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M Bronner-Fraser

Publications and source records attributed to M Bronner-Fraser.

At least 55 records · Page 3Linked to original sources
PubMed1998

Effects of Shh and Noggin on neural crest formation demonstrate that BMP is required in the neural tube but not ectoderm.

To define the timing of neural crest formation, we challenged the fate of presumptive neural crest cells by grafting notochords, Sonic Hedgehog- (Shh) or Noggin-secreting cells at different stages of neurulation in chick embryos. Notochords or Shh-secreting cells are able to prevent neural crest formation at open neural plate levels, as assayed by DiI-labeling and expression of the transcription factor, Slug, suggesting that neural crest cells are not committed to their fate at this time. In contrast, the BMP signaling antagonist, Noggin, does not repress neural crest formation at the open neural plate stage, but does so if injected into the lumen of the closing neural tube. The period of Noggin sensitivity corresponds to the time when BMPs are expressed in the dorsal neural tube but are down-regulated in the non-neural ectoderm. To confirm the timing of neural crest formation, Shh or Noggin were added to neural folds at defined times in culture. Shh inhibits neural crest production at early stages (0-5 hours in culture), whereas Noggin exerts an effect on neural crest production only later (5-10 hours in culture). Our results suggest three phases of neurulation that relate to neural crest formation: (1) an initial BMP-independent phase that can be prevented by Shh-mediated signals from the notochord; (2) an intermediate BMP-dependent phase around the time of neural tube closure, when BMP-4 is expressed in the dorsal neural tube; and (3) a later pre-migratory phase which is refractory to exogenous Shh and Noggin.

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PubMed1997

The origins of the neural crest. Part II: an evolutionary perspective.

The neural crest and cranial ectodermal placodes are traditionally thought to be unique to vertebrates; however, they must have had evolutionary precursors. Here, we review recent evidence suggesting that such ancestral cell types can be identified in modern non-vertebrate chordates, such as amphioxus (a cephalochordate) and ascidians (urochordates). Hence, migratory neuroectodermal cells may well have been present in the common ancestor of the chordates, such that the possibility of their existence in non-chordate deuterostomes (hemichordates and echinoderms) must also be considered. Finally, we discuss the various non-neuronal cell types produced by the neural crest in order to demonstrate that it is plausible that these different cell types evolved from an ancestral population that was neuronal in nature.

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PubMed1997

The origins of the neural crest. Part I: embryonic induction.

Neural crest cells form at the border between the neural plate and the epidermis. The tissue interactions that underlie neural crest cell induction have been investigated primarily by heterotopic grafting experiments in vivo and by conjugating different tissues in vitro. Three models have been proposed to explain the induction of neural crest cells at the neural plate border, i.e. (1) the influence of signals from the mesoderm, (2) changes in ectodermal competence and (3) local interactions between neural and non-neural ectoderm. The weight of the evidence supports the last model, although there are data that suggest a role for signals from the mesoderm. FGFs seem to be necessary but not sufficient for neural crest cell induction. BMP-4 is sufficient to induce neural crest cells from chick neural explants in vitro and intermediate levels of BMP-4-signalling induce neural crest cell markers in Xenopus animal cap assays. These data suggest a gradient model in which neural crest cells are induced by a particular range of BMP-4 activity, although a single-signal model may be too simplistic. Neural crest cell induction may be an ongoing process, in which an initial induction at the neural plate border is followed by further induction within the dorsal neural tube.

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PubMed1997

Differentiation of the vertebrate neural tube.

The vertebrate nervous system arises through a series of inductive interactions, beginning with the induction of the neural plate and the rostrocaudal patterning of the neural tube. The process continues with dorsoventral patterning of the neural tube, during which floor plate cells and motor neurons are induced ventrally by interactions of the neural tube with the notochord, and dorsal cell types are induced via neural plate/ectodermal interactions. Later interactions result in the formation of interneurons as well as neuronal migrations. Recent progress, guided in part by knowledge of evolutionary conservation of transcription factors and signaling pathways, is beginning to reveal the cellular and molecular bases of each of these steps in neuronal patterning.

