Autonomous epithelial folding induced by an intracellular mechano-polarity feedback loop

Epithelial tissues form folded structures during embryonic development and organogenesis. Whereas substantial efforts have been devoted to identifying mechanical and biochemical mechanisms that induce folding, whether and how their interplay synergistically shapes epithelial folds remains poorly understood. Here we propose a mechano-biochemical model for dorsal fold formation in the early Drosophila embryo, an epithelial folding event induced by shifts of cell polarity. Based on experimentally observed apical domain homeostasis, we couple cell mechanics to polarity and find that mechanical changes following the initial polarity shifts alter cell geometry, which in turn influences the reaction-diffusion of polarity proteins, thus forming a feedback loop between cell mechanics and polarity. This model can induce spontaneous fold formation in silico, recapitulate polarity and shape changes observed in vivo, and confer robustness to tissue shape change against small fluctuations in mechanics and polarity. These findings reveal emergent properties of a developing epithelium under control of intracellular mechano-polarity coupling.

Rigidity percolation uncovers a structural basis for embryonic tissue phase transitions

Embryo morphogenesis is impacted by dynamic changes in tissue material properties, which have been proposed to occur via processes akin to phase transitions (PTs). Here, we show that rigidity percolation provides a simple and robust theoretical framework to predict material/structural PTs of embryonic tissues from local cell connectivity. By using percolation theory, combined with directly monitoring dynamic changes in tissue rheology and cell contact mechanics, we demonstrate that the zebrafish blastoderm undergoes a genuine rigidity PT, brought about by a small reduction in adhesion-dependent cell connectivity below a critical value. We quantitatively predict and experimentally verify hallmarks of PTs, including power-law exponents and associated discontinuities of macroscopic observables. Finally, we show that this uniform PT depends on blastoderm cells undergoing meta-synchronous divisions causing random and, consequently, uniform changes in cell connectivity. Collectively, our theoretical and experimental findings reveal the structural basis of material PTs in an organismal context.

Fluidization-mediated tissue spreading by mitotic cell rounding and non-canonical Wnt signalling

Tissue morphogenesis is driven by mechanical forces that elicit changes in cell size, shape and motion. The extent by which forces deform tissues critically depends on the rheological properties of the recipient tissue. Yet, whether and how dynamic changes in tissue rheology affect tissue morphogenesis and how they are regulated within the developing organism remain unclear. Here, we show that blastoderm spreading at the onset of zebrafish morphogenesis relies on a rapid, pronounced and spatially patterned tissue fluidization. Blastoderm fluidization is temporally controlled by mitotic cell rounding-dependent cell-cell contact disassembly during the last rounds of cell cleavages. Moreover, fluidization is spatially restricted to the central blastoderm by local activation of non-canonical Wnt signalling within the blastoderm margin, increasing cell cohesion and thereby counteracting the effect of mitotic rounding on contact disassembly. Overall, our results identify a fluidity transition mediated by loss of cell cohesion as a critical regulator of embryo morphogenesis.

Modeling and Identification of Amnioserosa Cell Mechanical Behavior by Using Mass-Spring Lattices

Various mechanical models of live amnioserosa cells during Drosophila melanogaster's dorsal closure are proposed. Such models account for specific biomechanical oscillating behaviors and depend on a different set of parameters. The identification of the parameters for each of the proposed models is accomplished according to a least-squares approach in such a way to best fit the cellular dynamics extracted from live images. For the purpose of comparison, the resulting models after identification are validated to allow for the selection of the most appropriate description of such a cell dynamics. The proposed methodology is general and it may be applied to other planar biological processes.

The Physical Basis of Coordinated Tissue Spreading in Zebrafish Gastrulation

Embryo morphogenesis relies on highly coordinated movements of different tissues. However, remarkably little is known about how tissues coordinate their movements to shape the embryo. In zebrafish embryogenesis, coordinated tissue movements first become apparent during "doming," when the blastoderm begins to spread over the yolk sac, a process involving coordinated epithelial surface cell layer expansion and mesenchymal deep cell intercalations. Here, we find that active surface cell expansion represents the key process coordinating tissue movements during doming. By using a combination of theory and experiments, we show that epithelial surface cells not only trigger blastoderm expansion by reducing tissue surface tension, but also drive blastoderm thinning by inducing tissue contraction through radial deep cell intercalations. Thus, coordinated tissue expansion and thinning during doming relies on surface cells simultaneously controlling tissue surface tension and radial tissue contraction.

Retinoic acid promotes expression of germline-specific genes in chicken blastoderm cells by stimulating Smad1/5 phosphorylation in a feeder-free culture system

BACKGROUND

Producing transgenic chickens with chicken blastodermal cells (cBCs) is inefficient due to the extremely low germline transmission capacity of cBCs. As chicken primordial germ cells (PGCs) have been reported as an efficient method for producing transgenic chickens, the inefficiency of cBCs could potentially be resolved by inducing them to differentiate into germ cells. However, whether chemical inducers are able to enhance cBCs germline competence in vitro is unknown and the molecular mechanisms of differentiation of chicken pluripotent cells into germ cells are poorly understood.

RESULTS

We cultured cBCs with a monolayer morphology in E8 medium, a xeno- and feeder-free medium. We showed that retinoic acid (RA) treatment increased expression of germ cell-specific genes in cBCs. Using western blot, we determined that RA stimulated Smad1/5 phosphorylation. Moreover, Smad1/5 activation regulates the expression of germ cell-specific genes, as co-treatment with a Smad1/5 phosphorylation inhibitor or activator alters expression of these genes. We also demonstrate that Smad1/5 is required for RA-induced differentiation by RNA interference knockdown.

CONCLUSION

Our results demonstrated that E8 medium is able to maintain cBC growth for weeks and RA treatment induced germ cell differentiation of cBCs through the BMP-Smad1/5 signaling pathway.

Perspectives on avian stem cells for poultry breeding

Stem cells have prulipotency to differentiate into many types of cell lineages. Recent progress of avian biotechnology enabled us to analyze the developmental fate of the stem cells: embryonic stem cells / primordial germ cells (PGCs). The stem cells were identified in the central area of the area pellucida of the stage X blastoderms. These cells could be applied for production of germline chimeras and organ regeneration. Generation of medical substrate in transgenic chickens has considerable interests in pharmaceuticals. Sex alteration of the offspring should be enormously beneficial to the poultry industry. Fertilization of the sex-reversed sperm could lead to sexual alteration of the offspring. These strategies using stem cells / PGCs should be one of the most powerful tools for future poultry breeding.

[The gastrulation in Cnidaria: A key to understanding phylogeny or the chaos of secondary modifications?]

