Gastrulation and larval pattern in Xenopus after blastocoelic injection of a Xenopus-derived inducing factor: experiments testing models for the normal organization of mesoderm

Patterning of the Vertebrate Head in Time and Space by BMP Signaling

How head patterning is regulated in vertebrates is yet to be understood. In this study, we show that frog embryos injected with Noggin at different blastula and gastrula stages had their head development sequentially arrested at different positions. When timed BMP inhibition was applied to BMP-overexpressing embryos, the expression of five genes: xcg-1 (a marker of the cement gland, which is the front-most structure in the frog embryo), six3 (a forebrain marker), otx2 (a forebrain and mid-brain marker), gbx2 (an anterior hindbrain marker), and hoxd1 (a posterior hindbrain marker) were sequentially fixed. These results suggest that the vertebrate head is patterned from anterior to posterior in a progressive fashion and may involve timed actions of the BMP signaling.

Microinjection manipulations in the elucidation of Xenopus brain development

Microinjection has a long and distinguished history in Xenopus and has been used to introduce a surprisingly diverse array of agents into embryos by both intra- and intercellular means. In addition to nuclei, investigators have variously injected peptides, antibodies, biologically active chemicals, lineage markers, mRNA, DNA, morpholinos, and enzymes. While enumerating many of the different microinjection approaches that can be taken, we will focus upon the mechanical operations and options available to introduce mRNA, DNA, and morpholinos intracellularly into early stage embryos for the study of neurogenesis.

Visualizing long-range movement of the morphogen Xnr2 in the Xenopus embryo

One way in which cells acquire positional information during embryonic development is by measuring the local concentration of a signaling factor, or morphogen, that is secreted by an organizing center . The ways in which morphogen gradients are established, particularly in vertebrates, remain obscure, although various suggestions have been made for the mechanisms by which signaling molecules traverse fields of cells. These include simple diffusion, "cytonemes", filopodia, "argosomes", and "transcytosis". In this study, we use a functional EGFP-tagged ligand to visualize long-range signaling in the Xenopus embryo in real time. Our results show that the TGF-beta family member Xnr2 is secreted efficiently from embryonic cells, and a new method of tissue recombination allows us to investigate the way in which the morphogen traverses multiple cell diameters. This reveals that Xnr2 exerts long-range effects by diffusing rapidly through the extracellular milieu of nonexpressing cells. No evidence has been obtained for long-range signaling through cytonemes, filopodia, argosomes, or transcytosis. In demonstrating that long-range signaling in the early Xenopus embryo occurs by diffusion rather than by these alternative routes, our results suggest that different morphogens in different developmental contexts use different means of transport.

Timed interactions between the Hox expressing non-organiser mesoderm and the Spemann organiser generate positional information during vertebrate gastrulation

We report a novel developmental mechanism. Anterior-posterior positional information for the vertebrate trunk is generated by sequential interactions between a timer in the early non-organiser mesoderm and the organiser. The timer is characterised by temporally colinear activation of a series of Hox genes in the early ventral and lateral mesoderm (i.e., the non-organiser mesoderm) of the Xenopus gastrula. This early Hox gene expression is transient, unless it is stabilised by signals from the Spemann organiser. The non-organiser mesoderm and the Spemann organiser undergo timed interactions during gastrulation which lead to the formation of an anterior-posterior axis and stable Hox gene expression. When separated from each other, neither non-organiser mesoderm nor the Spemann organiser is able to induce anterior-posterior pattern formation of the trunk. We present a model describing that convergence and extension continually bring new cells from the non-organiser mesoderm within the range of organiser signals and thereby create patterned axial structures. In doing so, the age of the non-organiser mesoderm, but not the age of the organiser, defines positional values along the anterior-posterior axis. We postulate that the temporal information from the non-organiser mesoderm is linked to mesodermal Hox expression.

The initiation of Hox gene expression in Xenopus laevis is controlled by Brachyury and BMP-4

Hox genes encode a family of transcription factors that specify positional identities along the anterior-posterior (AP) axis during the development of vertebrate embryos. The earliest Hox expression in vertebrates is during gastrulation, at a position distant from the organiser or its equivalent. However, the mechanism that initiates this early expression is still not clear. Guided by the expression pattern, we identified upstream regulators in Xenopus laevis. The mesodermal transcription factor brachyury (Xbra) controls the early Hox expression domain in the animal-vegetal direction and the secreted growth factor BMP-4 limits it in the organiser/non-organiser direction. The overlap of these two signals, indicated by a Cartesian coordinate system, defines the initial Hox expression domain. We postulate that this system is a general mechanism for the activation of all Hox genes expressed during gastrulation.