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PubMed1997

Dorsal hindbrain ablation results in rerouting of neural crest migration and changes in gene expression, but normal hyoid development.

Our previous studies have shown that hindbrain neural tube cells can regulate to form neural crest cells for a limited time after neural fold removal (Scherson, T., Serbedzija, G., Fraser, S. E. and Bronner-Fraser, M. (1993). Development 188, 1049-1061; Sechrist, J., Nieto, M. A., Zamanian, R. T. and Bronner-Fraser, M. (1995). Development 121, 4103-4115). In the present study, we ablated the dorsal hindbrain at later stages to examine possible alterations in migratory behavior and/or gene expression in neural crest populations rostral and caudal to the operated region. The results were compared with those obtained by misdirecting neural crest cells via rhombomere rotation. Following surgical ablation of dorsal r5 and r6 prior to the 10 somite stage, r4 neural crest cells migrate along normal pathways toward the second branchial arch. Similarly, r7 neural crest cells migrate primarily to the fourth branchial arch. When analogous ablations are performed at the 10-12 somite stage, however, a marked increase in the numbers of DiI/Hoxa-3-positive cells from r7 are observed within the third branchial arch. In addition, some DiI-labeled r4 cells migrate into the depleted hindbrain region and the third branchial arch. During their migration, a subset of these r4 cells up-regulate Hoxa-3, a transcript they do not normally express. Krox20 transcript levels were augmented after ablation in a population of neural crest cells migrating from r4, caudal r3 and rostral r3. Long-term survivors of bilateral ablations possess normal neural crest-derived cartilage of the hyoid complex, suggesting that misrouted r4 and r7 cells contribute to cranial derivatives appropriate for their new location. In contrast, misdirecting of the neural crest by rostrocaudal rotation of r4 through r6 results in a reduction of Hoxa-3 expression in the third branchial arch and corresponding deficits in third arch-derived structures of the hyoid apparatus. These results demonstrate that neural crest/tube progenitors in the hindbrain can compensate by altering migratory trajectories and patterns of gene expression when the adjacent neural crest is removed, but fail to compensate appropriately when the existing neural crest is misrouted by neural tube rotation.

Age Factors↗
PubMed1997

Early- and late-migrating cranial neural crest cell populations have equivalent developmental potential in vivo.

We present the first in vivo study of the long-term fate and potential of early-migrating and late-migrating mesencephalic neural crest cell populations, by performing isochronic and heterochronic quail-to-chick grafts. Both early- and late-migrating populations form melanocytes, neurons, glia, cartilage and bone in isochronic, isotopic chimeras, showing that neither population is lineage-restricted. The early-migrating population distributes both dorsally and ventrally during normal development, while the late-migrating population is confined dorsally and forms much less cartilage and bone. When the late-migrating population is substituted heterochronically for the early-migrating population, it contributes extensively to ventral derivatives such as jaw cartilage and bone. Conversely, when the early-migrating population is substituted heterochronically for the late-migrating population, it no longer contributes to the jaw skeleton and only forms dorsal derivatives. When the late-migrating population is grafted into a late-stage host whose neural crest had previously been ablated, it migrates ventrally into the jaws. Thus, the dorsal fate restriction of the late-migrating mesencephalic neural crest cell population in normal development is due to the presence of earlier-migrating neural crest cells, rather than to any change in the environment or to any intrinsic difference in migratory ability or potential between early- and late-migrating cell populations. These results highlight the plasticity of the neural crest and show that its fate is determined primarily by the environment.

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PubMed1997

Coordinate actions of BMPs, Wnts, Shh and noggin mediate patterning of the dorsal somite.

