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PubMed2003

Mechanisms of cell positioning during C. elegans gastrulation.

Cell rearrangements are crucial during development. In this study, we use C. elegans gastrulation as a simple model to investigate the mechanisms of cell positioning. During C. elegans gastrulation, two endodermal precursor cells move from the ventral surface to the center of the embryo, leaving a gap between these ingressing cells and the eggshell. Six neighboring cells converge under the endodermal precursors, filling this gap. Using an in vitro system, we observed that these movements occurred consistently in the absence of the eggshell and the vitelline envelope. We found that movement of the neighbors towards each other is not dependent on chemotactic signaling between these cells. We further found that C. elegans gastrulation requires intact microfilaments, but not microtubules. The primary mechanism of microfilament-based motility does not appear to be through protrusive structures, such as lamellipodia or filopodia. Instead, our results suggest an alternative mechanism. We found that myosin activity is required for gastrulation, that the apical sides of the ingressing cells contract, and that the ingressing cells determine the direction of movement of their neighboring cells. Based on these results, we propose that ingression is driven by an actomyosin-based contraction of the apical side of the ingressing cells, which pulls neighboring cells underneath. We conclude that apical constriction can function to position blastomeres in early embryos, even before anchoring junctions form between cells.

Actin Cytoskeleton↗
PubMed2003

Acquisition of Hox codes during gastrulation and axial elongation in the mouse embryo.

Early sequential expression of mouse Hox genes is essential for their later function. Analysis of the relationship between early Hox gene expression and the laying down of anterior to posterior structures during and after gastrulation is therefore crucial for understanding the ontogenesis of Hox-mediated axial patterning. Using explants from gastrulation stage embryos, we show that the ability to express 3' and 5' Hox genes develops sequentially in the primitive streak region, from posterior to anterior as the streak extends, about 12 hours earlier than overt Hox expression. The ability to express autonomously the earliest Hox gene, Hoxb1, is present in the posterior streak region at the onset of gastrulation, but not in the anterior region at this stage. However, the posterior region can induce Hoxb1 expression in these anterior region cells. We conclude that tissues are primed to express Hox genes early in gastrulation, concomitant with primitive streak formation and extension, and that Hox gene inducibility is transferred by cell to cell signalling. Axial structures that will later express Hox genes are generated in the node region in the period that Hox expression domains arrive there and continue to spread rostrally. However, lineage analysis showed that definitive Hox codes are not fixed at the node, but must be acquired later and anterior to the node in the neurectoderm, and independently in the mesoderm. We conclude that the rostral progression of Hox gene expression must be modulated by gene regulatory influences from early on in the posterior streak, until the time cells have acquired their stable positions along the axis well anterior to the node.

Animals↗
PubMed1989

The specification of heart mesoderm occurs during gastrulation in Xenopus laevis.

The establishment of heart mesoderm during Xenopus development has been examined using an assay for heart differentiation in explants and explant combinations in culture. Previous studies using urodele embryos have shown that the heart mesoderm is induced by the prospective pharyngeal endoderm during neurula and postneurula stages. In this study, we find that the specification of heart mesoderm must begin well before the end of gastrulation in Xenopus embryos. Explants of prospective heart mesoderm isolated from mid- or late neurula stages were capable of heart formation in nearly 100% of cases, indicating that the specification of heart mesoderm is complete by midneurula stages. Moreover, inclusion of pharyngeal endoderm had no statistically significant effect upon either the frequency of heart formation or the timing of the initiation of heartbeat in explants of prospective heart mesoderm isolated after the end of gastrulation. When the superficial pharyngeal endoderm was removed at the beginning of gastrulation, experimental embryos formed hearts, as did explants of prospective heart mesoderm from such embryos. These results indicate that the inductive interactions responsible for the establishment of heart mesoderm occur prior to the end of gastrulation and do not require the participation of the superficial pharyngeal endoderm.

Animals↗
PubMed1989

Retarded gastrulation and altered subsequent development of neural tissues in heparin-injected Xenopus embryos.