The data revealed by comparative embryology of the basal (diploblastic) metazoans is traditionally considered a valuable potential source of information on the origin and early evolution of the animal kingdom and its major clades. Special attention is paid to the fundamental morphogenetic process of gastrulation during which the cells of the early embryo differentiate into the germ layers and the primary body plan is formed. Comparative analysis of gastrulation in different cnidarian taxa reveals high level of intergroup, intragroup, and individual variation. With few exceptions, there is no robust correlation between the type of gastrulation and the taxon. Current data do not support the idea that morphogenetic processes underlying cnidarian gastrulation can be divided into several distinct types. Rather, there is a continuum of equifinal ontogenetic trajectories. In cnidarians, the mode of gastrulation apparently depends less on the macroevolutionary history of the species than on various evolutionary plastic features, such as the oocyte size, the amount of yolk, the number of cells at the blastula (or morula) stage, the presence of phototrophic symbionts, or the ecology of the larva. Thus, in cnidarians, morphogenetic basis of gastrulation contains only a very weak phylogenetic signal and can have only limited application in phylogenetic reconstructions. On the other hand, comparative studies of the ontogeny of the basal metazoans shed light on the general rules of the evolution of morphogenetic processes that is crucial for understanding the early history of the animal kingdom.

[Variability of Quantitative Morphogenetic Parameters during Early Morphogenesis of the Loach, Missgurnus fossilis L]

Analysis of normal variation in quantitative morphological characters during the early embryonic development of the loach, based on fixed material and individual developmental trajectories of living embryos, shows that the dorsoventral differentiation of the blastoderm proceeds in two stages. Initially, at the onset of epiboly, the sagittal (short) and transverse (long) blastoderm meridians are marked off, and only then, upon germ ring (GR) formation, differentiation between the opposite poles of the sagittal meridian takes place. The embryonic shield (ES) usually appears in the segment of the blastoderm where the radius of its external curvature reaches a maximum and, therefore, the active surface tension at the blastoderm boundary with the YSL (peri-blast) and yolk is the highest. In this case, the convergence of inner cells toward the future dorsal segment (leading to ES formation) is a mechanical consequence of surface tension anisotropy. The normal course of epiboly is associated with periodic changes in the curvature of the blastoderm external surface, with new structures (the dorsal segment, GR, and ES) are marked off only when the surface curvature becomes maximally uniform. Although the ES in most embryos appears within the initial dorsal segment, individual developmental trajectories have been traced where the GR starts to form at the dorsal pole of the blastoderm but the ES develops on its opposite site, at the point of GR closure. In both cases, GR formation is initiated at the point of convergence of centrifugal cell migration flows that arise in the marginal zone of the blastoderm upon GR initiation or closure.

Left-right patterning in Xenopus conjoined twin embryos requires serotonin signaling and gap junctions

A number of processes operating during the first cell cleavages enable the left-right (LR) axis to be consistently oriented during Xenopus laevis development. Prior work showed that secondary organizers induced in frog embryos after cleavage stages (i.e. conjoined twins arising from ectopic induced primary axes) correctly pattern their own LR axis only when a primary (early) organizer is also present. This instructive effect confirms the unique LR patterning functions that occur during early embryogenesis, but leaves open the question: which mechanisms that operate during early stages are also involved in the orientation of later-induced organizers? We sought to distinguish the two phases of LR patterning in secondary organizers (LR patterning of the primary twin and the later transfer of this information to the secondary twin) by perturbing only the latter process. Here, we used reagents that do not affect primary LR patterning at the time secondary organizers form to inhibit each of 4 mechanisms in the induced twin. Using pharmacological, molecular-genetic, and photo-chemical tools, we show that serotonergic and gap-junctional signaling, but not proton or potassium flows, are required for the secondary organizer to appropriately pattern its LR axis in a multicellular context. We also show that consistently-asymmetric gene expression begins prior to ciliary flow. Together, our data highlight the importance of physiological signaling in the propagation of cleavage-derived LR orientation to multicellular cell fields.

Extensive divergence of transcription factor binding in Drosophila embryos with highly conserved gene expression

To better characterize how variation in regulatory sequences drives divergence in gene expression, we undertook a systematic study of transcription factor binding and gene expression in blastoderm embryos of four species, which sample much of the diversity in the 40 million-year old genus Drosophila: D. melanogaster, D. yakuba, D. pseudoobscura and D. virilis. We compared gene expression, measured by mRNA-seq, to the genome-wide binding, measured by ChIP-seq, of four transcription factors involved in early anterior-posterior patterning. We found that mRNA levels are much better conserved than individual transcription factor binding events, and that changes in a gene's expression were poorly explained by changes in adjacent transcription factor binding. However, highly bound sites, sites in regions bound by multiple factors and sites near genes are conserved more frequently than other binding, suggesting that a considerable amount of transcription factor binding is weakly or non-functional and not subject to purifying selection.

Lack of tailless leads to an increase in expression variability in Drosophila embryos

Developmental processes are robust, or canalised: dynamic patterns of gene expression across space and time are regulated reliably and precisely in the presence of genetic and environmental perturbations. It remains unclear whether canalisation relies on specific regulatory factors (such as heat-shock proteins), or whether it is based on more general redundancy and distributed robustness at the network level. The latter explanation implies that mutations in many regulatory factors should exhibit loss of canalisation. Here, we present a quantitative characterisation of segmentation gene expression patterns in mutants of the terminal gap gene tailless (tll) in Drosophila melanogaster. Our analysis provides new insights into the dynamic mechanisms underlying gap gene regulation, and reveals significantly increased variability of gene expression in the mutant compared to the wild-type background. We show that both position and timing of posterior segmentation gene expression domains vary strongly from embryo-to-embryo in tll mutants. This variability must be caused by a vulnerability in the regulatory system which is hidden or buffered in the wild-type, but becomes uncovered by the deletion of tll. Our analysis provides evidence that loss of canalisation in mutants could be more widespread than previously thought.

Heterotrimeric G protein signaling governs the cortical stability during apical constriction in Drosophila gastrulation

During gastrulation in Drosophila melanogaster, coordinated apical constriction of the cellular surface drives invagination of the mesoderm anlage. Forces generated by the cortical cytoskeletal network have a pivotal role in this cellular shape change. Here, we show that the organisation of cortical actin is essential for stabilisation of the cellular surface against contraction. We found that mutation of genes related to heterotrimeric G protein (HGP) signaling, such as Gβ13F, Gγ1, and ric-8, results in formation of blebs on the ventral cellular surface. The formation of blebs is caused by perturbation of cortical actin and induced by local surface contraction. HGP signaling mediated by two Gα subunits, Concertina and G-iα65A, constitutively regulates actin organisation. We propose that the organisation of cortical actin by HGP is required to reinforce the cortex so that the cells can endure hydrostatic stress during tissue folding.

Variation in mesoderm specification across Drosophilids is compensated by different rates of myoblast fusion during body wall musculature development

Background

It has been shown that species separated by relatively short evolutionary distances may have extreme variations in egg size and shape. Those variations are expected to modify the polarized morphogenetic gradients that pattern the dorso-ventral axis of embryos. Currently, little is known about the effects of scaling over the embryonic architecture of organisms. We began examining this problem by asking if changes in embryo size in closely related species of Drosophila modify all three dorso-ventral germ layers or only particular layers, and whether or not tissue patterning would be affected at later stages.

Principal Findings

Here we report that changes in scale affect predominantly the mesodermal layer at early stages, while the neuroectoderm remains constant across the species studied. Next, we examined the fate of somatic myoblast precursor cells that derive from the mesoderm to test whether the assembly of the larval body wall musculature would be affected by the variation in mesoderm specification. Our results show that in all four species analyzed, the stereotyped organization of the body wall musculature is not disrupted and remains the same as in D. melanogaster. Instead, the excess or shortage of myoblast precursors is compensated by the formation of individual muscle fibers containing more or less fused myoblasts.