Shortly after their formation, somites of vertebrate embryos differentiate along the dorsoventral axis into sclerotome, myotome and dermomyotome. The dermomyotome is then patterned along its mediolateral axis into medial, central and lateral compartments, which contain progenitors of epaxial muscle, dermis and hypaxial muscle, respectively. Here, we used Wnt-11 as a molecular marker for the medial compartment of dermomyotome (the 'medial lip') to demonstrate that BMP in the dorsal neural tube indirectly induces formation of the medial lip by up-regulating Wnt-1 and Wnt-3a (but not Wnt-4) expression in the neural tube. Noggin in the dorsal somite may inhibit the direct action of BMP on this tissue. Wnt-11 induction is antagonized by Sonic Hedgehog, secreted by the notochord and the floor plate. Together, our results show that the coordinated actions of the dorsal neural tube (via BMP and Wnts), the ventral neural tube/notochord (via Shh) and the somite itself (via noggin) mediates patterning of the dorsal compartment of the somite.

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PubMed1997

Neural tube-ectoderm interactions are required for trigeminal placode formation.

Cranial sensory ganglia in vertebrates develop from the ectodermal placodes, the neural crest, or both. Although much is known about the neural crest contribution to cranial ganglia, relatively little is known about how placode cells form, invaginate and migrate to their targets. Here, we identify Pax-3 as a molecular marker for placode cells that contribute to the ophthalmic branch of the trigeminal ganglion and use it, in conjunction with DiI labeling of the surface ectoderm, to analyze some of the mechanisms underlying placode development. Pax-3 expression in the ophthalmic placode is observed as early as the 4-somite stage in a narrow band of ectoderm contiguous to the midbrain neural folds. Its expression broadens to a patch of ectoderm adjacent to the midbrain and the rostral hindbrain at the 8- to 10-somite stage. Invagination of the first Pax-3-positive cells begins at the 13-somite stage. Placodal invagination continues through the 35-somite stage, by which time condensation of the trigeminal ganglion has begun. To challenge the normal tissue interactions leading to placode formation, we ablated the cranial neural crest cells or implanted barriers between the neural tube and the ectoderm. Our results demonstrate that, although the presence of neural crest cells is not mandatory for Pax-3 expression in the forming placode, a diffusible signal from the neuroectoderm is required for induction and/or maintenance of the ophthalmic placode.

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PubMed1996

Inhibition of cranial neural crest adhesion in vitro and migration in vivo using integrin antisense oligonucleotides.

Although it is well-established that beta1 integrins play a functional role in the migration of cranial neural crest cells, little is known about the number or importance of their associated alpha subunits. Here, we have utilized antisense oligonucleotides (aONs) against various mammalian integrin alpha subunits to functionally "knock out" integrins in vitro and in vivo. First, we examined the attachment in vitro of cranial neural crest cells to fibronectin and laminin in the presence of antisense or reversed-sense oligonucleotides using a quantitative adhesion assay. We found three alpha integrin aONs that blocked attachment to fibronectin substrates only, one that blocked attachment to laminin substrates only, and one that blocked attachment to both fibronectin and laminin. As expected, an aON to chick beta1 integrin reduced attachment to both fibronectin and laminin substrates. These results suggest that there are three or more functionally distinct integrin heterodimers on avian cranial neural crest cells. Second, we examined the ability of aONs against various alpha integrin subunits to perturb cranial neural crest migration in vivo by injecting the oligonucleotides into the cranial mesenchyme through which neural crest cells migrate. Those alpha aONs that inhibited cell attachment in vitro also caused neural crest and/or neural tube abnormalities after injection in vivo. In addition, two aONs that had no effect in vitro did affect emigration of neural crest cells in vivo. Immunoprecipitations revealed that some integrin subunits were depleted after treatment with antisense but not reversed-sense oligonucleotides both in vivo and in vitro. The results suggest that integrin alpha subunits are required for cranial neural crest cell attachment and emigration.

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PubMed1996

The genesis of avian neural crest cells: a classic embryonic induction.