Gastrulation was examined in Xenopus embryos injected with various polysaccharides into the blastocoel cavity. The progression of gastrulation was assessed by observing pigmentation and yolk plug size in vegetative view embryos. In heparin- or dextran-sulphate-injected embryos, gastrulation was significantly retarded. This was further confirmed in tissue sections of embryos. In contrast, no such retardation was found in embryos injected with hyaluronic acid or chondroitin sulphate. A quantitative analysis showed that the extent of retardation in heparin- or dextran-sulphate-injected embryos was dose-dependent and that, after the initial retardation of up to 2-3 h, gastrulation progressed at a similar rate to controls. At the time when untreated sibling embryos hatched, embryos injected with heparin or dextran sulphate showed abnormalities in their external appearance and swimming behavior in a dose-dependent manner. When these embryos were examined histologically or immunohistochemically using tissue-specific monoclonal antibodies, it was found that central nervous system (CNS), especially the brain and eye structures, were most severely damaged. The extent of damage was again dose-dependent. In contrast, neural-crest-derived melanophores were abundant even in aneural larvae. No such change was found in embryos injected with hyaluronic acid or chondroitin sulphate.

Animals↗
PubMed1990

Mesoderm induction and the control of gastrulation in Xenopus laevis: the roles of fibronectin and integrins.

Exposure of isolated Xenopus animal pole ectoderm to the XTC mesoderm-inducing factor (XTC-MIF) causes the tissue to undergo gastrulation-like movements. In this paper, we take advantage of this observation to investigate the control of various aspects of gastrulation in Xenopus. Blastomeres derived from induced animal pole regions are able, like marginal zone cells, but unlike control animal pole blastomeres, to spread and migrate on a fibronectin-coated surface. Dispersed animal pole cells are also able to respond to XTC-MIF in this way; this is one of the few mesoderm-specific responses to induction that has been observed in single cells. The ability of induced animal pole cells to spread on fibronectin is abolished by the peptide GRGDSP. However, the elongation of intact explants is unaffected by this peptide. This may indicate that fibronectin-mediated cell migration is not required for convergent extension. We have investigated the molecular basis of XTC-MIF-induced gastrulation-like movements by measuring rates of synthesis of fibronectin and of the integrin beta 1 chain in induced and control explants. No significant differences were observed, and this suggests that gastrulation is not initiated simply by control of synthesis of these molecules. In future work, we intend to investigate synthesis of other integrin subunits and to examine possible post-translational modifications to fibronectin and the integrins.

Animals↗
PubMed1995

Nodal-related signals induce axial mesoderm and dorsalize mesoderm during gastrulation.

Mouse embryos homozygous for a null mutation in nodal arrest development at early gastrulation and contain little or no embryonic mesoderm. Here, two Xenopus nodal-related genes (Xnr-1 and Xnr-2) are identified and shown to be expressed transiently during embryogenesis, first within the vegetal region of late blastulae and later in the marginal zone during gastrulation, with enrichment in the dorsal lip. Xnrs and mouse nodal function as dose-dependent dorsoanterior and ventral mesoderm inducers in whole embryos and explanted animal caps. Using a plasmid vector to produce Xnr proteins during gastrulation, we show that, in contrast to activin and other TGF beta-like molecules, Xnr-1 and Xnr-2 can dorsalize ventral marginal zone explants and induce muscle differentiation. Xnr signalling also rescues a complete embryonic axis in UV-ventralized embryos. The patterns of Xnr expression, the activities of the proteins and the phenotype of mouse nodal mutants, all argue strongly that a signaling pathway involving nodal, or nodal-related peptides, is an essential conserved element in mesoderm differentiation associated with vertebrate gastrulation and axial patterning.

Amino Acid Sequence↗
PubMed1996

Vegetal egg cytoplasm promotes gastrulation and is responsible for specification of vegetal blastomeres in embryos of the ascidian Halocynthia roretzi.