Conclusions

Our data suggest that changes in embryonic scaling often lead to expansions or retractions of the mesodermal domain across Drosophila species. At later stages, two compensatory cellular mechanisms assure the formation of a highly stereotyped larval somatic musculature: an invariable selection of 30 muscle founder cells per hemisegment, which seed the formation of a complete array of muscle fibers, and a variable rate in myoblast fusion that modifies the number of myoblasts that fuse to individual muscle fibers.

Pabp binds to the osk 3'UTR and specifically contributes to osk mRNA stability and oocyte accumulation

RNA localization is tightly coordinated with RNA stability and translation control. Bicaudal-D (Bic-D), Egalitarian (Egl), microtubules and their motors are part of a Drosophila transport machinery that localizes mRNAs to specific cellular regions during oogenesis and embryogenesis. We identified the Poly(A)-binding protein (Pabp) as a protein that forms an RNA-dependent complex with Bic-D in embryos and ovaries. pabp also interacts genetically with Bic-D and, similar to Bic-D, pabp is essential in the germline for oocyte growth and accumulation of osk mRNA in the oocyte. In the absence of pabp, reduced stability of osk mRNA and possibly also defects in osk mRNA transport prevent normal oocyte localization of osk mRNA. pabp also interacts genetically with osk and lack of one copy of pabp(+) causes osk to become haploinsufficient. Moreover, pointing to a poly(A)-independent role, Pabp binds to A-rich sequences (ARS) in the osk 3'UTR and these turned out to be required in vivo for osk function during early oogenesis. This effect of pabp on osk mRNA is specific for this RNA and other tested mRNAs localizing to the oocyte are less and more indirectly affected by the lack of pabp.

MicroRNA-mediated posttranscriptional regulation is required for maintaining undifferentiated properties of blastoderm and primordial germ cells in chickens

MicroRNAs (miRNAs) play a critical role in determining the differentiation fate of pluripotent stem cells and germ cells in mammals. However, the mechanism(s) of miRNA-mediated posttranscriptional regulation with regard to lineage specification and differentiation in chick development require further investigation. Therefore, we conducted miRNA expression profiling to explore specific miRNA signatures in undifferentiated blastoderm and primordial germ cells (PGCs). We identified seven miRNAs that are highly expressed in blastoderm and 10 that are highly expressed in PGCs. In this study, miR-302a and miR-456 for blastoderm and miR-181a* for PGCs were analyzed further for their target transcripts and regulatory pathways. Both miR-302a and miR-456 bound directly to the sex-determining region Y box 11 transcript and could act as posttranscriptional coregulators to maintain the undifferentiated state of the chicken blastoderm through the suppression of somatic gene expression and differentiation. Moreover, miR-181a* showed a bifunctional role in PGCs by binding to two different transcripts. miR-181a* inhibited the somatic differentiation of PGCs by silencing homeobox A1 expression. Additionally, miR-181a* prevented PGCs from entering meiosis through the repression of the nuclear receptor subfamily 6, group A, member 1 transcript. Collectively, our data demonstrate that in chickens miRNAs intrinsically regulate the differentiation fate of blastoderms and PGCs and that the specific timing of germ cell meiosis is controlled through miRNA expression.

Tissue distribution of cells derived from the area opaca in heterospecific quail-chick blastodermal chimeras

The objective of the current study was to determine the tissue distribution of cells derived from the area opaca in heterospecific quail-chick blastodermal chimeras. Quail-chick chimeras were constructed by transferring dissociated cells from the area opaca of the stage X-XII (EG&K) quail embryo into the subgerminal cavity of the unincubated chick blastoderm. The distribution of quail cells in embryonic as well as extra-embryonic tissues of the recipient embryo were examined using the QCPN monoclonal antibody after 6 days of incubation in serial sections taken at 100-mum intervals. Data gathered in the present study demonstrated that, when introduced into the subgerminal cavity of a recipient embryo, cells of the area opaca are able to populate not only extra-embryonic structures such as the amnion and the yolk sac, but also various embryonic tissues derived from the ectoderm and less frequently the mesoderm. Ectodermal chimerism was confined mainly to the head region and was observed in tissues derived from the neural ectoderm and the surface ectoderm, including the optic cup, diencephalon and lens. Although the possibility of random incorporation of transplanted cells into these embryonic structures cannot be excluded, these results would suggest that area opaca, a peripheral ring of cells in the avian embryo destined to form the extra-embryonic ectoderm and endoderm of the yolk sac, might harbor cells that have the potential to give rise to various cell types in the recipient chick embryo, including those derived from the surface ectoderm and neural ectoderm.

Injection of lentivirus into stage-X blastoderm for the production of transgenic quail

This protocol describes how to generate transgenic quail by injecting a lentiviral vector into freshly laid Japanese quail eggs at stage X. The lentivirus infects primordial germ cells originating in the area pellucida.

[3D-X-ray microcomputer tomography and optical coherence tomography as methods for the localization of the blastoderm in the newly laid unincubated chicken egg]

The routine culling of the male offspring of hybrid layer type chickens is met with increasing public disapproval for both ethical and legal reasons. Until now practice-oriented methods for reliable sex diagnosis prior to hatch could not be developed. Molecular genetical analysis of blastodermic cells can be used for sex determination in unincubated eggs; however, knowledge of the precise localization of the germinal disc is crucial for the extraction of a carefully directed cell biopsy. In principle, 3D-X-ray micro computed tomography (3D-CT) has been proven a suitable method to localize the germinal disk in the unincubated egg without damaging the egg shell. No negative effects on embryogenesis and hatching rate of irradiated hatching eggs were established. The pictorial representation of the germinal disk using optical coherence tomography (OCT) failed in the unopened egg. The egg shell formed an impenetrable barrier for the currently available measuring method which utilized near infrared (NIR) wavelength regions. After opening the egg shell, the germinal disk could be visualized without any difficulties. In conclusion, technical possibilities for localization of the germinal disk in the unincubated egg already exist, but regarding technical parameters, the procedures have to be adapted to the specific purpose.

Hyalin is a cell adhesion molecule involved in mediating archenteron-blastocoel roof attachment

The US National Institutes of Health has designated the sea urchin embryo as a model organism because around 25 discoveries in this system have led to insights into the physiology of higher organisms, including humans. Hyalin is a large glycoprotein in the hyaline layer of sea urchin embryos that functions to maintain general adhesive relationships in the developing embryo. It consists of the hyalin repeat domain that has been identified in organisms as diverse as bacteria, worms, flies, mice, sea urchins and humans. Here we show, using a polyclonal antibody raised against the 11.6 S species of hyalin, that it localizes at the tip of the archenteron and on the roof of the blastocoel exactly where these two structures bond in an adhesive interaction that has been of interest for over a century. In addition, the antibody blocks the interaction between the archenteron tip and blastocoel roof. These results, in addition to other recent findings from this laboratory that will be discussed, suggest that hyalin is involved in mediating this cellular interaction. This is the first demonstration that suggests that hyalin functions as a cell adhesion molecule in many organisms, including humans.

