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PubMed2004

Hypoblast controls mesoderm generation and axial patterning in the gastrulating rabbit embryo.

Gastrulation in higher vertebrate species classically commences with the generation of mesoderm cells in the primitive streak by epithelio-mesenchymal transformation of epiblast cells. However, the primitive streak also marks, with its longitudinal orientation in the posterior part of the conceptus, the anterior-posterior (or head-tail) axis of the embryo. Results obtained in chick and mouse suggest that signals secreted by the hypoblast (or visceral endoderm), the extraembryonic tissue covering the epiblast ventrally, antagonise the mesoderm induction cascade in the anterior part of the epiblast and thereby restrict streak development to the posterior pole (and possibly initiate head development anteriorly). In this paper we took advantage of the disc-shape morphology of the rabbit gastrula for defining the expression compartments of the signalling molecules Cerberus and Dickkopf at pre-gastrulation and early gastrulation stages in a mammal other than the mouse. The two molecules are expressed in novel expression compartments in a complementary fashion both in the hypoblast and in the emerging primitive streak. In loss-of-function experiments, carried out in a New-type culturing system, hypoblast was removed prior to culture at defined stages before and at the beginning of gastrulation. The epiblast shows a stage-dependent and topographically restricted susceptibility to express Brachyury, a T-box gene pivotal for mesoderm formation, and to transform into (histologically proven) mesoderm. These results confirm for the mammalian embryo that the anterior-posterior axis of the conceptus is formed first as a molecular prepattern in the hypoblast and then irrevocably fixed, under the control of signals secreted from the hypoblast, by epithelio-mesenchymal transformation (primitive streak formation) in the epiblast.

Amino Acid Sequence↗
PubMed1984

Morphogenesis of the head of a newt: mesodermal segments, neuromeres, and distribution of neural crest.

Segmentation of the mesoderm in the head of a newt embryo is revealed by scanning electron microscopy. By the end of gastrulation, the newt embryo is already segmented from one end to the other, with additional segments added later by the tail bud. This metameric segmentation appears long before the first "somite" can be seen in the late neurula by light microscopy. The six segments found in the newt head look much like the six most-cranial segments described decades ago in shark embryos. Mesodermal segments in the newt head are similar to somitomeres in amniote embryos, but in amniote embryos, the numbers and relationships of head segments are quite different from those of the newt. In both amniote and newt, the first segment abuts the prosencephalon, but for each more caudal head segment, where the newt embryo has one segment, the amniote has two. Although the pattern and distribution of cranial neural crest is quite similar in newt and amniote embryos, there are different relationships between migrating crest masses and mesodermal segments due to the doubling of most of the cranial segments in amniotes. It now appears that all vertebrate embryos, regardless of their mode of gastrulation, form similar mesodermal segments from one end of the embryo to the other, and this metameric pattern is established during gastrulation.

Animals↗
PubMed1985

Experimental evidence for a proteinaceous presegmental wave required for morphogenesis of axolotl mesoderm.

Mesoderm of axolotl embryos at various developmental stages was briefly exposed to a calcium-free 0.01% trypsin solution by temporary removal of the epidermis. This treatment was found to disrupt somite segmentation in a localized region and the pronephric duct was unable to migrate through this region. The affected area, consisting of 3.91 +/- 1.04 somites, traveled through the embryo in synchrony with, and 3.55 +/- 0.69-somite widths ahead of segmentation. Trypsinization in the presence of 340 microM calcium resulted in normal duct migration while somite segmentation was still affected. These results demonstrate the existence of a trypsin-sensitive region in the somitic mesoderm and the lateral mesoderm of the duct path that travels in advance of somite segmentation and in synchrony with it. In addition, the trypsin sensitivity of the duct path is calcium dependent whereas that of the somitic mesoderm is not.

Ambystoma↗
PubMed1986

Precardiac cell migration: fibronectin localization at mesoderm-endoderm interface during directional movement.