An animal-vegetal axis exists in the unfertilized eggs of the ascidian Halocynthia roretzi. The first phase of ooplasmic segregation brings the egg cortex to the vegetal pole very soon after fertilization. In the present study, when 5-8% of the egg cytoplasm in the vegetal pole region was removed between the first and second phase of segregation, most embryos exhibited failure of gastrulation, as reported previously in Styela by Bates and Jeffery (Dev. Biol, 124, 65-76, 1987). The embryos that were deficient in vegetal pole cytoplasm (VC-deficient embryos) developed into permanent blastulae. They consisted for the most part of epidermal cells and most lacked the derivatives of vegetal blastomeres, such as endoderm, muscle and notochord. Removal of cytoplasm from other regions did not affect embryogenesis. The cleavage of the VC-deficient embryos not only exhibited radial symmetry along the animal-vegetal axis but the pattern of the cleavage was also identical in the animal and vegetal hemispheres. Examination of the developmental fates of early blastomeres of VC-deficient embryos revealed that the vegetal blastomeres had assumed the fate of animal cells. These results suggested that the VC-deficient embryos had been totally animalized. When vegetal pole cytoplasm was transplanted to the animal pole or equatorial position of VC-deficient eggs, gastrulation occurred, starting at the site of the transplantation and tissues derived from vegetal blastomeres formed. Therefore, it appears that vegetal pole cytoplasm specifies the site of gastrulation and the cytoplasm is responsible for the specification of vegetal blastomeres. It is suggested that during the second phase of ooplasmic segregation, cytoplasmic factors responsible for gastrulation spread throughout the entire vegetal hemisphere.

Acetylcholinesterase↗
PubMed1997

Murine gastrulation requires HNF-4 regulated gene expression in the visceral endoderm: tetraploid rescue of Hnf-4(-/-) embryos.

Immediately prior to gastrulation the murine embryo consists of an outer layer of visceral endoderm (VE) and an inner layer of ectoderm. Differentiation and migration of the ectoderm then occurs to produce the three germ layers (ectoderm, embryonic endoderm and mesoderm) from which the fetus is derived. An indication that the VE might have a critical role in this process emerged from studies of Hnf-4(-/-) mouse embryos which fail to undergo normal gastrulation. Since expression of the transcription factor HNF-4 is restricted to the VE during this phase of development, we proposed that HNF-4-regulated gene expression in the VE creates an environment capable of supporting gastrulation. To address this directly we have exploited the versatility of embryonic stem (ES) cells which are amenable to genetic manipulation and can be induced to form VE in vitro. Moreover, embryos derived solely from ES cells can be generated by aggregation with tetraploid morulae. Using Hnf-4(-/-) ES cells we demonstrate that HNF-4 is a key regulator of tissue-specific gene expression in the VE, required for normal expression of secreted factors including alphafetoprotein, apolipoproteins, transthyretin, retinol binding protein, and transferrin. Furthermore, specific complementation of Hnf-4(-/-) embryos with tetraploid-derived Hnf-4(+/+) VE rescues their early developmental arrest, showing conclusively that a functional VE is mandatory for gastrulation.

Animals↗
PubMed1998

Role of caudal in hindgut specification and gastrulation suggests homology between Drosophila amnioproctodeal invagination and vertebrate blastopore.

During early embryogenesis in Drosophila, caudal mRNA is distributed as a gradient with its highest level at the posterior of the embryo. This suggests that the Caudal homeodomain transcription factor might play a role in establishing the posterior domains of the embryo that undergo gastrulation and give rise to the posterior gut. By generating embryos lacking both the maternal and zygotic mRNA contribution, we show that caudal is essential for invagination of the hindgut primordium and for further specification and development of the hindgut. These effects are achieved by the function of caudal in activating different target genes, namely folded gastrulation, which is required for invagination of the posterior gut primordium, and fork head and wingless, which are required to promote development of the internalized hindgut primordium. caudal is not sufficient for hindgut gastrulation and development, however, as it does not play a significant role in activating expression of the genes tailless, huckebein, brachyenteron and bowel. We argue that caudal and other genes expressed at the posterior of the Drosophila embryo (fork head, brachyenteron and wingless) constitute a conserved constellation of genes that plays a required role in gastrulation and gut development.