Characterization of the Drosophila segment determination morphome

Here we characterize the expression of the full system of genes which control the segmentation morphogenetic field of Drosophila at the protein level in one dimension. The data used for this characterization are quantitative with cellular resolution in space and about 6 min in time. We present the full quantitative profiles of all 14 segmentation genes which act before the onset of gastrulation. The expression patterns of these genes are first characterized in terms of their average or typical behavior. At this level, the expression of all of the genes has been integrated into a single atlas of gene expression in which the expression levels of all genes in each cell are specified. We show that expression domains do not arise synchronously, but rather each domain has its own specific dynamics of formation. Moreover, we show that the expression domains shift position in the direction of the cephalic furrow, such that domains in the anlage of the segmented germ band shift anteriorly while those in the presumptive head shift posteriorly. The expression atlas of integrated data is very close to the expression profiles of individual embryos during the latter part of the blastoderm stage. At earlier times gap gene domains show considerable variation in amplitude, and significant positional variability. Nevertheless, an average early gap domain is close to that of a median individual. In contrast, we show that there is a diversity of developmental trajectories among pair-rule genes at a variety of levels, including the order of domain formation and positional accuracy. We further show that this variation is dynamically reduced, or canalized, over time. As the first quantitatively characterized morphogenetic field, this system and its behavior constitute an extraordinarily rich set of materials for the study of canalization and embryonic regulation at the molecular level.

[Efficacy of various methods of a reporter gene transfer to chicken embryonic cells]

The methods of transfection ofa plasmid with a reporter gene involving DNA injection into chicken embryonic cells were studied. The parameters of the efficient transfection of chicken blastodermal cells with a foreign gene have been determined (20-24 and up to 40% in culture and embryos, respectively). A high efficiency of transfection of primordial germ cells isolated from the gonads has been obtained after DNA injection into the dorsal aorta of 2.5-day-old chicken embryos.

[Spatial-temporal dynamics of morphogenetic blastoderm potencies in early embryogenesis of the loach]

The degree of differentiation of axial structures (notochord, neuroectoderm, and somites) in 24-hour explants (a total of 380) of the loach embryonic blastoderm was determined on histological sections according to a developed scale of estimates. Before the beginning of epiboly, axial structures were formed only from fragments of the dorsal sector of the blastoderm marginal zone. Its other sectors acquired the capacity of forming axial structure only with the beginning of epiboly, as the germ ring was formed in the marginal zone, unlike the cells outside the germ ring. The degree of differentiation of axial structures in the dorsal sector of marginal zone increased reliably with the appearance of embryonic shield, i.e. area of the convergence of cell flows. Here, statistically significant regional differences in morphogenetic potencies of the marginal zone first appeared, which corresponded to the differences in prospective significance of its materials; notochord and neuroectoderm better differentiate from the dorsal sector material, while somites better differentiate from the ventral sector material. Thus, distribution of morphogenetic potencies reflects precisely the spatial-temporal dynamics of collective movement of the blastoderm cells during the normal course of morphogenesis.

Differential adhesion in morphogenesis: a modern view

The spreading of one embryonic tissue over another, the sorting out of their cells when intermixed and the formation of intertissue boundaries respected by the motile border cells all have counterparts in the behavior of immiscible liquids. The 'differential adhesion hypothesis' (DAH) explains these liquid-like tissue behaviors as consequences of the generation of tissue surface and interfacial tensions arising from the adhesion energies between motile cells. The experimental verification of the DAH, the recent computational models simulating adhesion-mediated morphogenesis, and the evidence concerning the role of differential adhesion in a number of morphodynamic events, including teleost epiboly, the specification of boundaries between rhombomeres in the developing vertebrate hindbrain, epithelial-mesenchymal transitions in embryos, and malignant invasion are reviewed here.

Three-dimensional morphology and gene expression in the Drosophila blastoderm at cellular resolution II: dynamics

A new spatio-temporal coordinate framework for studying three-dimensional patterns of gene expression in the Drosophila blastoderm is presented that takes account of previously undetected morphological movements.

Background

To accurately describe gene expression and computationally model animal transcriptional networks, it is essential to determine the changing locations of cells in developing embryos.

Results

Using automated image analysis methods, we provide the first quantitative description of temporal changes in morphology and gene expression at cellular resolution in whole embryos, using the Drosophila blastoderm as a model. Analyses based on both fixed and live embryos reveal complex, previously undetected three-dimensional changes in nuclear density patterns caused by nuclear movements prior to gastrulation. Gene expression patterns move, in part, with these changes in morphology, but additional spatial shifts in expression patterns are also seen, supporting a previously proposed model of pattern dynamics based on the induction and inhibition of gene expression. We show that mutations that disrupt either the anterior/posterior (a/p) or the dorsal/ventral (d/v) transcriptional cascades alter morphology and gene expression along both the a/p and d/v axes in a way suggesting that these two patterning systems interact via both transcriptional and morphological mechanisms.

Conclusion

Our work establishes a new strategy for measuring temporal changes in the locations of cells and gene expression patterns that uses fixed cell material and computational modeling. It also provides a coordinate framework for the blastoderm embryo that will allow increasingly accurate spatio-temporal modeling of both the transcriptional control network and morphogenesis.

Three-dimensional morphology and gene expression in the Drosophila blastoderm at cellular resolution I: data acquisition pipeline

A suite of methods that provide the first quantitative three-dimensional description of gene expression and morphology with cellular resolution in whole Drosophila embryos is described.

Background

To model and thoroughly understand animal transcription networks, it is essential to derive accurate spatial and temporal descriptions of developing gene expression patterns with cellular resolution.

Results

Here we describe a suite of methods that provide the first quantitative three-dimensional description of gene expression and morphology at cellular resolution in whole embryos. A database containing information derived from 1,282 embryos is released that describes the mRNA expression of 22 genes at multiple time points in the Drosophila blastoderm. We demonstrate that our methods are sufficiently accurate to detect previously undescribed features of morphology and gene expression. The cellular blastoderm is shown to have an intricate morphology of nuclear density patterns and apical/basal displacements that correlate with later well-known morphological features. Pair rule gene expression stripes, generally considered to specify patterning only along the anterior/posterior body axis, are shown to have complex changes in stripe location, stripe curvature, and expression level along the dorsal/ventral axis. Pair rule genes are also found to not always maintain the same register to each other.

Conclusion

The application of these quantitative methods to other developmental systems will likely reveal many other previously unknown features and provide a more rigorous understanding of developmental regulatory networks.

