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C D Stern

Publications and source records attributed to C D Stern.

At least 37 records · Page 2Linked to original sources
PubMed1999

Neural induction. A bird's eye view.

Since the discovery of the phenomenon of neural induction by Spemann and Mangold in 1924, considerable effort has been invested in identifying the signals produced by the organizer that are responsible for diverting the fate of cells from epidermal to neural. Substantial progress has been made only recently by the finding in amphibians that BMP4 is a neural inhibitor and epidermal inducer, and that endogenous antagonists of BMPs are secreted by the organizer. However, recent results in the chick point to the existence of other, upstream events required before BMP inhibition stabilizes neural fates. Here we take a critical view of the evidence for and against the view that BMP inhibition is a sufficient trigger for neural induction in different vertebrates.

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PubMed1999

Establishment and maintenance of the border of the neural plate in the chick: involvement of FGF and BMP activity.

We have investigated the cell interactions and signalling molecules involved in setting up and maintaining the border between the neural plate and the adjacent non-neural ectoderm in the chick embryo at primitive streak stages. msx-1, a target of BMP signalling, is expressed in this border at a very early stage. It is induced by FGF and by signals from the organizer, Hensen's node. The node also induces a ring of BMP-4, some distance away. By the early neurula stage, the edge of the neural plate is the only major site of BMP-4 and msx-1 expression, and is also the only site that responds to BMP inhibition or overexpression. At this time, the neural plate appears to have a low level of BMP antagonist activity. Using in vivo grafts and in vitro assays, we show that the position of the border is further maintained by interactions between non-neural and neural ectoderm. We conclude that the border develops by integration of signals from the organizer, the developing neural plate, the paraxial mesoderm and the non-neural epiblast, involving FGFs, BMPs and their inhibitors. We suggest that BMPs act in an autocrine way to maintain the border state.

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PubMed1999

SWiP-1: novel SOCS box containing WD-protein regulated by signalling centres and by Shh during development.

We describe a novel chick WD-protein, cSWiP-1, expressed in somitic mesoderm and developing limb buds as well as in other embryonic structures where Hedgehog signalling has been shown to play a role. Using embryonic manipulations we show that in somites cSWiP-1 expression integrates two signals originating from structures adjacent to the segmental mesoderm: a positive signal from the notochord and a negative signal from intermediate and/or lateral mesoderm. In explant cultures of somitic mesoderm, Shh protein induces cSWiP-1, while a blocking antibody to Shh inhibits the induction of cSWiP-1 by the notochord. These results show that the positive signal from the notochord is mediated by Shh. We also show that in limb buds cSWiP-1 is upregulated by ectopic Shh. This occurs in about the same time period as upregulation of BMP2, placing cSWiP-1 among the earliest markers for the change of limb pattern caused by ectopic Shh. We also describe a human homologue of cSWiP-1 and a mouse gene, mSWiP-2, that is more distantly related to SWiP-1, suggesting that SWiP-1 belongs to a novel subfamily of WD-proteins.

Amino Acid Sequence↗
PubMed1999

Mesoderm patterning and somite formation during node regression: differential effects of chordin and noggin.

In Xenopus, one of the properties defining Spemann's organizer is its ability to dorsalise the mesoderm. When placed ajacent to prospective lateral/ventral mesoderm (blood, mesenchyme), the organizer causes these cells to adopt a more axial/dorsal fate (muscle). It seems likely that a similar property patterns the primitive streak of higher vertebrate embryos, but this has not yet been demonstrated clearly. Using quail/chick chimaeras and a panel of molecular markers, we show that Hensen's node (the amniote organizer) can induce posterior primitive streak (prospective lateral plate) to form somites (but not notochord) at the early neurula stage. We tested two BMP antagonists, noggin and chordin (both of which are expressed in the organizer), for their ability to generate somites and intermediate mesoderm from posterior streak, and find that noggin, but not chordin, can do this. Conversely, earlier in development, chordin can induce an ectopic primitive streak much more effectively than noggin, while neither BMP antagonist can induce neural tissue from extraembryonic epiblast. Neurulation is accompanied by regression of the node, which brings the prospective somite territory into a region expressing BMP-2, -4 and -7. One function of noggin at this stage may be to protect the prospective somite cells from the inhibitory action of BMPs. Our results suggest that the two BMP antagonists, noggin and chordin, may serve different functions during early stages of amniote development.

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PubMed1999

Gata2 and Gata3: novel markers for early embryonic polarity and for non-neural ectoderm in the chick embryo.

We have investigated in detail the expression patterns of two Gata genes, cGata2 and cGata3, during early chick development. In addition to confirming previously described expression of these two genes in developing brain, kidney and blood islands, this study reveals several important novel expression domains during very early stages of development. cGata2 is expressed in the area opaca in pre-primitive streak stages, forming a gradient along the A-P axis (strongest anteriorly). Both genes are expressed strongly in the entire non-neural ectoderm from stage 4+, and neither is expressed in prospective neural plate at any stage. Unlike other previously described non-neural markers, neither gene is expressed in the dorsal neural tube. We also describe dynamic expression of cGata2 and cGata3 during eye, ear and gut development.