The pathway of directional movement of chick precardiac mesoderm cells was studied by indirect immunofluorescence and by scanning electron microscopy. Directional movement of the precardiac cells begins at stage 6 from the lateral sides of the embryo at the level of Hensen's node. The cells move anteriorly in an arc to the embryo's midline. By stage 8 the cells arrive at the lateral sides of the anterior intestinal portal and movement ceases. The interval of this directional movement is approximately 10 hr. During migration the precardiac cells are in close association with the underlying endoderm. As migration proceeds, the cells encounter increasing amounts of fibrils in the substratum at the mesoderm-endoderm interface. Concomitant with increasing fibril formation there is an increase in fibronectin (FN) in the heart-forming region. During stage 5 FN first appears in the lateral heart-forming regions and increases in amount during the period of cell migration. By stage 7 a concentration difference of FN is apparent in the lateral regions with more FN cephalad and decreasing amounts caudad. At stages 7 and 8 large amounts of extracellular FN-associated fibrils are observed at the lateral sides of the anterior intestinal portal where the cells stop moving. The precardiac cells moving into this region are oriented perpendicular to the anterior intestinal portal and in close association with these fibrils. There is no evidence that the fibrillar meshwork forming the substratum of the precardiac mesoderm cells is physically oriented as a guide for directional movement. The correlations between FN distribution at the mesoderm-endoderm interface and directional cell movement suggest that the precardiac cells may migrate by haptotaxis, i.e., by moving along the substratum toward areas of greater adhesiveness.

Animals↗
PubMed1988

Retinoic acid promotes proliferation and chondrogenesis in the distal mesodermal cells of chick limb bud.

Distal and proximal mesoderm of chick limb bud was respectively dissociated and cultured in the medium containing various concentrations of retinoic acid (RA). At low concentrations (5-50 ng/ml), RA promoted proliferation and chondrogenesis in the distal mesodermal cells. The distal cells of stage 20-24 limb buds were responsive to RA, although those of stages 25-27 were unresponsive. Both the cells of anterior and posterior regions of the distal mesoderm were responsive to RA, while the cells of proximal mesoderm were unresponsive. At higher concentrations, the growth-promoting effect of RA was reduced and chondrogenesis in the distal cells was rather inhibited. These results were discussed in relation to the role of RA as the morphogen in normal limb development and experimental duplicate formation.

Animals↗
PubMed1989

Clonal analysis of mesoderm induction in Xenopus laevis.

Acidic fibroblast growth factor (aFGF) has been used to induce mesoderm from single animal pole cells of midblastula stage Xenopus embryos. The cells are individually cultured in a completely defined medium and are able to differentiate as small clones in a high proportion of cases. FGF-treated cells can give rise to several mesodermal cell types, while untreated cells show only epidermal or neural differentiation. Mesodermal differentiation can occur in clones of as few as eight cells, indicating that any additional cell-cell interactions required for mesodermal differentiation can be met by the medium used.

Animals↗
PubMed1989

Leech neurogenesis. II. Mesodermal control of neuronal patterns.

This paper reports analyses of the effects of eliminating mesoderm from one or both sides of embryos of the glossiphoniid leech Theromyzon rude on the differentiation and distribution of ectodermal cells, especially identified peripheral neurons and central 5-hydroxytryptamine (5-HT)-containing neurons arising from the bilateral pair of cell lines called the n bandlets (n-kinship cells). In mesoderm-deprived regions, no segmental hemiganglia formed, and identified neurons were not organized into recognizable patterns, although 5-HT neurons underwent neurochemical differentiation and grew axons. In unilaterally mesoderm-deprived embryos, segmental hemiganglia were formed in a midbody experimental zone, and cells that had abnormally crossed the ventral midline from the deprived side gave rise to identified neurons that were incorporated as supernumeraries into the normal organization of hemiganglia on the nondeprived side. In a posterior experimental zone, ganglionic morphology was disrupted on both sides. We conclude that precursor cells are committed to specific neuronal fates regardless of whether they occupy normal positions and that mesodermal tissues provide positional cues necessary for such precursor cells to find positions appropriate to their fates.