Mosaic versus regulation development in avian blastoderms depends on the spatial distribution of Rauber's sickle material

We describe how to prepare unincubated avian eggs to obtain a greater number of clearly visible Rauber's sickles for experimental embryology. After hemi-sectioning of unincubated chicken (Gallus domesticus) blastoderms and cultivating both halves in vitro, two kinds of development can be discerned: (1) when the unincubated blastoderms were hemi-sectioned according to the plane of bilateral symmetry, going through the middle region of Rauber's sickle, we obtained two hemi-embryos (a left and a right one). Each contained a half primitive streak, localized at the cut edge (starting from the most median part of Rauber's sickle) giving rise to a half mesoblast mantle and half area vasculosa, thus indicating mosaic development (each part of the whole fertilized egg would be able to form independently on its own). (2) When the unincubated blastoderm is hemi-sectioned more obliquely, going through a more lateral part of Rauber's sickle (sickle horn), two complete bilaterally symmetrically miniature embryos will form, indicating the so-called regulation phenomena. We demonstrate that these two types of development are in reality due to the different spreading and concentration of Rauber's sickle tissue (containing gamma ooplasm) around the area centralis. Embryonic regulation thus must not be considered as a kind of totipotent regeneration capacity of isolated parts of the unincubated avian blastoderm, but depends on the spatial distribution of a kind of extraembryonic tissue (Rauber's sickle) built up by the oblique uptake of gamma ooplasm (ooplasmic mosaicism) at the moment of bilateral symmetrization (Callebaut [1994] Eur Arch Biol 105:111-123; Callebaut [2005] Dev Dyn 233:1194-1216).

Establishment of an in vitro culture system for chicken preblastodermal cells

To develop an alternative source for chicken pluripotent cells, we examined (1) whether undifferentiated preblastodermal cells could be subcultured in vitro for an extended period and (2) how subculturing affected the physiological properties of preblastodermal cells. The average number of preblastodermal cells was 2,397 in stage V embryos and 36,345 in stage VII embryos; stage X embryos had an average of 53,857 blastodermal cells. The average cell size decreased significantly (70.63-18.83 microm in diameter; P < 0.0001) as the embryo grew; this was closely related to a reduction in the size and number of lipid vesicles in the cell cytoplasm. The culture conditions were optimized for the stage V preblastodermal cells and the control stage X blastodermal cells. On STO feeder cells, the preblastodermal cells achieved stable growth in vitro only in HES medium or a mixed medium of the Knockout DMEM and HES media. However, more than 10 passages of preblastodermal cells at intervals of 3-4 days was possible only by using the Knockout/HES mixed medium and BRL cell-conditioned HES medium for the primary cultures and subcultures, respectively. Colony-forming preblastodermal cells had well-delineated cytoplasm, which was positively stained for stem cell-specific markers by anti-stage-specific embryo antigen-1 antibody, periodic acid-Schiff's solution, and alkaline phosphatase. When preblastodermal cells with or without culturing were transferred into the blastodermal cavity of stage X embryos, only in vitro-cultured preblastodermal cells at stage V (4/5 = 80%) and stage VII (2/8 = 25%) induced somatic chimerism in recipient chickens. In conclusion, undifferentiated preblastodermal cells could be subcultured, and only the colony-forming preblastodermal cells that stained positively for stem cell markers could induce somatic chimerism.

The importance of the posterior midline region for axis initiation at early stages of the avian embryo

The avian blastoderm acts during its early stages of development as an integrative system programmed to form a single embryonic axis. Here, I report the results of a variety of transplantation experiments of the midline region at stages X-XII, which were carried out to study their relevance for axis initiation. The results of the experimental series discussed herein emphasizes the importance of the posterior midline region (including the marginal zone and Koller's sickle) for axis initiation. This ability resides mainly at stage X in the posterior side of a narrow midline region, while at stages XI-XII it is exhibited at the region which is located more anterior and lateral to the posterior midline region. This posterior midline region has developmental abilities which allow it to initiate a single embryonic axis and at the same time to prevent other regions that also have such abilities to do so. Therefore, in normal development only one embryonic axis develops in the avian blastoderm. It is proposed that the cells which are important to initiate the avian embryonic axis are concentrated mainly at the region of the posterior midline region. These cells may have organizer properties which determine the initiation site of the axis in the avian embryo.

The ostrich (Struthio camelus) blastoderm and embryo development following storage of eggs at various temperatures

1. The gross morphology of blastoderms in fresh unstored ostrich eggs and in eggs subjected to different regimen of storage and incubation was studied. Then the effects of storage duration of eggs (1, 2 and 3 weeks) and storage temperature (15, 20 and 25 degrees C) on blastoderm and embryo development were investigated.2. Only incubation following overnight storage at 18 degrees C advanced blastoderm development (1.5-fold increase in diameter) to a stage comparable to hypoblast. 3. Storage of eggs at 15 or 20 degrees C did not affect blastoderm stage and size whereas, at 25 degrees C, the blastoderm doubled in size and appeared to have advanced to a primitive streak stage. Embryo development was reduced after 2 weeks of storage regardless of the storage temperature. 4. After oviposition and during pre-incubation storage the ostrich blastoderm develops progressively over time in a temperature-dependent manner towards the hypoblast stage and beyond but the viability of the blastoderm and embryo development is seriously compromised by 2 weeks of storage.

Onset of the DNA replication checkpoint in the early Drosophila embryo

The Drosophila embryo is a promising model for isolating gene products that coordinate S phase and mitosis. We have reported before that increasing maternal Cyclin B dosage to up to six copies (six cycB) increases Cdk1-Cyclin B (CycB) levels and activity in the embryo, delays nuclear migration at cycle 10, and produces abnormal nuclei at cycle 14. Here we show that the level of CycB in the embryo inversely correlates with the ability to lengthen interphase as the embryo transits from preblastoderm to blastoderm stages and defines the onset of a checkpoint that regulates mitosis when DNA replication is blocked with aphidicolin. A screen for modifiers of the six cycB phenotypes identified 10 new suppressor deficiencies. In addition, heterozygote dRPA2 (a DNA replication gene) mutants suppressed only the abnormal nuclear phenotype at cycle 14. Reduction of dRPA2 also restored interphase duration and checkpoint efficacy to control levels. We propose that lowered dRPA2 levels activate Grp/Chk1 to counteract excess Cdk1-CycB activity and restore interphase duration and the ability to block mitosis in response to aphidicolin. Our results suggest an antagonistic interaction between DNA replication checkpoint activation and Cdk1-CycB activity during the transition from preblastoderm to blastoderm cycles.

Progress Toward the Culture and Transformation of Chicken Blastodermal Cells

Chicken blastodermal cells can be cultured for short periods of time and retain the ability to contribute to somatic and germline tissues when injected into gamma-irradiated stage X embryos. Such a method has yet to yield a germline transgenic bird, in part due to the low rate of transgene integration into the avian genome. In addition, the short culture period precludes the identification and expansion of those cells that carry an integrated transgene. In this study, two methods were developed that produced blastodermal cells isolated from stage X Barred Plymouth Rock embryos bearing an integrated transgene. Addition of chick embryo extract to the culture medium enabled expansion of single colonies for multiple passages. Southern blot analysis indicated that the transgenes had integrated as a single copy in most of the clones. Cells from passaged, transgenic embryo cells were injected into irradiated stage X White Leghorn embryos, producing hatched chicks that bore the donor cells in their somatic tissues. Transgene sequences were detected in sperm DNA; however, breeding of chimeras did not result in germline transmission of the transgene, indicating that the contribution of the transgenic cells to the germline was either nonexistent or very low.

Avian Germplasm Preservation: Embryonic Stem Cells or Primordial Germ Cells?