Neural crest cells arise from the ectoderm and are first recognizable as discrete cells in the chicken embryo when they emerge from the neural tube. Despite the classical view that neural crest precursors are a distinct population lying between epidermis and neuroepithelium, our results demonstrate that they are not a segregated population. Cell lineage analyses have demonstrated that individual precursor cells within the neural folds can give rise to epidermal, neural crest, and neural tube derivatives. Interactions between the neural plate and epidermis can generate neural crest cells, since juxtaposition of these tissues at early stages results in the formation of neural crest cells at the interface. Inductive interactions between the epidermis and neural plate can also result in "dorsalization" of the neural plate, as assayed by the expression of the Wnt transcripts characteristic of the dorsal neural tube. The competence of the neural plate changes with time, however, such that interaction of early neural plate with epidermis generates only neural crest cells, whereas interaction of slightly older neural plate with epidermis generates neural crest cells and Wnt-expressing cells. At cranial levels, neuroepithelial cells can regulate to generate neural crest cells when the endogenous neural folds are removed, probably via interaction of the remaining neural tube with the epidermis. Taken together, these experiments demonstrate that: (i) progenitor cells in the neural folds are multipotent, having the ability to form multiple ectodermal derivatives, including epidermal, neural crest, and neural tube cells; (ii) the neural crest is an induced population that arises by interactions between the neural plate and the epidermis; and (iii) the competence of the neural plate to respond to inductive interactions changes as a function of embryonic age.

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PubMed1996

Age-dependent inhibition of neural crest migration by the notochord correlates with alterations in the S103L chondroitin sulfate proteoglycan.

In avian embryos, the notochord inhibits neural crest migration, resulting in the absence of neural crest cells from the perinotochordal space. Here, we test whether temporal changes in the ability of the notochord to inhibit neural crest migration correlate with alterations in the S1O3L chondroitin sulfate proteoglycan (CSPG). Because CSPGs are abundant in the perinotochordal space and the inhibitory effects of the notochord are chondroitinase sensitive both in vivo and in vitro, we examined the distribution and biochemical nature of a large CSPG whose core protein is recognized by the S103L antibody. The S103L CSPG is specific to the perinotochordal space during the course of neural crest migration and codistributes with the HNK-1 carbohydrate. Biochemical characterization reveals that the S103L CSPG bears the HNK-1 epitope and is the only HNK-l immunoreactive proteoglycan present around the notochord at these stages. Following neural crest migration, the S103L CSPG staining is maintained in the perinotochordal region and also is expressed later in cartilage. In 4-day-old embryos, however, the S103L CSPG undergoes a reduction of HNK-1 immunoreactivity. To examine the temporal nature of the notochord's inhibitory ability, we assayed the effects, on neural crest migration of grafting notochords from 2- to 5-day-old donor quail embryos into 2-day-old host chick embryos. Donor notochords from 2- to 3-day-old embryos inhibit neural crest cell migration, whereas the degree of inhibition is reduced or absent when notochords are derived from > or = 4-day-old donors. This suggests that older notochords lose their inhibitory ability. Interestingly, preincubation of younger notochords with the HNK-1 antibody blocks the inhibitory effect, suggesting that glycosylation of the perinotochordal matrix may be important. The time when the notochord loses its inhibitory ability as assessed by our in vivo grafting assay correlates with the biochemical and immunocytochemical changes in the notochordal S103L antigen. These data suggest that a species of S103L CSPG, which is expressed by the early notochord and bears the HNK-1 epitope, may be important for the inhibition of neural crest migration.

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PubMed1996

Digit induction by Hensen's node and notochord involves the expression of shh but not RAR-beta 2.

It is well established that Hensen's nodes can induce the formation of supernumerary digits after grafting into the anterior margin of the developing limb bud. The recent finding that distinct mesodermal cell populations are segregated within the node has made it possible to isolate different prospective cell types in an attempt to correlate digit-inducing ability with cell fate. We find that the prospective notochord cells contained within Hensen's node are able to induce supernumerary digits, whereas presumptive somite cells cannot. This early difference in inducing ability persists into later stages of development: epithelial somites are unable to induce while notochord from all lengths of the neuraxis continues to induce. Using probes to retinoic acid receptor-beta 2 and sonic hedgehog (shh) we find no evidence to support the idea that inducing tissues generate extra digits by releasing retinoic acid into adjacent limb tissue but find that the inducing ability of a tissue correlates with its expression of shh.