Integration of IGF, FGF, and anti-BMP signals via Smad1 phosphorylation in neural induction

How do very diverse signaling pathways induce neural differentiation in Xenopus? Anti-BMP (Chordin), FGF8, and IGF2 signals are integrated in the embryo via the regulation of Smad1 phosphorylation. Neural induction results from the combined inhibition of BMP receptor serine/threonine kinases and activation of receptor tyrosine kinases that signal through MAPK and phosphorylate Smad1 in the linker region, further inhibiting Smad1 transcriptional activity. This hard-wired molecular mechanism at the level of the Smad1 transcription factor may help explain the opposing activities of IGF, FGF, and BMP signals not only in neural induction, but also in other aspects of vertebrate development.

Early development of Xenopus embryos is affected by simulated gravity

Early amphibian (Xenopus laevis) development under clinostat-simulated weightlessness and centrifuge-simulated hypergravity was studied. The results revealed significant effects on (i) "morphological patterning" such as the cleavage furrow pattern in the vegetal hemisphere at the eight-cell stage and the shape of the dorsal lip in early gastrulae and (ii) "the timing of embryonic events" such as the third cleavage furrow completion and the dorsal lip appearance. Substantial variations in sensitivity to simulated force fields were observed, which should be considered in interpreting spaceflight data.

Transforming growth factor-beta in cardiac ontogeny and adaptation

The transforming growth factor-beta (TGF-beta) superfamily comprises a set of regulatory peptides with multiple effects on cell growth and differentiation. The elaborate regulation of TGF-beta s during embryonic development of the heart, the upregulation of TGF-beta after hemodynamic stress, and the impact of TGF-beta on cardiac gene expression together imply a prominent functional role for this family of growth factors in cardiac organogenesis and hypertrophy. Basal and TGF-beta-induced expression of skeletal alpha-actin, one of several genes specifically associated with developing or hypertrophied myocardium, each are contingent on transcriptional activation by serum response factor. A truncated form of the type II TGF-beta receptor, created by deletion of the cytoplasmic kinase domain, acts as a dominant suppressor of TGF-beta signal transduction in cultured cardiac muscle cells and may provide a suitable means to establish the functions of TGF-beta in vivo.

Analysis of gastrulation: different types of gastrulation movement are induced by different mesoderm-inducing factors in Xenopus laevis

In this paper we analyze the control of gastrulation in Xenopus laevis. Our approach takes advantage of the observation that mesoderm-inducing factors such as activin, FGF and BMP-4 induce presumptive ectodermal cells to undergo gastrulation-like movements. Activin, for example, makes intact animal pole regions undergo convergent extension and causes individual cells to spread and migrate on a fibronectin-coated substrate. By varying the concentrations of the growth factors to which animal pole cells are exposed, and by applying them in different combinations, we show how graded distributions of a combination of factors could establish the correct spatial and temporal patterns of gastrulation in the Xenopus embryo. The distributions we propose support and develop the model previously suggested by Green et al. (1992) to account for the spatial patterns of gene activation in the early embryo.

The frog prince-ss: a molecular formula for dorsoventral patterning in Xenopus

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Patterns of cell motility in the organizer and dorsal mesoderm of Xenopus laevis

In a companion paper (Shih, J. and Keller, R. (1992) Development 116, 901–914), we described a sequence of cell behaviors, called mediolateral intercalation behavior (MIB), that produces mediolateral cell intercalation, the process that drives convergence and extension of the axial and paraxial mesoderm of Xenopus. In this paper, we describe the pattern of expression of MIB in the mesoderm during gastrulation, using video image processing and recording of cell behavior in ‘shaved’, open-faced explants of the marginal zone. At midgastrula stage (10.5), MIB begins at two dorsolateral sites in the prospective anterior mesoderm and progresses medially along two arcs that lengthen toward and meet at the midline to form a single arc of cells expressing MIB, called the vegetal alignment zone (VgAZ). The notochordal-somitic mesodermal boundary forms within the VgAZ at stage 11, and then progresses animally and laterally, along the prospective anterior-posterior axis, eventually bounding a trapezoidal area the shape of the fate-mapped notochord. Meanwhile, from its origin in the VgAZ, MIB spreads in the prospective posterior direction along the lateral boundaries of both the notochordal and somitic mesoderm. From there it spreads medially in both tissues. Subsequently, vacuolation of notochord cells, and segmentation and expression of a somite-specific marker repeat the progression of mediolateral intercalation behavior. Thus cells in the posterior, medial regions of the notochordal and the somitic territories are the last to express mediolateral intercalation behavior and subsequent tissue differentiations. In explants that do not converge, these cells neither express mediolateral intercalation behavior nor differentiate. These facts suggest that progressions of MIB in the anterior-posterior and lateral-medial directions may be organized by signals emanating from the lateral somitic and notochordal boundaries. These signals may have limited range and may be dependent on convergence, driven by mediolateral cell intercalation, to bring cells within their range. In the embryo, the posterior progression of MIB results in arcs of convergence, anchored in the vegetal endoderm at each end, acting on the inside of the blastoporal lip to produce involution of the IMZ.