Presently, avian genetic resources are best maintained as living collections of birds. Unfortunately, these stocks have been under constant pressure to be destroyed because of the decline in the number of Poultry Science Departments and pressures to cut costs at land grant institutions. Cryopreservation of semen is often suggested as a means to bank avian germplasm. However, this is only applicable for single-gene traits and does not allow for full reconstitution of the genetics of the original line. Over the last 15 yr, advances in the manipulation of the early chick embryo, manipulation of primordial germ cells (PGC), and the culture of embryonic stem cells (ESC) suggests that cryopreservation of blastodermal cells, ESC, or PGC might offer a means to preserve the entire genome of highly selected, specialized stocks of poultry. Freezing each of these cell types is possible with varying degrees of efficiency. Similarly, the effectiveness of generating germ line chimeras using blastodermal cells, ESC, or PGC also varies greatly. Other factors that must be considered include the choice of the recipient lines to develop the germ line chimeras and the number of individuals needed to reconstitute the line. Finally, the low efficiency rate of reconstitution and the high cost associated with current technologies makes these approaches prohibitive. Significant challenges remain to be overcome before the entire genome of poultry stocks can be routinely cryoperserved and reconstituted.

Gastrulation in the sea anemone Nematostella vectensis occurs by invagination and immigration: an ultrastructural study

The sea anemone Nematostella vectensis has recently been established as a new model system for the understanding of the evolution of developmental processes. In particular, the evolutionary origin of gastrulation and its molecular regulation are the subject of intense investigation. However, while molecular data are rapidly accumulating, no detailed morphological data exist describing the process of gastrulation. Here, we carried out an ultrastructural study of different stages of gastrulation in Nematostella using transmission electron microscope and scanning electron microscopy techniques. We show that presumptive endodermal cells undergo a change in cell shape, reminiscent of the bottle cells known from vertebrates and several invertebrates. Presumptive endodermal cells organize into a field, the pre-endodermal plate, which undergoes invagination. In parallel, the endodermal cells decrease their apical cell contacts but remain loosely attached to each other. Hence, during early gastrulation they display an incomplete epithelial-mesenchymal transition (EMT). At a late stage of gastrulation, the cells eventually detach and fill the interior of the blastocoel as mesenchymal cells. This shows that gastrulation in Nematostella occurs by a combination of invagination and late immigration involving EMT. The comparison with molecular expression studies suggests that cells expressing snailA undergo EMT and become endodermal, whereas forkhead/brachyury expressing cells at the ectodermal margin of the blastopore retain their epithelial integrity throughout gastrulation.

Effect of novel monoclonal antibodies on LIF-induced signaling in chicken blastodermal cells

Leukemia-inhibitory factor (LIF) is indispensable for maintaining the undifferentiated state when propagating mouse embryonic stem (ES) cells. We previously cloned chicken LIF (chLIF) cDNA and demonstrated that it maintained chicken ES cell cultures in an undifferentiated state. Here, we developed two monoclonal antibodies, HUL-1 and HUL-2, against chLIF, which specifically recognized recombinant chLIF (rchLIF) produced by Escherichia coli and Chinese hamster ovary K1 cells, in enzyme-linked immunosorbent assays and Western blot analysis. In addition, HUL-2 inhibited the phosphorylation of signal transducer and activator of transcription 3 by rchLIF in chicken blastodermal cells (CBCs), but not that of mitogen-activated protein kinase kinase. Furthermore, the addition of HUL-2 to CBC cultures resulted in embryoid bodies forming earlier than in normal cultures. These results indicated that HUL-2 recognized not only rchLIF but also native chLIF, and suggested that CBCs in culture produce LIF, which functions in autocrine signaling.

NeuroM and MyoD are expressed in separate subpopulations of cells in the pregastrulating epiblast

Epiblast cells form skeletal muscle and neurons in culture and some express mRNA for the skeletal muscle specific transcription factor MyoD in vivo. The following experiments were designed to determine whether the neurogenic transcription factor NeuroM is expressed in the epiblast and if NeuroM and MyoD are present in separate subpopulations of epiblast cells that can differentiate into neurons and muscle, respectively. In situ hybridization revealed that NeuroM was present in the anterior region of the pregastrulating epiblast. Some cells with NeuroM were proliferating and expressed two molecules present in neurogenic cells, NCAM and the Zn-12/HNK-1 carbohydrate. The G8 antibody labeled cells with MyoD but not NeuroM. When G8 positive cells were isolated by magnetic cell sorting and placed in culture, nearly all differentiated into skeletal muscle in serum free medium. A subpopulation of cells isolated with antibodies that bound to cells expressing NeuroM formed neurons when cultured in medium supplemented with sera and embryo extract. These experiments demonstrate that NeuroM and MyoD are present in separate subpopulations of cells in the pregastrulating epiblast. Epiblast cells with NeuroM are more dependent on exogenous factors to differentiate than those with MyoD.

[Dynamics and variability of early morphogenesis in the loach according to observations of individual developmental trajectories]

The dynamics and variability of quantitative morphological characters (morphological variables), which undergo changes upon epiboly, were studied by means of vital observations and measurements of developing loach (Misgurnus fossilis L.) embryos within equal time intervals. None of morphological variables, which characterize the dynamics of blastoderm shape, had monotonous dependence on time. In each individual embryo, the intervals of changes in morphological variables in the "normal" direction corresponding to the change of their mean values during the normal course of epiboly alternated with arrests, as well as with the changes of morphological variables in the reverse direction. The dynamics of morphological variables in time, which reflect the sequence of morphological states of the same embryo, and those of individual variations (variations of morphological states of different embryos on the same temporal section) had identical structure. This suggests instability of individual trajectories of morphogenesis or, strictly speaking, their actual absence. It was shown for the first time on the basis of analysis of individual trajectories of morphogenesis that its dynamics corresponded to so-called "determined chaos", which was previously discussed only as a theoretical possibility. The data obtained suggest that upon approach to the equatorial area of the embryo, the blastoderm marginal zone was elongated in the longitudinal direction and contracted across the axis of its movement. As a result, a positive feedback arises between the cooperated cell movement and the change of shape of the surface, over which the cells move. This leads, due to unstable radial symmetry of this movement, to the formation of embryonic shield.

Analysis of an approach to oviduct-specific expression of modified chicken lysozyme genes

The -2.7 kb enhancer (E) element of the chicken lysozyme gene domain appears to govern expression of the gene in macrophages but not in oviduct tubular gland cells, the only other site of lysozyme expression. The ultimate goal of our research was to determine whether lysozyme domain variants could be developed that would mainly be expressed in the oviduct so that transgenic birds could be produced that would deposit exogenous protein in the egg white. Accordingly, precise mutations were made by poxvirus-mediated gene targeting in FEF/PU.1 and CCAAT/enhancer-binding protein (C/EBP) transcription factor binding sites in the -2.7 kb E of cloned copies of a specific lysozyme gene variant that includes a hydrophobic pentapeptide tail encoding sequence inserted immediately prior to the stop codon. This variant contains the entire lysozyme domain and is cloned in a lambda bacteriophage vector (lambdaDIILys-HT); the novel tail sequence enables distinction in cell-based expression systems between transcripts of the variant and those of the endogenous gene. These various lysozyme domain mutants, in bacteriophage vector form, were tested for expression in cultured chicken blastodermal cells cotransfected with plasmids encoding the transcription factors C/EBP and v-Myb. In the absence of these plasmids, barely detectable levels of endogenous lysozyme gene transcription resulted in the blastodermal cells. In the presence of the plasmids, however, transcripts of the endogenous gene could be detected as well as varying levels (as evaluated by quantitative real-time PCR) of transcripts of all of the lysozyme domain mutants. These results are discussed in the context of the known role and occurrence of various transcription factors involved in gene expression in differentiating macrophage cells. The ultimate test of expression of the variants in macrophages vs. oviduct cells will be to use them to produce transgenic birds.