Animals↗
PubMed2002

Low proliferative and high migratory activity in the area of Brachyury expressing mesoderm progenitor cells in the gastrulating rabbit embryo.

General mechanisms initiating the gastrulation process in early animal development are still elusive, not least because embryonic morphology differs widely among species. The rabbit embryo is revived here as a model to study vertebrate gastrulation, because its relatively simple morphology at the appropriate stages makes interspecific differences and similarities particularly obvious between mammals and birds. Three approaches that centre on mesoderm specification as a key event at the start of gastrulation were chosen. (1) A cDNA fragment encoding 212 amino acids of the rabbit Brachyury gene was cloned by RT-PCR and used as a molecular marker for mesoderm progenitors. Whole-mount in situ hybridisation revealed single Brachyury-expressing cells in the epiblast at 6.2 days post conception, i.e. several hours before the first ingressing mesoderm cells can be detected histologically. With the anterior marginal crescent as a landmark, these mesoderm progenitors are shown to lie in a posterior quadrant of the embryonic disc, which we call the posterior gastrula extension (PGE), for reasons established during the following functional analysis. (2) Vital dye (DiI) labelling in vitro suggests that epiblast cells arrive in the PGE from anterior parts of the embryonic disc and then move within this area in a complex pattern of posterior, centripetal and anterior directions to form the primitive streak. (3) BrdU labelling shows that proliferation is reduced in the PGE, while the remaining anterior part of the embryonic disc contains several areas of increased proliferation. These results reveal similarities with the chick with respect to Brachyury expression and cellular migration. They differ, however, in that local differences in proliferation are not seen in the pre-streak avian embryo. Rather, rabbit epiblast cells start mesoderm differentiation in a way similar to Drosophila, where a transient downregulation of proliferation initiates mesoderm differentiation and, hence, gastrulation.

Amino Acid Sequence↗
PubMed2005

Measuring cell adhesion forces of primary gastrulating cells from zebrafish using atomic force microscopy.

During vertebrate gastrulation, progenitor cells of different germ layers acquire specific adhesive properties that contribute to germ layer formation and separation. Wnt signals have been suggested to function in this process by modulating the different levels of adhesion between the germ layers, however, direct evidence for this is still lacking. Here we show that Wnt11, a key signal regulating gastrulation movements, is needed for the adhesion of zebrafish mesendodermal progenitor cells to fibronectin, an abundant extracellular matrix component during gastrulation. To measure this effect, we developed an assay to quantify the adhesion of single zebrafish primary mesendodermal progenitors using atomic-force microscopy (AFM). We observed significant differences in detachment force and work between cultured mesendodermal progenitors from wild-type embryos and from slb/wnt11 mutant embryos, which carry a loss-of-function mutation in the wnt11 gene, when tested on fibronectin-coated substrates. These differences were probably due to reduced adhesion to the fibronectin substrate as neither the overall cell morphology nor the cell elasticity grossly differed between wild-type and mutant cells. Furthermore, in the presence of inhibitors of fibronectin-integrin binding, such as RGD peptides, the adhesion force and work were strongly decreased, indicating that integrins are involved in the binding of mesendodermal progenitors in our assay. These findings demonstrate that AFM can be used to quantitatively determine the substrate-adhesion of cultured primary gastrulating cells and provide insight into the role of Wnt11 signalling in modulating cell adhesion at the single cell scale.

Animals↗
PubMed1992

Specification of the body plan during Xenopus gastrulation: dorsoventral and anteroposterior patterning of the mesoderm.