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PubMed1999

Axial mesendoderm refines rostrocaudal pattern in the chick nervous system.

There has long been controversy concerning the role of the axial mesoderm in the induction and rostrocaudal patterning of the vertebrate nervous system. Here we investigate the neural inducing and regionalising properties of defined rostrocaudal regions of head process/prospective notochord in the chick embryo by juxtaposing these tissues with extraembryonic epiblast or neural plate explants. We localise neural inducing signals to the emerging head process and using a large panel of region-specific neural markers, show that different rostrocaudal levels of the head process derived from headfold stage embryos can induce discrete regions of the central nervous system. However, we also find that rostral and caudal head process do not induce expression of any of these molecular markers in explants of the neural plate. During normal development the head process emerges beneath previously induced neural plate, which we show has already acquired some rostrocaudal character. Our findings therefore indicate that discrete regions of axial mesendoderm at headfold stages are not normally responsible for the establishment of rostrocaudal pattern in the neural plate. Strikingly however, we do find that caudal head process inhibits expression of rostral genes in neural plate explants. These findings indicate that despite the ability to induce specific rostrocaudal regions of the CNS de novo, signals provided by the discrete regions of axial mesendoderm do not appear to establish regional differences, but rather refine the rostrocaudal character of overlying neuroepithelium.

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PubMed1998

Clocked gene expression in somite formation.

A recent paper describes a striking expression pattern during somite formation for a chick ortholog of the fly hairy gene. Before segmentation, c-hairy1 mRNA oscillates in the presomitic mesoderm such that three distinct spatial patterns are seen. The authors use a series of ingenious manipulations to show that these phases follow each other in time, adding up to a 90-minute periodicity in c-hairy1 expression. The discovery of this clock of gene expression emphasizes the importance of temporally regulated events in the establishment of spatial patterns.

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PubMed1998

Induction of primitive streak and Hensen's node by the posterior marginal zone in the early chick embryo.

In the preprimitive streak chick embryo, the search for a region capable of inducing the organizer, equivalent to the Nieuwkoop Center of the amphibian embryo, has focused on Koller's sickle, the hypoblast and the posterior marginal zone. However, no clear evidence for induction of an organizer without contribution from the inducing tissue has been provided for any of these structures. We have used DiI/DiO labeling to establish the fate of midline cells in and around Koller's sickle in the normal embryo. In the epiblast, the boundary between cells that contribute to the streak and those that do not lies at the posterior edge of Koller's sickle, except at stage X when it lies slightly more posteriorly in the epiblast. Hypoblast and endoblast (a second lower layer formed under the streak) have distinct origins in the lower layer, and goosecoid expression distinguishes between them. We then used anterior halves of chick prestreak embryos as recipients for grafts of quail posterior marginal zone; quail cells can be identified subsequently with a quail-specific antibody. Anterior halves alone usually formed a streak, most often from the posterior edge. Quail posterior marginal zones without Koller's sickle were grafted to the anterior side of anterior halves. These grafts were able to increase significantly the frequency of streaks arising from the anterior pole of stage X-XI anterior halves without contributing to the streak or node. Stage XII anterior halves no longer responded. A goosecoid-expressing hypoblast did not form under the induced streak, indicating that it is not required for streak formation. We conclude that the marginal zone posterior to Koller's sickle can induce a streak and node, without contributing cells to the induced streak.

Animals↗
PubMed1998

Chordin regulates primitive streak development and the stability of induced neural cells, but is not sufficient for neural induction in the chick embryo.

We have investigated the role of Bone Morphogenetic Protein 4 (BMP-4) and a BMP antagonist, chordin, in primitive streak formation and neural induction in amniote embryos. We show that both BMP-4 and chordin are expressed before primitive streak formation, and that BMP-4 expression is downregulated as the streak starts to form. When BMP-4 is misexpressed in the posterior area pellucida, primitive streak formation is inhibited. Misexpression of BMP-4 also arrests further development of Hensen's node and axial structures. In contrast, misexpression of chordin in the anterior area pellucida generates an ectopic primitive streak that expresses mesoderm and organizer markers. We also provide evidence that chordin is not sufficient to induce neural tissue in the chick. Misexpression of chordin in regions outside the future neural plate does not induce the early neural markers L5, Sox-3 or Sox-2. Furthermore, neither BMP-4 nor BMP-7 interfere with neural induction when misexpressed in the presumptive neural plate before or after primitive streak formation. However, chordin can stabilise the expression of early neural markers in cells that have already received neural inducing signals. These results suggest that the regulation of BMP signalling by chordin plays a role in primitive streak formation and that chordin is not sufficient to induce neural tissue.

Amino Acid Sequence↗
PubMed1997

Competence for neural induction: HGF/SF, HGF1/MSP and the c-Met receptor.