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PubMed1996

Expression of the avian alpha 7-integrin in developing nervous system and myotome.

Integrins are cell surface receptors for a variety of extracellular matrix molecules including fibronectin, laminin and collagens. Although their role in development is not completely understood, they are likely to have important functions in cell migration and axon guidance. To characterize the types of integrins expressed in the developing nervous system, we have used monoclonal antibodies against alpha 7- and alpha v-integrin subunits to examine the distribution of these subunits in the early chick embryo. Low levels of alpha 7 immunoreactivity were first observed in the neural tube and developing myotome of stage 17 embryos (E2.5). Although low levels of alpha 7 expression were associated with most neuroepithelial cells, distinct alpha 7 immunoreactivity was first detected in the ventrolateral portions of the neural tube at a stage corresponding to the time when the first neurons differentiate. Its distribution pattern overlapped with that of commissural neurons in the developing spinal cord. alpha 7 was also prominently localized to the motor neurons and their axons emanating from the neural tube. In addition, alpha 7 immunoreactivity was observed on a subpopulation of trunk neural crest cells migrating through the somitic sclerotome. At later stages, alpha 7 expression was observed in other nervous system structures such as the pigmented retinal epithelial cells. In addition to its distribution in the developing nervous system, alpha 7 immunoreactivity was associated with early myotomal cells shortly after myotome formation and its expression persisted throughout myotome development. In contrast to alpha 7, alpha v-integrin had a limited distribution in the nervous system, being expressed only at low levels in the neural tube. However, alpha v displayed prominent immunoreactivity in the myotome and in endothelial cells of the dorsal aorta. The results suggest that alpha 7-integrin is one of the prevalent integrin subunits on neurons and axons in the developing spinal cord.

Animals↗
PubMed1996

Inhibitory effects of PG-H/aggrecan and PG-M/versican on avian neural crest cell migration.

Aggrecans and PG-M/versicans represent two newly defined families of hyaluronan-binding proteoglycans for which the function is still poorly understood. Using the avian neural crest as a model system, we have examined the molecular mechanisms entailed in the cell-proteoglycan interaction during embryonic cell motility. Both the primary cartilage aggrecan of the avian embryo (PG-H/aggrecan) and the largest variant of the avian mesenchymal versican (PG-M/versican VO) failed to support neural crest cell adhesion and migration when topographically immobilized onto the substrate. Conversely, solely the PG-H/aggrecan, and similar aggrecans from other species, counteracted the migration-promoting effect of a number of matrix molecules lacking proteoglycan affinity. This inhibitory effect was not reproduced by the isolated glycosaminoglycan chains, the isolated core protein, the reduced and alkylated macromolecule, or the aggrecan in which the G1 hyaluronan-binding domain had been inactivated with hyaluronan fragments or antibodies. Limited depolymerization of the side chains and preincubation of the PG-H/aggrecan with anti-glycosaminoglycan antibodies differentially reduced the inhibitory activity of the proteoglycan on cell motility. The results demonstrate a diverse inhibitory effect of aggrecans and PG-M/versicans on embryonic cell movement and show that the inhibitory action of aggrecans is independent of substrate binding, is dependent on a G1 domain-mediated association of the intact proteoglycan with cell surface-bound hyaluronan, and is differentially mediated by its glycosaminoglycan side chains.

Aggrecans↗
PubMed1996

Rhombomere of origin determines autonomous versus environmentally regulated expression of Hoxa-3 in the avian embryo.