Although the mesoderm itself is induced at the blastula stage, its subdivision mainly occurs in response to further inductive signals during gastrulation. In the late blastula, most of the mesoderm has a ventral-type commitment except for the small organizer region which extends about 30 degrees on each side of the dorsal midline. During gastrulation, dorsal convergence movements bring the cells of the lateroventral marginal zone up near the dorsal midline and into the range of the dorsalizing signal emitted by the organizer. This dorsalizing signal operates throughout gastrulation, can cross a Nuclepore membrane, and is not mimicked by lithium, FGFs or activin. Anteroposterior specification also takes place during gastrulation and is probably controlled by a dominant region at the posterior end of the forming axis. We have studied the expression patterns in Xenopus of three members of the FGF family: bFGF, int-2 and a newly discovered species, eFGF. These all have mesoderm inducing activity on isolated animal caps, but are likely also to be involved with the later interactions. RNAase protections and in situ hybridizations show that the int-2 and eFGF mRNAs are concentrated at the posterior end, while bFGF is expressed as a posterior to anterior gradient from tailbud to head. Studies of embryos in which bFGF is overexpressed from synthetic mRNA show that biological activity is far greater when a functional signal sequence is provided. This suggests that int-2 and eFGF, which possess signal sequences, are better candidates for inducing factors in vivo than is bFGF.

Activins↗
PubMed1990

Fibronectin-rich fibrillar extracellular matrix controls cell migration during amphibian gastrulation.

We have reviewed the evidence supporting the notion that the fibrillar extracellular matrix on the basal surface of the blastocoel roof in amphibian embryos directs and guides mesodermal cell migration during gastrulation. Based on extensive experimental evidence in several different systems, we conclude the following: (i) the fibrillar extracellular matrix contains fibronectin (FN) and laminin. (ii) The fibrils are oriented in such a way as to promote directional migration of mesodermal cells during migration. (iii) We have used several different probes to disrupt the interaction between migrating mesodermal cells and the fibrillar extracellular matrix. These probes include: (a) nucleocytoplasmic and interspecific hybridization. Such embryos have defects in FN synthesis and gastrulation. (b) Fab' fragments of anti-FN and anti-integrin VLA-5 IgGs prohibit mesodermal cell adhesion both in vitro and in vivo and gastrulation is arrested. (c) Peptides containing the RGDS sequence specifically inhibit interactions between migrating mesodermal cells and the FN-fibrillar matrix. (d) Tenascin blocks cell adhesion to FN in vitro and gastrulation in vivo. (e) Antibodies against the cytoplasmic domain of beta 1 integrin, when injected into blastomeres, prevent FN-fibrillogenesis in progeny of injected blastomeres and delay mesodermal cell migration selectively in the progeny of injected blastomeres but not in the uninjected blastomere progeny.

Amphibians↗
PubMedDate not supplied

[Computer modelling of gastrulation and neurulation in amphibian embryos based on mechanical tension fields].

The values of cell wall tensions were calculated with an assumption of mechanical equilibrium of every cell apex on schematic diagrams of histological sections of the common frog embryonic tissues. The maps of the main (the strongest) tensions were drawn for the early gastrula and early neurula. The further course of gastrulation and neurulation was simulated with an assumption that the cell apices are displaced due to active contractions of the most tensed cell walls (variant A of the model). In addition, a suggestion was studied that the capacity for contraction falls in the most extended cell walls (variant B). Up to seven steps of model morphogenesis were simulated and the tension field was recalculated at every step. The course of gastrulation and neurulation was reproduced during simulation with sufficient details, including regional peculiarities of neurulation in the trunk and head regions. Both variants gave roughly similar results for gastrulation, whereas variant B ensured a faster course of model morphogenesis for neurulation. A conclusion was drawn that the mechanical tension fields established by the onset of gastrulation and neurulation represent a sufficient informational base for their further course.

Amphibians↗
PubMedDate not supplied

[Spatial-temporal patterns of the cell cycles in embryos of the clawed toad during gastrulation and neurulation].

The dynamics of DNA synthesis and duration of G2-phases in different dorso-ventral zones and cell layers were studied using 3H-thymidine in the X. laevis embryos during gastrulation and neurulation. Gradients of the index of labelled nuclei and of the rate of 3H-thymidine incorporation decreasing in the ventro-dorsal direction were observed during the whole period of gastrulation. In the end of gastrulation a wave of increase of the index of labelled nuclei and of the rate of 3H-thymidine incorporation proceeds in the same direction and, as a result, both the gradients are inverted. A high positive correlation was found between the simultaneously registered indices of labelled nuclei in different cell layers of the same dorso-ventral zones. The data were obtained suggesting the ventro-dorsal migration of cells in G2-phase. A conclusion is drawn that the regulation of cell cycles during gastrulation--neurulation obeys integral laws among which the waves of cell entrance in S-phase play an important role.

