Maternal cytoplasmic factors for generation of unique cleavage patterns in animal embryos

Simulations of sea urchin early development delineate the role of oriented cell division in the morula-to-blastula transition

The sea urchin morula to blastula transition has long been thought to require oriented cell divisions and blastomere adherence to the enveloping hyaline layer. In a computer simulation model, cell divisions constrained by a surface plane division rule are adequate to effect morphological transition. The hyaline membrane acts as an enhancer but is not essential. The model is consistent with the orientation of micromere divisions and the open blastulae of direct developing species. The surface plane division rule precedes overt epithelization of surface cells and acts to organize the developing epithelium. It is a universal feature of early metazoan development and simulations of non-echinoid cleavage patterns support its role throughout Metazoa. The surface plane division rule requires only local cues and cells need not reference global positional information or embryonic axes.

DNA and RNA pattern of staining during oogenesis in zebrafish (Danio rerio): a confocal microscopy study

Oogenesis involves a sequence of cellular divisions and developmental changes leading to the formation of oocytes, whose role in development is to transfer genomic information to the next generation. During this process, the gene expression pattern changes considerably concomitant with genome remodeling, while genomic information is maintained. The development of the gonad in zebrafish is unique in that it goes through an initial ovarian phase and subsequently into either ovarian or testicular phases. How the germ cells choose to commit to an oogenic fate and enter meiosis or alternatively not to enter meiosis and commit to a spermatogenetic fate remains a key question in development. Lack of suitable markers has hampered the understanding of the principles controlling sex differentiation in zebrafish. The current study was aimed at finding substantive cytochemical markers to identify specific oocyte stages primarily focusing on the DNA and RNA component of cells, using fluorescent dyes: acridine orange and propidium iodide. The pattern of synthesis and appearance of nucleoli was stage specific and may be used to identify stages of oogenesis. A distinguishing and possibly diagnostic feature of the staining pattern observed was the low level of chromatin staining compared to other cellular structures. This may be related to the more diffuse state of chromatin that occurs prior to thickening of chromosomes from the pachytene stage onwards. Although the fluorescent dyes may be useful in determining the localization of nucleic acids in tissue sections, it was not possible to quantify the relative contribution of the DNA and RNA components of specific stages of oocyte growth.

Polo-like kinase 1 is required for localization of Posterior End Mark protein to the centrosome-attracting body and unequal cleavages in ascidian embryos

In ascidian embryos, the posterior-localized maternal factor Posterior End Mark (PEM) is responsible for patterning embryos along the anterior-posterior axis with regard to both cleavage pattern involving unequal cell divisions and gene expression. Although PEM plays important roles in embryogenesis, its mechanism of action is still unclear because PEM has no known functional domain. In the present study, we explored the candidate of PEM partner proteins in Halocynthia roretzi using yeast two-hybrid screening. We isolated a homologue of Polo-like kinase 1 (Plk1), a key regulator of cell division and highly conserved in eukaryotes, as the first potential binding partner of PEM. We biochemically confirmed that interaction occurred between the Plk1 and PEM proteins. Immunostaining showed that Plk1 protein concentrates in the centrosome-attracting body (CAB) at the posterior pole, where PEM protein is also localized. The CAB is a subcellular structure that plays an important role in generating the posterior cleavage pattern. Plk1 localization to the CAB was dependent on the cell cycle phases during unequal cleavage. Inhibition of Plk1 with specific drugs resulted in failure of the nucleus to migrate towards the posterior pole and formation of a microtubule bundle between the CAB and a centrosome, similarly to inhibition of PEM function, suggesting that both proteins are involved in the same process of unequal cleavages. This interrupted nuclear migration was rescued by overexpression of PEM. In Plk1-inhibited embryos, the localization of PEM protein to the CAB was impaired, indicating that Plk1 is required for appropriate localization of PEM.

Localized PEM mRNA and protein are involved in cleavage-plane orientation and unequal cell divisions in ascidians

BACKGROUND

Orientation and positioning of the cell division plane are essential for generation of invariant cleavage patterns and for unequal cell divisions during development. Precise control of the division plane is important for appropriate partitioning of localized factors, spatial arrangement of cells for proper intercellular interactions, and size control of daughter cells. Ascidian embryos show complex but invariant cleavage patterns mainly due to three rounds of unequal cleavage at the posterior pole.