Animals↗
PubMed1992

Xenopus blastulae show regional differences in competence for mesoderm induction: correlation with endogenous basic fibroblast growth factor levels.

Single cells from the animal cap and marginal zone (MZ) of mid-blastula stage embryos can undergo mesodermal or ectodermal differentiation as small clones under defined conditions in culture. Here we report that cells treated with Xenopus basic fibroblast growth factor (XbFGF), a mesoderm-inducing factor, usually differentiated into muscle. MZ cells, which normally give rise to most of the mesoderm, responded to lower concentrations of XbFGF than animal pole (AP) presumptive ectoderm cells. This difference in sensitivity correlated with immunocytochemical staining patterns that showed much greater levels of endogenous bFGF within MZ than AP cells in early embryos. At the mid-late blastula stage, nuclei of MZ cells were strongly immunoreactive. Nuclear staining persisted during gastrula and neurula stages, and extracellular bFGF also became apparent. Subsequently in somites, immunoreactivity of nuclei declined while that of the extracellular matrix was retained during tailbud stages. Nuclear localization of bFGF appeared to be temporally correlated with new transcription of muscle-specific genes, and extracellular bFGF was present during morphological differentiation. The results suggest that a cell's competence for mesoderm induction is related to its position in the embryo.

Animals↗
PubMed1992

Motile behavior and protrusive activity of migratory mesoderm cells from the Xenopus gastrula.

During Xenopus gastrulation, the mesoderm migrates across a fibronectin (FN)-containing substrate, the inner surface of the blastocoel roof (BCR). A possible role for FN is to promote the extension of cytoplasmic processes which serve as locomotory organelles for mesoderm cells. To test this idea, the interaction of prospective head mesoderm (HM) cells with FN was examined in vitro. Nonattached HM cells extend filiform processes from an active region of the cell surface. This spontaneous activity is modulated by cell attachment to FN. Additional active regions appear, and cytoplasmic lamellae extend from these sites, leading to cell spreading and translocation. Thus, although FN seems not to induce processes de novo, it modulates a spontaneous protrusive activity to yield the extension of lamellae along the substrate surface. As putative locomotory organelles, HM cell protrusions were characterized functionally. They adhere rapidly and selectively to in situ substrates, preferentially to FN, and retract upon attachment. During translocation, the passive cell body is moved by the activity of the protrusions. Lamellae continuously extend, retract, or split into parts. This leads to an intermittent, nonpersistent mode of translocation. The polarity of HM cells, as expressed in the arrangement of protrusions, bears no constant relationship to the orientation of the cell body, and a cell can change its direction of movement without a corresponding rotation of the cell body. This may be relevant with respect to the mechanism by which mesoderm cells translate guidance cues of the BCR into a polarized, oriented cell structure during directional migration in situ.

Animals↗
PubMed1980

The formation of mesodermal derivatives after induction with vegetalizing factor depends on secondary cell interactions.

Early amphibian gastrula ectoderm induced with vegetalizing factor for 6 h using the sandwich-method and cultured for up to 12 days, differentiated into mesodermal and endodermal tissues. Explants which were dissociated into single cells after the induction followed by immediate reaggregation and then cultured for 12 days likewise differentiated into mesodermal and endodermal tissues. However, if after induction and dissociation of the tissue, single cells are cultured for 20 h prior to the reaggregation, the reaggregated cell mass mainly differentiated into endoderm (liver and intestine) and irregularly shaped epidermis (formerly called 'atypical epidermis'). Blood cells and, in few cases, heart structures were the only mesodermal structures found in this series. The results suggest that an endodermal anlage is induced first. The differentiation of mesodermal derivatives depends on secondary cell interactions between endodermal induced and non-induced ectoderm. For this process permanent cell-to-cell contacts are necessary.

Animals↗
PubMed1989

Mix.1, a homeobox mRNA inducible by mesoderm inducers, is expressed mostly in the presumptive endodermal cells of Xenopus embryos.