DRhoGEF2 regulates actin organization and contractility in the Drosophila blastoderm embryo

Morphogenesis of the Drosophila melanogaster embryo is associated with a dynamic reorganization of the actin cytoskeleton that is mediated by small GTPases of the Rho family. Often, Rho1 controls different aspects of cytoskeletal function in parallel, requiring a complex level of regulation. We show that the guanine triphosphate (GTP) exchange factor DRhoGEF2 is apically localized in epithelial cells throughout embryogenesis. We demonstrate that DRhoGEF2, which has previously been shown to regulate cell shape changes during gastrulation, recruits Rho1 to actin rings and regulates actin distribution and actomyosin contractility during nuclear divisions, pole cell formation, and cellularization of syncytial blastoderm embryos. We propose that DRhoGEF2 activity coordinates contractile actomyosin forces throughout morphogenesis in Drosophila by regulating the association of myosin with actin to form contractile cables. Our results support the hypothesis that specific aspects of Rho1 function are regulated by specific GTP exchange factors.

Location of micropyles and early embryonic development of the two-spotted cricket Gryllus bimaculatus (Insecta, Orthoptera)

Early embryogenesis of the two-spotted cricket Gryllus bimaculatus was examined by scanning electron microscopy and several fluorescence staining methods, with special reference to these four issues: (i) the location of micropyles; (ii) the transfer of the female pronucleus following meiosis; (iii) the timing of cellularization; and (iv) the process of the germ primordium formation. Between two and four micropyles lie in the mid-ventral region of the egg. The egg nucleus is at the mid-dorsal periphery of the new laid egg, and meiosis resumes and is completed there. The female pronucleus moves to the mid-ventral side, and fertilization occurs there. Energid starts to proliferate and migrates to the periphery of the egg, initiating blastoderm formation. Actin caps surround each superficial nucleus. Cellularization occurs during the blastoderm stage. At a late blastoderm stage, nuclei aggregate in both the posterolateral patch-like regions of the egg to form a germ primordium. The germ primordium looks like a pair of dumbbells. Both the patches shift towards the ventral side and fuse into a germ primordium. The germ primordium contracts to produce a clearly delineated germ band. Observations on distribution patterns of F-actin indicate that, all through the process, the germ primordium retains that unity, and is not separated into two parts.

Research of blastocyte-like structure in chicken

The chicken embryo is a classic model used to investigate embryonic development, gene expression, and tissue differentiation, and is also an important research tool in studying the animal functional genomics. The whole blastoderms of fresh unincubated eggs from White Leghorn chickens were collected with a paper ring, mechanically broken into small pieces and cultured in medium. Then the small pieces would develop into blastocyte-like structures (BLS), which could be facilitated by an addition of fetal bovine serum (FBS) to the primary culture and their diameter was nearly doubled from 12 to 24 h. The additional yolk had no positive effect on the development in the first 12 h but encouraged the BLSs attaching and inner cells differentiating instead in 24 h. The inner cells of the BLS showing a high alkaline-phosphatase activity similar to those in mouse embryonic stem (ES) cells and also expressing a large amount of the specific stage embryonic antigen-1 (SSEA-1) on the surface, which was known to be the characteristic of non-differentiated mouse and avian ES cells, could finally differentiate into nerve-like cells, fibroblast cells and so on in the medium. Leukemia inhibitory factor (LIF) facilitated the cells' proliferation and prevented differentiation in the suspended culture of the BLSs. So we drew the conclusion that the BLS obtained from broken blastoderm can be used to amplify avian ES cells so as to initiate a new method of producing transgenic chickens.

Migration of primordial germ cells isolated from embryonic blood into the gonads after transfer to stage X blastoderms and detection of germline chimaerism by PCR

1. The present study was carried out to determine whether primordial germ cells isolated from embryonic blood can enter the bloodstream and successfully migrate to the germinal ridges of recipient embryos after transfer to stage X blastoderms, and also whether they can differentiate into blood cells, as is suggested in mice. 2. Primordial germ cells were transfected in vitro by lipofection and then transferred to stage X blastoderms. The introduced GFP gene was efficiently expressed in the gonads of 6-d incubated embryos. 3. Freshly collected primordial germ cells were transferred to stage X blastoderms. The fate of the transferred primordial germ cells was traced by detecting the single nucleotide polymorphism in the D-loop region of the mitochondrial DNA in White Leghorn and Barred Plymouth Rock chickens used in this study. The transferred donor primordial germ cell-derived cells were detected in the gonads, but not in the blood cells, of 17-d incubated embryos by PCR. 4. This procedure for primordial germ cell manipulation could provide a novel method of producing germline chimaeric chickens. 5. In conclusion, our findings indicate that primordial germ cells isolated from embryonic blood can migrate to the germinal ridges of recipient embryos after being transferred to stage X blastoderms. Although these transferred primordial germ cells differentiated into germ cells, no differentiation into blood cells was observed.

Kruppel is a gap gene in the intermediate germband insect Oncopeltus fasciatus and is required for development of both blastoderm and germband-derived segments

Segmentation in long germband insects such as Drosophila occurs essentially simultaneously across the entire body. A cascade of segmentation genes patterns the embryo along its anterior-posterior axis via subdivision of the blastoderm. This is in contrast to short and intermediate germband modes of segmentation where the anterior segments are formed during the blastoderm stage and the remaining posterior segments arise at later stages from a posterior growth zone. The biphasic character of segment generation in short and intermediate germ insects implies that different formative mechanisms may be operating in blastoderm-derived and germband-derived segments. In Drosophila, the gap gene Krüppel is required for proper formation of the central portion of the embryo. This domain of Krüppel activity in Drosophila corresponds to a region that in short and intermediate germband insects spans both blastoderm and germband-derived segments. We have cloned the Krüppel homolog from the milkweed bug, Oncopeltus fasciatus (Hemiptera, Lygaeidae), an intermediate germband insect. We find that Oncopeltus Krüppel is expressed in a gap-like domain in the thorax during the blastoderm and germband stages of embryogenesis. In order to investigate the function of Krüppel in Oncopeltus segmentation, we generated knockdown phenotypes using RNAi. Loss of Krüppel activity in Oncopeltus results in a large gap phenotype, with loss of the mesothoracic through fourth abdominal segments. Additionally, we find that Krüppel is required to suppress both anterior and posterior Hox gene expression in the central portion of the germband. Our results show that Krüppel is required for both blastoderm-derived and germband-derived segments and indicate that Krüppel function is largely conserved in Oncopeltus and Drosophila despite their divergent embryogenesis.