RESULTS

The ascidian embryo is an emerging model for studies of developmental and cellular processes. The maternal Posterior End Mark (PEM) mRNA is localized within the egg and embryo to the posterior region. PEM is a novel protein that has no known domain. Immunostaining showed that the protein is also present in the posterior cortex and the in centrosome-attracting body (CAB) and that the localization is extraction-resistant. Here we show that PEM of Halocynthia roretzi is required for correct orientation of early-cleavage planes and subsequent unequal cell divisions because it repeatedly pulls a centrosome toward the posterior cortex and the CAB, respectively, where PEM mRNA and protein are localized. When PEM activity is suppressed, formation of the microtubule bundle linking the centrosome and the posterior cortex did not occur. PEM possibly plays a role in anchoring microtubule ends to the cortex. In our model of orientation of the early-cleavage planes, we also amend the allocation of the conventional animal-vegetal axis in ascidian embryos, and discuss how the newly proposed A-V axis provides the rationale for various developmental events and the fate map of this animal.

CONCLUSIONS

The complex cleavage pattern in ascidian embryos can be explained by a simple rule of centrosome attraction mediated by localized PEM activity. PEM is the first gene identified in ascidians that is required for multiple spindle-positioning events.

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.

Maternal determinants and mRNAs in the cortex of ascidian oocytes, zygotes and embryos

The peripheral region of ascidian oocytes and zygotes contains five determinants for morphogenesis and differentiation of the embryo. The determinant for the 24 primary muscle cells of the tadpole, macho1, is one of several cortical mRNAs localized in a gradient along the animal-vegetal axis in the oocyte. After fertilization these mRNAs, together with cortical endoplasmic reticulum (cER) and a subcortical mitochondria-rich domain (myoplasm), relocate in two major reorganization phases forming the posterior plasm (postplasm) of the zygote. At the 8-cell stage cortical mRNAs concentrate in a macroscopic cortical structure called the centrosome-attracting body (CAB), forming a characteristic posterior end mark (PEM) in the two posterior vegetal blastomeres. We propose to call the numerous mRNAs showing this particular cortical localization in the posterior region of the embryo postplasmic/PEM RNAs and suggest a nomemclature. We do not know how postplasmic/PEM RNAs reach their polarized distribution in the oocyte cortex but at least PEM1 and macho1 (and probably others) bind to the network of cER retained in isolated cortical fragments. We propose that after fertilization, these postplasmic/PEM mRNAs move in the zygote cortex together with the cER network (cER/mRNA domain) via microfilament- and microtubule-driven translocations. The cER/mRNA domain is localized posteriorly at the time of first cleavage and distributed equally between the first two blastomeres. After the third cleavage, the cER/mRNA domain and dense particles compact to form the CAB in posterior vegetal blastomeres of the 8-cell stage. We discuss the identity of postplasmic/PEM RNAs, how they localize, anchor, relocate and may be translated. We also examine their roles in unequal cleavage and as a source of posterior morphogenetic and differentiation factors.

Evolution of development in the sea star genus Patiriella: clade-specific alterations in cleavage

Examination of early development in five species of the Patiriella sea star species complex indicates that the ancestral-type radial holoblastic cleavage (Type I) is characteristic of P. regularis and P. exigua, whereas cleavage in species from the calcar clade followed multiple alternatives (Types II-IV) from holoblastic to meroblastic. Considering that invariant radial cleavage is thought to play a role in embryonic axis formation in echinoderms, we documented the details of blastomere formation in Patiriella sp. and followed development of the embryos. In Type II cleavage, the first and second cleavage planes appeared simultaneously at one pole of the embryo, dividing it directly into four equally sized blastomeres. In Type III cleavage, the first and second cleavage planes appeared simultaneously, followed promptly by the third cleavage plane, dividing the embryo directly into eight equally sized blastomeres. In Type IV cleavage, numerous furrows appeared simultaneously at one end of the embryo, dividing it into 32-40 equally sized blastomeres. Confocal sections revealed that embryos with cleavage Types II-IV were initially syncytial. The timing of karyokinesis in embryos with Types II and III cleavage was similar to that seen in clutch mates with Type I cleavage. Karyokinesis in embryos with Type IV cleavage, however, differed in timing compared with Type I clutch mates. Alteration in cleavage was not associated with polarized distribution of maternally provided nutrients. For each cleavage type, development was normal to the competent larval stage. Although variable blastomere configuration in the calcar clade may be linked to possession of a lecithotrophic development, other Patiriella species with this mode of development have typical cleavage. The presence of variable cleavage in all calcar clade species indicates that phylogenetic history has played a role in the distribution of this embryonic trait in Patiriella. The plasticity in early cleavage in these sea stars indicates that this aspect of early development is not constrained against change and that there are many ways to achieve multicellularity.

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.