In frogs, mesoderm presumably derives from presumptive ectoderm by induction under the control of diffusible substances produced by the endoderm. To analyze the early phase of mesoderm induction, I have isolated cDNA copies of mRNAs induced in presumptive ectoderm by mesoderm inducing factor secreted by the XTC cell line. One of the inducible mRNAs encodes a homeodomain-containing protein that is likely to play a regulatory role in development. Mix.1 behaves as an immediate early response to induction, and its kinetics of expression suggest a major role for MBT in the control of inducible gene expression. Unexpectedly, Mix.1 is expressed mostly in the future endoderm, suggesting that endoderm may be formed by induction in a similar way as mesoderm.

Amino Acid Sequence↗
PubMed1994

Overexpression of cadherins and underexpression of beta-catenin inhibit dorsal mesoderm induction in early Xenopus embryos.

The cadherin-catenin complex has an important role in cell-cell adhesion and may also function in signaling pathways. We report that overexpression of three cadherin types in Xenopus embryos causes them to develop with reduced dorsal axial structures. The same phenotype is produced in embryos that have been depleted of maternal beta-catenin protein by an antisense oligodeoxynucleotide complementary to beta-catenin mRNA. They show an inhibition in the expression of dorsal mesodermal markers MyoD and goosecoid, but not of ventral and general mesodermal markers. They lack notochords, somites, and neural tubes and are defective in dorsal mesodermal signaling in Nieuwkoop assays. The phenotype can be rescued by the injection of beta-catenin mRNA and not by the injection of Xwnt-8 mRNA. These results show that beta-catenin has an important role in dorsal mesoderm induction. They directly demonstrate the activity of a maternal mRNA in axis specification.

Animals↗
PubMed1996

Cytogenetic findings in four malignant mixed mesodermal tumors of the ovary.

Short-term cultures of four malignant mixed mesodermal tumors of the ovary were cytogenetically analyzed. The primary tumor was examined in three cases, whereas in one case the sample was obtained from a residual tumor mass after chemotherapy. The tumor sampled after cytostatic treatment had a relatively simple karyotype with numerical changes that included pentasomy 12 and an i(1)(q10) as the only structural abnormality. Karyotypic analysis of the three primary tumors revealed extensive structural as well as numerical aberrations, i.e., a picture similar to that seen in the few malignant mixed mesodermal tumors with karyotypic anomalies described previously. Rearrangements of chromosome 1, leading to loss of distal 1p, and homogeneously staining regions have so far been the most frequent cytogenetic changes in this tumor type. Malignant mixed mesodermal tumors of the ovary thus seem to be karyotypically identical to the more numerous mixed mesodermal tumors of uterine origin, and they do not differ substantially in this respect from pure ovarian carcinomas.

Chromosome Aberrations↗
PubMed2004

Is mechano-sensitive expression of twist involved In mesoderm formation?

Mesoderm invagination, the first morphogenetic movement of gastrulation in the early Drososphila embryo, is controlled by the expression of the twist and snail genes. Our knowledge concerning epistatic relationships between these genes implies the existence of a poorly understood biochemical maintenance of twist expression during mesoderm invagination by the snail gene. In the light of a review detailing the role of these genes in the cell shape changes leading to invagination, and of recent findings showing the expression of twist as mechanically sensitive, we suggest that the expression of twist in the mesoderm could alternatively be maintained by mechanical strains developed during mesoderm invagination.

Animals↗
PubMed2003

Mesoangioblasts--vascular progenitors for extravascular mesodermal tissues.

Mesoangioblasts are multipotent progenitors of mesodermal tissues that express the key marker of angiopoietic progenitors, Flk1 (VEGF-receptor 2), and are physically associated with the embryonic dorsal aorta in avian and mammalian species. When transplanted in vivo, they give rise to multiple differentiated mesodermal phenotypes. Their ability to extensively self-renew in vitro, while retaining multipotency, qualifies mesoangioblasts as a novel class of stem cells. Mesoangioblasts disclose not only an unexpected source of progenitors for skeletal muscle and a variety of other mesoderm-derived tissues, but also establish a lineage kinship between progenitors of vascular and extravascular mesodermal tissues, with important basic and applicative implications.