Suramin changes the fate of Spemann's organizer and prevents neural induction in Xenopus laevis

Suramin, a polyanionic compound, which has previously shown to dissociate platelet derived growth factor (PDGF) from its receptor, prevents the differentiation of neural (brain) structures of recombinants of dorsal blastopore lip (Spemann's organizer) and competent neuroectoderm. Furthermore, the suramin treatment changes the prospective differentiation pattern of isolated blastopore lip. While untreated dorsal blastopore lip will differentiate into dorsal mesodermal structures (notochord and somites), suramin treated dorsal blastopore lip will form ventral mesoderm structures, especially heart structures. The results are discussed in the context of the current opinion about the mode of action of different growth factor superfamilies.

Ectopic mesoderm formation in Xenopus embryos caused by widespread expression of a Brachyury homologue

The Brachyrury (T) gene is required cell-autonomously for mesoderm formation in the posterior of the mouse embryo, and both is complementary DNA sequence and expression pattern closely resemble those of a Xenopus homologue (Xbra), suggesting that these genes have an evolutionarily conserved function in vertebrate development. Strong expression of Xbra messenger RNA is found in the ring of involuting mesoderm during Xenopus gastrulation, and the expression of Xbra is an immediate-early response of animal pole blastomeres to mesoderm-inducing factors. To assess the role of Xbra in mesoderm formation, we increased its domain of expression in the embryo by microinjection of Xbra transcripts into the animal pole of Xenopus embryos at the one-cell stage. We show that expression of Xbra by cells of the early embryo is sufficient to direct their development into differentiated mesodermal tissues. At the molecular level this response shows a sharp threshold of sensitivity to the dose of Xbra RNA delivered, and we suggest that Xbra may act as a genetic switch initiating posterior mesodermal specification during embryogenesis.

The marginal zone of the 32-cell amphibian embryo contains all the information required for chordamesoderm development

The formation of the amphibian organizer is evidenced by the ability of cells of the dorsal marginal zone (DMZ) to self-differentiate to form notochord and to induce the formation of other axial structures from neighboring regions of the embryo. We have attempted to determine when these abilities are acquired in the urodele, Ambystoma mexicanum (axolotl), and in the anuran, Xenopus laevis, by removing the mesodermalizing influence of the vegetal hemisphere at different stages of development and culturing the animal hemisphere isolate. This was possible, even at the 32 and 64-cell stage, through the use of embryos with rare cleavage patterns. Cultured isolates were analyzed for morphological differentiation of mesodermal and neural structures, and for biochemical differentiation of the tissue-specific enzyme, acetylcholinesterase (AChE). Large amounts of mesodermal and neural structures, and normal expression of AChE were found in isolates made as early as the 32-cell stage in both species. Only a small increase in the percentage of isolates developing mesoderm was detected when isolations were made at later cleavage or blastula stages. The amount of mesoderm formed did not depend on the stage of isolation. Mesoderm differentiation was usually limited to the notocord and muscle. The isolates rarely formed pronephros, mesothelium, or mesenchyme, derivatives of ventral mesoderm, during normal development. The results indicate that the marginal zone of the cleavage-stage embryo contains all of the information needed for the formation of the organizer. The formation of dorsal mesoderm does not require subsequent interaction with the cells of the vegetal hemisphere, although the presence of those cells is likely to play a role in normal pattern formation.

Inductive interactions in early embryonic development

This is an update of a previous review (Current Opinion in Cell Biology 2:969-974) in which we discussed recent work attempting to understand the sequence of inductive interactions responsible for establishing the body plan of the early embryo. As before, we concentrate on inductive interactions in amphibian embryos, where significant progress has been made in the past two years. In this update, however, we also consider recent embryological data obtained with amniote embryos such as the chick, together with complementary data provided by genetic analyses of mouse and Drosophila development.

Expression of a dominant negative mutant of the FGF receptor disrupts mesoderm formation in Xenopus embryos

Peptide growth factors may play a role in patterning of the early embryo, particularly in the induction of mesoderm. We have explored the role of fibroblast growth factor (FGF) in early Xenopus development by expressing a dominant negative mutant form of the FGF receptor. Using a functional assay in frog oocytes, we found that a truncated form of the receptor effectively abolished wild-type receptor function. Explants from embryos expressing this dominant negative mutant failed to induce mesoderm in response to FGF. In whole embryos the mutant receptor caused specific defects in gastrulation and in posterior development, and overexpression of a wild-type receptor could rescue these developmental defects. These results demonstrate that the FGF signaling pathway plays an important role in early embryogenesis, particularly in the formation of the posterior and lateral mesoderm.