Computing an organism: on the interface between informatic and dynamic processes

The importance of interactions between processes at diverse space and time scales in biological information processing is a recurrent theme in the work of Michael Conrad (BioSystems, 1995: 35, 157-160; BioSystems, 1999: 52, 99-100). In this paper I present some results of explicit computational models that aim to capture and exploit some of the essential features of such multi-level processes. In the model formulation, I try to minimize the explicit definition of inter-level interactions, while providing the possibility of such interactions to develop. As often argued by Conrad inter-level interactions limit programmability. Indeed, we use an evolutionary process to derive the specific models. We study morphogenesis. We show that the interplay between cell adhesion and cell differentiation provides interesting mechanisms for morphogenesis. We also show that the interplay can both reduce and enhance small random fluctuations. We show that unequal cell cleavage in the early embryo-genesis reduces inter-individual variation of the morphemes developed from the same 'genome'. Our results suggest that, during evolution, the interplay between levels is exploited while it is at the same time reduced so as to give a certain primacy to inherited information.

Developmental genetics in primitive chordates

Recent advances in the study of the genetics and genomics of urochordates testify to a renewed interest in this chordate subphylum, believed to be the most primitive extant chordate relatives of the vertebrates. In addition to their primitive nature, many features of their reproduction and early development make the urochordates ideal model chordates for developmental genetics. Many urochordates spawn large numbers of transparent and externally developing embryos on a daily basis. Additionally, the embryos have a defined and well-characterized cell lineage until the end of gastrulation. Furthermore, the genomes of the urochordates have been estimated to be only 5-10% of the size of the vertebrates and to have fewer genes and less genetic redundancy than vertebrates. Genetic screens, which are powerful tools for investigating developmental mechanisms, have recently become feasible due to new culturing techniques in ascidians. Because hermaphrodite ascidians are able to self-fertilize, recessive mutations can be detected in a single generation. Several recent studies have demonstrated the feasibility of applying modern genetic techniques to the study of ascidian biology.

Dispersion of cyclin B mRNA aggregation is coupled with translational activation of the mRNA during zebrafish oocyte maturation

Cyclin B mRNA stored in immature zebrafish oocytes is translationally activated upon the stimulation of 17alpha,20beta-dihydroxy-4-pregnen-3-one (17alpha,20beta-DP), an event prerequisite for initiating oocyte maturation in this species. We investigated localization of cyclin B mRNA in zebrafish oocytes. Cyclin B mRNA was found to be exclusively localized as an aggregation along the cytoplasm at the animal pole of full-grown immature oocytes. When oocytes were treated with 17alpha,20beta-DP, a meshwork of microfilaments in the oocyte cortex disappeared and the aggregation of cyclin B mRNA dispersed just prior to the initiation of cyclin B synthesis and germinal vesicle breakdown (GVBD). Cytochalasin B, but not nocodazole or taxol, deformed the aggregation of cyclin B mRNA, indicating the involvement of microfilaments in organizing this form. Like 17alpha,20beta-DP, cytochalasin B (10 microg/ml) induced both complete dispersion of the aggregation and translational activation of cyclin B mRNA, forcing the oocytes to undergo GVBD without 17alpha,20beta-DP. Conversely, disturbance of the aggregation of cyclin B mRNA with a low concentration (1 microg/ml) of cytochalasin B inhibited 17alpha,20beta-DP-induced GVBD. These results suggest that the direct change in cyclin B mRNA from the aggregated form to the dispersed form is responsible for translational activation of the mRNA during zebrafish oocyte maturation.

mRNA localization patterns in zebrafish oocytes

In both invertebrate and vertebrate systems, the localization of maternal mRNAs is a common mechanism used to influence developmental processes, including the establishment of the dorsal/ventral axis, anterior/posterior axis, and the germ line (for review, see Bashirullah et al., 1998. Annu. Rev. Biochem. 67, 335-394). While the existence of localized maternal mRNAs has been reported in the zebrafish, Danio rerio, the precise localization patterns of these molecules during oogenesis has not been determined. In this study, in situ hybridization experiments were performed on zebrafish ovaries and activated eggs to examine different mRNA localization patterns. The results establish that while some maternal mRNAs remain ubiquitously distributed throughout the oocyte, other mRNAs follow specific localization patterns, including localization to the animal pole, localization to the vegetal pole, and cortical localization. The animal/vegetal axis is first apparent in stage II oocytes when the earliest mRNA localization is seen. Unique patterns of localization are seen in mature eggs as well. Some mRNAs maintain their oocyte localization patterns, while others localize upon egg activation (fertilization).

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.