During the early stages of development of the vertebrate embryo, the nervous system is induced by a special region, Hensen's node (or 'organizer'), situated at the tip of the primitive streak during gastrulation. Neural induction is finely regulated both by the timing of inducing signals emitted by the organizer and by temporal and spatial changes in the responsiveness of the ectoderm ('competence'). Here we review the evidence that the glycoprotein L5-220 is a marker for competent cells, that it is involved directly in the response to neural inducing signals, and that its expression, as well as competence itself, are maintained and enhanced by HGF/SF. HGF/SF is expressed in the node itself, suggesting that the organizer maintains the responsiveness of neighbouring regions to inducing signals. We also speculate that HGF1/MSP may play a role in setting up the initial competent region at earlier stages of development.

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PubMed1997

Goosecoid misexpression alters the morphology and Hox gene expression of the developing chick limb bud.

The homeobox-containing gene goosecoid (gsc) has been implicated in a variety of embryonic processes from gastrulation to rib patterning. We have analyzed the role it plays during chick limb development. Expression is initially observed at stage 20 in a proximal-anterior-ventral domain of the early limb bud which expands during subsequent stages. Later in limb development a second domain of expression appears distally which resolves to regions which surround the condensing cartilage. In order to understand the function of gsc in limb development, we have examined the effect of misexpressing gsc throughout the limb. Two striking phenotypes are observed. The first, evident at stage 24, is an alteration in the angle of femur outgrowth from the main body axis. The second, which can be detected at day 10 of development, is an overall decrease in the size of the limb with bones that are small, misshapen and bent. These phenotypes correlate with a decrease in levels of Hox gene expression in gsc-infected limb buds. From these results we suggest that gsc may normally function to regulate growth and patterning of the limb, perhaps through regulation of Hox gene expression.

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PubMed1997

The prechordal region lacks neural inducing ability, but can confer anterior character to more posterior neuroepithelium.

The avian equivalent of Spemann's organizer, Hensen's node, begins to lose its ability to induce a nervous system from area opaca epiblast cells at stage 4+, immediately after the full primitive streak stage. From this stage, the node is no longer able to induce regions of the nervous system anterior to the hindbrain. Stage 4+ is marked by the emergence from the node of a group of cells, the prechordal mesendoderm. Here we have investigated whether the prechordal region possesses the lost functions of the organizer, using quail-chick chimaeras to distinguish graft- and host-derived cells, together with several region-specific molecular markers. We find that the prechordal region does not have neural inducing ability, as it is unable to divert extraembryonic epiblast cells to a neural fate. However, it can confer more anterior character to prospective hindbrain cells of the host, making them acquire expression of the forebrain markers tailless and Otx-2. It can also rescue the expression of Krox-20 and Otx-2 from nervous system induced by an older (stage 5) node in extraembryonic epiblast. We show that these properties reflect a true change of fate of cells rather than recruitment from other regions. The competence of neuroectoderm to respond to anteriorizing signals declines by stages 7-9, but both posteriorizing signals and the ability of neuroectoderm to respond to them persist after this stage.

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PubMed1997

Misexpression of chick Vg1 in the marginal zone induces primitive streak formation.

In the chick embryo, the primitive streak is the first axial structure to develop. The initiation of primitive streak formation in the posterior area pellucida is influenced by the adjacent posterior marginal zone (PMZ). We show here that chick Vg1 (cVg1), a member of the TGFbeta family of signalling molecules whose homolog in Xenopus is implicated in mesoderm induction, is expressed in the PMZ of prestreak embryos. Ectopic expression of cVg1 protein in the marginal zone chick blastoderms directs the formation of a secondary primitive streak, which subsequently develops into an ectopic embryo. We have used cell marking techniques to show that cells that contribute to the ectopic primitive streak change fate, acquiring two distinct properties of primitive streak cells, defined by gene expression and cell movements. Furthermore, naive epiblast explants exposed to cVg1 protein in vitro acquire axial mesodermal properties. Together, these results show that cVg1 can mediate ectopic axis formation in the chick by inducing new cell fates and they permit the analysis of distinct events that occur during primitive streak formation.

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PubMed1997

Preventing the loss of competence for neural induction: HGF/SF, L5 and Sox-2.

The response to neural induction depends on the presence of inducing signals and on the state of competence of the responding tissue. The epiblast of the chick embryo loses its ability to respond to neural induction by the organizer (Hensen's node) between stages 4 and 4+. We find that the pattern of expression of the L5(220) antigen closely mirrors the changes in competence of the epiblast in time and in space. For the first time, we describe an experiment that can extend the period of neural competence: when L5(220) expression is maintained beyond its normal time by implanting HGF/SF secreting cells, the competence to respond to Hensen's node grafts is retained. The host epiblast forms a non-regionalized neural tube, which expresses the pan-neural marker SOX-2 (a Sry-related transcription factor) but not any region-specific markers for the forebrain, hindbrain or spinal cord. Although HGF/SF secreting cells can mimic signals from Hensen's node that maintain L5 expression, they cannot rescue the ability of the node to induce anterior structures (which is normally lost after stage 4). The ectoderm may acquire stable neural characteristics during neural induction by going through a hierarchy of states: competence, neuralization and regionalization. Our findings allow us to start to define these different states at a molecular level, and show that the competence to respond to neural induction is not entirely autonomous to the responding cells, but can be regulated by extracellular signalling molecules.

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