Animals↗
PubMed2005

XBP1 forms a regulatory loop with BMP-4 and suppresses mesodermal and neural differentiation in Xenopus embryos.

The active form of the Xenopus X-box binding protein 1 (xXBP1) partially synergizes and partially antagonizes with BMP-4 signaling. xXBP1 overexpression inhibits mesoderm differentiation and formation of neural tissues. A functional knockdown promotes differentiation of lateral and dorsal mesoderm but not of ventral mesoderm and of neuroectoderm. We show that the active form of xXBP1 in gastrula and early neurula stage embryos is generated by removal of exon 4 and not by an endoribonuclease activity in the endoplasmic reticulum. The N-terminal region of xXBP1 which contains the basic leucine-zipper also contains a nuclear localization signal and both, the N-terminal as well as the C-terminal regions are required for xXBP1 function. The effects of xXBP1 are in part correlated to a regulatory loop between xXBP1 and BMP-4. xXBP1 and BMP-4 stimulate mutually the transcription of each other, but xXBP1 inhibits the BMP-4 target gene, Xvent-2. Both, in vitro and in vivo assays demonstrate that xXBP1 interacts with BMP-4 and Xvent-2B promoters. GST-pulldown assays reveal that xXBP1 can interact with c-Jun, the transcriptional co-activator p300 and with the BMP-4 responsive Smad1. On the other hand, xXBP1 also binds to the inhibitory Smads, Smad6 and Smad7, that can act as transcriptional co-repressors. Based on these data, we conclude that xXBP1 might function as an inhibitor of mesodermal and neural tissue formation by acting either as transcriptional activator or as repressor. This dual activity depends upon binding of co-factors being involved in the formation of distinct transcription complexes.

Alternative Splicing↗
PubMed2004

Tracking mesoderm formation and specification to the hemangioblast in vitro.

During development of the mouse embryo, blood cells are generated from mesodermal precursors at specific times and locations. Using various in vivo and in vitro systems, we are now starting to understand the cascade of molecular events leading to the commitment of mesoderm and the formation of the first blood precursors, the hemangioblast. The in vitro differentiation of embryonic stem (ES) cells has proved to be an invaluable model to study lineage commitment, because one can access and easily manipulate large quantities of early progenitor cells. To help us track mesodermal subpopulations and study their specification toward blood lineages, we have engineered an ES cell line that expresses the green fluorescence protein exclusively in the mesoderm germ layer, under the control of the Brachyury regulatory sequences.

Animals↗
PubMed2004

Inhibition of mesodermal fate by Xenopus HNF3beta/FoxA2.

The winged-helix transcription factor HNF3beta/FoxA2 is expressed in embryonic organizing centers of the gastrulating mouse, frog, fish, and chick. In the mouse, HNF3beta is required for the formation of the mammalian node and notochord, and can induce ectopic floor plate formation when misexpressed in the developing neural tube; HNF3beta expression in the extraembryonic endoderm is also necessary for the proper morphogenesis of the mammalian primitive streak. In the frog Xenopus laevis, several lines of evidence suggest that the related winged-helix factor Pintallavis functions as the ortholog of mammalian HNF3beta in both axial mesoderm and neurectoderm; the role of Xenopus HNF3beta itself, however, has not been clearly defined, and is the subject of this study. HNF3beta is widely expressed in the vegetal pole but, as previously suggested, is excluded from the gastrula-stage mesoderm. We find that expression of an HNF3beta-Engrailed repressor fusion protein induces ectopic axes and inhibits head formation in Xenopus embryos, while ectopic HNF3beta inhibits mesoderm and anterior endoderm formation in explant assays and in vivo. Our studies suggest that HNF3beta target genes function to limit the extent of mesoderm formation in the Xenopus gastrula, and point to related roles for Xenopus HNF3beta and the extraembryonic component of mammalian HNF3beta during vertebrate gastrulation.

Animals↗