Identification of a potent Xenopus mesoderm-inducing factor as a homologue of activin A

The first inductive interaction in amphibian development is mesoderm induction, when a signal from the vegetal hemisphere of the blastula induces mesoderm from overlying equatorial cells. Recently, several 'mesoderm-inducing factors' (MIFs) have been discovered. These cause isolated Xenopus animal caps to form mesodermal cell types such as muscle, instead of their normal fate of epidermis. The MIFs fall into two classes. One comprises members of the fibroblast growth factor (FGF) family, and the other members of the transforming growth factor type beta (TGF-beta) family. Of the latter group, the most potent is XTC-MIF, a protein produced by Xenopus XTC cells. Here we show that XTC-MIF is the homologue of mammalian activin A. Activins modulate the release of follicle-stimulating hormone from cultured anterior pituitary cells and cause the differentiation of two erythroleukaemia cell lines. Our results indicate that these molecules may also act in early development during formation of the mesoderm.

Mesoderm induction by fibroblast growth factor in early Xenopus development

In early amphibian development the mesoderm is formed around the equator of the blastula in response to inductive signals from the endoderm. At the time of its formation the mesoderm consists of a large 'ventral type' zone and a small 'organizer' zone. A screen of candidate substances showed that a small group of heparin binding growth factors (HBGFs) were active as mesoderm inducing agents in vitro. The fibroblast growth factors (aFGF and bFGF) and embryonal carcinoma derived growth factor (ECDGF) all show similar potency and can produce ventral inductions at concentrations above about 100 pm. Single blastula ectoderm cells can be induced and will differentiate in a defined medium to form mesodermal tissues and all inner blastula cells are competent to respond to the factors. Inducing activity can be extracted from Xenopus blastulae and can be purified by heparin affinity chromatography. Antibody neutralization and Western blotting experiments identify this activity as bFGF. The amounts present are small but would be sufficient to evoke ventral inductions in vivo. It is not yet known whether the bFGF is localized to the endoderm, although it is known that inducing activity secreted by endodermal cells can be neutralized by heparin. The competence of ectoderm to respond to FGF rises from about the 128-cell-stage and falls again by the onset of gastrulation. This change is paralleled by a rise and fall of binding of 125I-labelled aFGF. Chemical cross-linking reveals that this binding is attributable to a receptor of molecular mass about 130 kilodaltons (kDa). The receptor is present both in the marginal zone, which responds to the signal in vivo, and in the animal pole region, which is not induced in vivo but which will respond to HBGFs in vitro. In intact embryos we believe that the ventral type mesoderm forms the somites, kidney and other intermediate structures as well as the blood islands of the ventral midline. These intermediate structures are induced as a function of distance from the organizer in a process called 'dorsalization'. Lithium salts have a dorsalizing effect on whole embryos and also on explants from the ventral marginal zone, causing them to form large blocks of muscle. Lithium will also cause large muscle blocks to form when applied to ectoderm explants together with FGF. It is difficult to extend these results directly to mammalian embryos, but we have shown that the products of the murine int-2 gene and of the human k-fgf genes are active as mesoderm inducing factors.

Interaction between peptide growth factors and homoeobox genes in the establishment of antero-posterior polarity in frog embryos

The expression of the Xenopus homoeobox gene xhox3 is an early response to mesoderm induction by peptide growth factors and the level of xhox3 expression marks the antero-posterior character of the induced mesoderm. Different peptide growth factors specify different antero-posterior mesodermal cell fates as seen by the level of xhox3 expression and the capacity to induce specific secondary neural/epidermal structures. These factors and homoeobox genes thus form part of the mechanism necessary for establishing antero-posterior polarity in the frog embryo.

Experimental reversal of the normal dorsal-ventral timing of blastopore formation does not reverse axis polarity in Xenopus laevis embryos

During gastrulation in Xenopus laevis, the dorsal lip of the blastopore normally appears before the ventral lip. Metabolic gradient models propose that the dorsal lip develops from the region of highest metabolic activity and somehow dominates other regions to prevent them from becoming dorsal. To test these ideas, I applied a temperature gradient of 12 degrees C across the embryo. Localized heating of the prospective ventral vegetal region from early in the first cleavage period until gastrulation causes the blastopore lip to form first by 2 hr at the prospective ventral meridian rather than at the prospective dorsal meridian. Despite this reversal of the timing of blastopore formation, gastrulation is completed, and the neural plate forms at its usual position on the prospective dorsal meridian. This demonstrates that the earliest gastrulating regions of the blastopore do not necessarily become dorsal, nor do they inhibit dorsal development by other regions. It is unlikely that axis polarity is based on regional differences in energy metabolism.