We have investigated the pattern and regulation of Hoxa3 expression in the hindbrain and associated neural crest cells in the chick embryo, using whole mount in situ hybridization in conjunction with DiI labeling of neural crest cells and microsurgical manipulations. Hoxa3 is expressed in the neural plate and later in the neural tube with a rostral border of expression corresponding to the boundary between rhombomeres (r) 4 and 5. Initial expression is diffuse and becomes sharp after boundary formation. Hoxa3 exhibits uniform expression within r5 after formation of rhombomeric borders. Cell marking experiments reveal that neural crest cells migrating caudally, but not rostrally, from r5 and caudally from r6 express Hoxa3 in normal embryo. Results from transposition experiments demonstrate that expression of Hoxa3 in r5 neural crest cells is not strictly cell-autonomous. When r5 is transposed with r4 by rostrocaudal rotation of the rhomobomeres, Hoxa3 is expressed in cells migrating lateral to transposed r5 and for a short time, in condensing ganglia, but not by neural crest within the second branchial arch. Since DiI-labeled cells from transposed r5 are present in the second arch, Hoxa3-expressing neural crest cells from r5 appear to down-regulate their Hoxa3 expression in their new environment. In contrast, when r6 is transposed to the position of r4 after boundary formation, Hoxa3 is maintained in both migrating neural crest cells and those positioned within the second branchial arch and associated ganglia. These results suggest that Hoxa3 expression is cell-autonomous in r6 and its associated neural crest. Our results suggest that neural crest cells expressing the same Hox gene are not eqivalent; they respond differently to environmental signals and exhibit distinct degrees of cell autonomy depending upon their rhombomere of origin.

Animals↗
PubMed1995

Relationship between spatially restricted Krox-20 gene expression in branchial neural crest and segmentation in the chick embryo hindbrain.

Previous studies have suggested that the rostrocaudal patterning of branchial arches in the vertebrate embryo derives from a coordinate segmental specification of gene expression in rhombomeres (r) and neural crest. However, expression of the Krox-20 gene is restricted to neural crest cells migrating to the third branchial arch, apparently from r5, whereas this rhombomere contributes cells to both the second and third arches. We examined in the chick embryo how this spatially restricted expression is established. Expression occurs in precursors in both r5 and r6, and we show by cell labelling that both rhombomeres contribute to Krox-20-expressing neural crest, emigration occurring first from r6 and later caudally from r5. Krox-20 transcripts are not detected in some precursors in rostral r5, presaging the lack of expression in cells migrating rostrally from this rhombomere. After transposition of r6 to the position of r4 or r5, many Krox-20-expressing cells migrate rostral to the otic vesicle, whereas when r5 is transplanted to the position of r4, only a small number of migrating cells express Krox-20. These results indicate that, in the chick, Krox-20 expression in branchial neural crest does not correlate with rhombomeric segmentation, and that there may be intrinsic differences in regulation between the r5 and r6 Krox-20-expressing populations.

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PubMed1995

Origin of the avian neural crest.

Neural crest cells are derived from a population of multipotent stem cells within the neural tube. They emerge shortly after neural tube closure, migrate extensively in the embryo and localize in numerous sites, where they differentiate into neurons and glia of the peripheral nervous system, cartilage and bone of the face, melanocytes and various other cell types. This review summarizes recent experiments from our laboratory delineating the origin and lineage of avian neural crest cells. Neural crest cells arise from the ectoderm, which also gives rise to presumptive epidermal, placodal and neural tube cells. Fate mapping experiments have demonstrated that the neural crest arises at the juncture between presumptive epidermis and the neural plate. Inductive interactions between these two early tissues can generate neural crest cells, suggesting that signals travel through the epidermis to generate neural crest cells prior to neural tube closure. Injection of lineage tracer into individual cells reveals that a single neural fold can form all ectodermal derivatives (i.e., epidermis, neural tube, neural crest). Even after neural tube closure, neuroepithelial cells have the capacity to form multiple neural crest and neural tube derivatives, including both dorsal and ventral phenotypes, suggesting that neural tube and neural crest cells share a common precursor. Further evidence that neural crest and neural tube cells are intimately related comes from experiments in which the cranial neural folds are ablated. The remaining neural tube cells have the capacity to regulate, at least for a limited time, to compensate for missing neural crest cells. These experiments suggest that the early neuroepithelium has no clear segregation with respect to the neural tube or neural crest. With time, dorsalizing and ventralizing signals may cause neural tube cells to acquire specific cell fates.

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