Animals↗
PubMed1994

The evolution of vertebrate gastrulation.

The availability of molecular markers now permits the analysis of the common elements of vertebrate gastrulation. While gastrulation appears to be very diverse in the vertebrates, by analyzing a head-organizer marker, goosecoid, and a marker common to all forming mesoderm, Brachyury, we attempt to identify homologous structures and equivalent stages in Xenopus, zebrafish, chick and mouse gastrulation. Using a tail-organizer marker, Xnot-2, we also discuss how the late stages of gastrulation lead to the formation of the postanal tail, a structure characteristic of the chordates.

Animals↗
PubMed1995

Gastrulation inducing potencies of endophyll and Rauber's sickle in isolated caudocranially oriented prestreak avian blastoderm quadrants (or fragments) in vitro.

Unincubated quail and prestreak chicken blastoderms were sectioned into cranial, caudal and lateral quadrants and cultured in vitro. Whilst in the caudal and lateral quadrants (containing both endophyll and Rauber's sickle material) a primitive streak (PS) developed, in the cranial quadrants (where merely central endophyll is present) only an ectoblastic thickening (early gastrula: phase I of gastrulation) appeared. If, however, sickle material was grafted on a cranial quadrant then a PS developed (phase II of gastrulation). In the anti-sickle region (lacking sickle material and endophyll) no gastrular differentiation occurred spontaneously. The grafting of endophyll and/or Rauber's sickle evokes here also phase I or II of gastrulation. When the endophyll was removed from the caudal quadrant no PS appeared. If, however, endophyll was replaced, a PS and neurula developed. Our study demonstrates that the presence of endophyll is necessary to obtain an early gastrular development (phase I of gastrulation). It seems also to have an influence on the formation of the future neurectodermal region by induction of a thickening of the cranial upper layer. Only when both endophyll and sickle material were present, a PS formed in caudal quadrants. In the caudal quadrant, the endophyll (which seems to represent the vegetative pole of the avian blastula) does not regenerate, neither from the upper layer, nor from Rauber's sickle (containing blastoporal material: Callebaut and Van Nueten: 1994).

Animals↗
PubMed1997

Each member of the Id gene family exhibits a unique expression pattern in mouse gastrulation and neurogenesis.

We have performed a detailed comparative in situ hybridization analysis to examine the patterns of expression of all the members of the Id gene family (Id1-4) during murine gastrulation and neurogenesis. During gastrulation, both Id1 and Id3 are expressed in the tissues derived from the inner cell mass from 5.5 dpc onward, whereas Id2 is expressed in tissues derived from trophoblasts. Id4 expression is absent during this period of development. Embryonic Id1 messages are detected during gastrulation on the proximal side of the embryonic ectoderm, which is the border between the embryo proper and the extraembryonic tissues, and the expression of Id3 is found throughout the entire embryo proper. This unique pattern of expression of the different members of the Id family suggests a nonredundant role for these genes in antagonizing the activity of bHLH transcription factors during very early mouse development. During neurogenesis, the expression of each member of the Id family is present in an unique pattern along the dorsal-ventral axis of the neural tube: In the early stages of spinal cord development, both Id1 and Id2 are expressed in the roof plate, whereas Id3 is expressed both in the roof and the floor plates. As development progresses, the expression of both Id1 and Id3 is detected in the dividing neuroblasts, whereas Id2 and 4 are expressed in presumptive neurons which are undergoing maturation. The expression patterns of all the members of the Id gene family persist throughout the entire CNS, both in the spinal cord and in the brain. In addition, the characteristic expression of Id2 and Id4 in more mature neurons is reiterated both in the PNS and in the neurons of some of the sensory organs. These data suggest that the expression of different subgroups of the Id gene family may have different physiological consequences and thereby contributes in unique ways to specify the differentiation state of neuronal cells during development.

Animals↗