Dynamic control of positional information in the early Drosophila embryo

Morphogen gradients contribute to pattern formation by determining positional information in morphogenetic fields. Interpretation of positional information is thought to rely on direct, concentration-threshold-dependent mechanisms for establishing multiple differential domains of target gene expression. In Drosophila, maternal gradients establish the initial position of boundaries for zygotic gap gene expression, which in turn convey positional information to pair-rule and segment-polarity genes, the latter forming a segmental pre-pattern by the onset of gastrulation. Here we report, on the basis of quantitative gene expression data, substantial anterior shifts in the position of gap domains after their initial establishment. Using a data-driven mathematical modelling approach, we show that these shifts are based on a regulatory mechanism that relies on asymmetric gap-gap cross-repression and does not require the diffusion of gap proteins. Our analysis implies that the threshold-dependent interpretation of maternal morphogen concentration is not sufficient to determine shifting gap domain boundary positions, and suggests that establishing and interpreting positional information are not independent processes in the Drosophila blastoderm.

[Achievements and applications in making chicken chimeras using BCs]

The technology of producing chicken chimeras using blastodermal cells is very important not only in the field of transgenic chicken bioreactor but also in searching for efficient ways to conserve avian genetic resource. The basic processes for producing chicken chimeras consist of: (1) Setting up the color model; (2) Separating and dissociating of donor embryos; (3) Compromising of the recipient embryos; (4) Windowing and recovering the recipient eggs; (5) Cells injecting; (6) Method of hatching. The progress, obstacles and prospects of producing chicken chimeras via BCs were discussed in this paper.

In vivo time-lapse imaging in medaka – n-heptanol blocks contractile rhythmical movements

Medaka is an ideal model system for developmental studies as it combines the advantages of powerful genetics and classical embryology. Due to the accessibility, transparency and fast development, embryogenesis and morphogenesis can be followed in vivo. Microscopic time-lapse imaging, however, requires the immobilization of the object to be observed. In medaka rhythmical contractile movements of the blastoderm during early development hampered time-lapse studies, as they cause the embryo to rotate vividly. Here we show that the contractile movements can be reduced by continuous treatment with the gap-junction uncoupling agent n-heptanol up to the 12-somite stage (stage 23) without interfering with development. This allows for the first time to perform high-resolution time-lapse studies in medaka.

Single cell lineage and regionalization of cell populations during Medaka neurulation

To study the movement of individual cells and development of cell grouping during neurogenesis, we labeled single cells in early Medaka gastrula at stage 13 [13 hours post-fertilization (hpf)] with a fluorescent vital dye, and analyzed cells and their descendants using time-lapse live recording up to stage 24 (44 hpf). At stage 13, all future neural cells were located in a dorsal 140° sector of the embryo, and migrated toward the vegetal pole;but during stage 15 to 16, they converged towards the midline. Cells that contributed to later neural subdivisions initially formed overlapping populations, but after stage 16+ they formed non-overlapping cell groups having characteristics of tissue `compartments', preceding development of morphologically distinct neural subdivisions. In early retinal development, a single compartment for future retinal cells was formed superficial to telencephalic and diencephalic compartments, but it was split into left and right eye components at stage 17 in parallel with anterodorsal movement of the diencephalic compartment. At stage 16+, when these compartments were established, Pax6 expression initiated, but only in the laterally located subpopulation of the retina precursor. These observations revise the current view of bilateral retinal development. Continuous live recording of labeled single precursor cells and computer graphics-assisted data analysis,which are presented for the first time in this study, provide excellent means with which to analyze essential cellular processes in organogenesis.

[Dorsoventral differences of morphogenetic potencies of the loach blastoderm in experiments with alteration of the mass of explanted fragments]

We studied the influence of doubling the mass of explanted fragments of the dorsal and ventral loach blastoderm at the early gastrula stage on their capacity for differentiation of axial structures. The dorsoventral differences are as follows: the differentiation of somites correlates, according to the results of factor analysis, with the shape complication only in double dorsal explants, while the notochord is more differentiated in the ventral fragments, if it is present, than in the dorsal ones. Doubling of the mass of dorsal fragments of the blastoderm enhances their morphogenetic potencies and shifts differentiation towards the formation of trunk axial structures. The increased mass of ventral fragments does not affect their differentiation and morphogenesis, but disturbs the correlation of these processes.

Anterior identity is established in chick epiblast by hypoblast and anterior definitive endoderm

Previous studies of head induction in the chick have failed to demonstrate a clear role for the hypoblast and anterior definitive endoderm (ADE) in patterning the overlying ectoderm, whereas data from both mouse and rabbit suggest patterning roles for anterior visceral endoderm (AVE) and ADE. Based on similarity of gene expression patterns, fate and a dual role in 'protecting' the prospective forebrain from caudalising influences of the organiser, the chick hypoblast has been suggested to be the homologue of the mouse anterior visceral endoderm. In support of this, when transplanted to chick embryos, the rabbit AVE induces anterior markers in the chick epiblast. To reevaluate the role of the hypoblast/ADE (lower layer) in patterning the chick ectoderm, we used rostral blastoderm isolates (RBIs) as an assay, that is, rostral regions of blastoderms transected at levels rostral to the node. RBIs are, therefore, free from the influences of Hensen's node and ingressing axial mesoderm - tissues that are able to induce Ganf, the earliest specific marker of anterior neural plate. We demonstrate, using such RBIs (or RBIs dissected to remove the lower layer with or without tissue replacement), that the hypoblast/ADE (lower layer) is required and sufficient for patterning anterior positional identity in the overlying ectoderm, leading to expression of Ganf in neuroectoderm. Our results suggest that patterning of anterior positional identity and specification of neural identity are separable events operating to pattern the rostral end of the early chick embryo. Based on this new evidence we propose a revised model for establishing anteroposterior polarity, neural specification and head patterning in the early chick that is consonant with that occurring in other vertebrates.

Expression of exogenous genes in blastodermal cells of chicken in vitro

Chicken blastodermal cells (BCs) from stage X embryos produce both somatic and germline chimeras when injected into the subgerminal cavity of recipient embryos. Transfection of the donor cells in vitro could lead to the production of chimeras capable of transmitting the transgene to their offspring. The aim of this study was to transfer and express foreign genes under control of the ovalbumin promoter in the BCs. The results showed that luciferase activity in the BCs reached a plateau value with a 2.0:1.0 or 5.0:1.0 liposome-DNA ratio and using 1 microg of DNA. Under this same condition, no difference was found in relative activity between the pGL-control and pOVALUC plasmid. The expression of other exogenous genes (green fluorescent protein and interferon alpha2a) driven by the chicken ovalbumin promoter in cultured chicken blastodermal cells in vitro is possible by this assay. Hatchability of recipient embryos after injection of 1,500 or 800 transfected BCs was compared. The advantage of using a smaller number (800) of injected transfected BCs was that early embryonic mortality was reduced and resulted in higher (P<0.01) hatchability (24.5%) than in the case of 1,500 BCs injected.