essential roles of a novel structure, called the centrosome-attracting body (CAB), which exists in the posterior pole cortex of cleaving embryos, in generating unequal cleavages. By removing and transplanting posterior egg cytoplasm and by treatment with sodium dodecyl sulfate, we demonstrated that loss of the CAB resulted in abolishment of unequal cleavage, while ectopic formation of the CAB caused ectopic unequal cleavages to occur. Experiments with a microtubule inhibitor demonstrated that the centrosome and nucleus were attracted toward the posterior cortex, where the CAB is located, by shortening of microtubule bundles formed between the centrosome and the CAB. Consequently, the mitotic apparatus was positioned asymmetrically, resulting in unequal cleavage. Immunohistochemistry provided evidence that a microtubule motor protein, a kinesin or kinesin-like molecule, may be associated with the CAB. Formation of the CAB during the early cleavage stage was resistant to treatment with the microtubule inhibitor. In contrast, the integrity of the CAB was lost upon treatment with a microfilament inhibitor. We propose that the CAB plays key roles in the orientation and positioning of cleavage planes during unequal cell division.

Cytokinesis in eukaryotes: a mechanistic comparison

Cytokinesis is a crucial but poorly understood process of cell proliferation. Recently, molecular genetic analyses of fungal cytokinesis have led to an appreciation of contractile mechanisms in simple eukaryotes, and studies in animal and plant cells have led to new insights into the role of microtubules in the cleavage process. These findings suggest that fundamental mechanisms of cytokinesis may be highly conserved among eukaryotic organisms.

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.