Avian neural crest cell fate decisions: a diffusible signal mediates induction of neural crest by the ectoderm.

During neurulation, a region of central ectoderm becomes thickened to form the neural plate which then folds upon itself to generate the neural tube, from which all neurons and glia cells of the central nervous system arise. Neural crest cells form at the border of the neural plate, where it abuts the prospective epidermis. The neural crest is a transient population of cells that undergo an epithelial-mesenchymal transition, become highly migratory and subsequently differentiate into most of the peripheral nervous systems as well as numerous other derivatives. The origin of neural crest cells at the epidermal-neural plate border suggests that an interaction between these two tissues may be involved in neural crest formation. By experimentally juxtaposing prospective epidermis with naive neural plate, we previously showed that an inductive interaction between these tissues can generate neural crest cells. Here, we further characterize the nature of this inductive interaction by co-culturing isolated neural plate and prospective epidermis on opposing sides of polycarbonate filters with differing pore sizes. We find that neural crest cells are generated even when epidermis and neural plate are separated by filters that do not allow cell contact. These results suggest that the epidermal inducer is a diffusible, secreted molecule. We discuss the developmental potential of neural crest precursors and lineage decisions that effect their differentiation into numerous derivatives.

Timing and competence of neural crest formation.

Neural crest cells can be induced by an interaction between neural plate and ectoderm. To clarify the timing and nature of these inductive interactions, we have examined the time of competence of the neural plate to become neural crest as well as the time of neural fold specification. The neural plate is competent to respond to inductive interactions with the nonneural ectoderm for a limited period, rapidly losing its responsive ability after stage 10. In contrast, nonneural ectoderm from numerous stages retains the ability to induce neural crest cells from competent neural plate. When neural folds are explanted to test their ability to produce neural crest without further tissue interactions, we find that folds derived from all rostrocaudal levels of the open neural plate are already specified to express the neural crest marker Slug. However, additional signals may be required for maintenance of Slug expression, since the transcript is later down-regulated in vitro in the absence of tissue interactions. Taken together, these results suggest that there are multiple stages of neural crest induction. The earliest induction must have occurred by the end of gastrulation, since the newly formed neural fold population is already specified to form neural crest. However, isolated neural folds eventually down-regulate Slug, suggesting a second phase that maintains neural crest formation. Thus, induction of the neural crest may involve multiple and sustained tissue interactions.

Analysis of cranial neural crest migratory pathways in axolotl using cell markers and transplantation.

We have examined the ability of normal and heterotopically transplanted neural crest cells to migrate along cranial neural crest pathways in the axolotl using focal DiI injections and in situ hybridization with the neural crest marker, AP-2. DiI labeling demonstrates that cranial neural crest cells migrate as distinct streams along prescribed pathways to populate the maxillary and mandibular processes of the first branchial arch, the hyoid arch and gill arches 1-4, following migratory pathways similar to those observed in other vertebrates. Another neural crest marker, the transcription factor AP-2, is expressed by premigratory neural crest cells within the neural folds and migrating neural crest cells en route to and within the branchial arches. Rotations of the cranial neural folds suggest that premigratory neural crest cells are not committed to a specific branchial arch fate, but can compensate when displaced short distances from their targets by migrating to a new target arch. In contrast, when cells are displaced far from their original location, they appear unable to respond appropriately to their new milieu such that they fail to migrate or appear to migrate randomly. When trunk neural folds are grafted heterotopically into the head, trunk neural crest cells migrate in a highly disorganized fashion and fail to follow normal cranial neural crest pathways. Importantly, we find incorporation of some trunk cells into branchial arch cartilage despite the random nature of their migration. This is the first demonstration that trunk neural crest cells can form cartilage when transplanted to the head. Our results indicate that, although cranial and trunk neural crest cells have inherent differences in ability to recognize migratory pathways, trunk neural crest can differentiate into cranial cartilage when given proper instructive cues.

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.

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.

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.

Dorsalization of the neural tube by the non-neural ectoderm.

The patterning of cell types along the dorsoventral axis of the spinal cord requires a complex set of inductive signals. While the chordamesoderm is a well-known source of ventralizing signals, relatively little is known about the cues that induce dorsal cell types, including neural crest. Here, we demonstrate that juxtaposition of the non-neural and neural ectoderm is sufficient to induce the expression of dorsal markers, Wnt-1, Wnt-3a and Slug, as well as the formation of neural crest cells. In addition, the competence of neural plate to express Wnt-1 and Wnt-3a appears to be stage dependent, occurring only when neural tissue is taken from stage 8-10 embryos but not from stage 4 embryos, regardless of the age of the non-neural ectoderm. In contrast to the induction of Wnt gene expression, neural crest cell formation and Slug expression can be induced when either stage 4 or stage 8-10 neural plates are placed in contact with the non-neural ectoderm. These data suggest that the non-neural ectoderm provides a signal (or signals) that specifies dorsal cell types within the neural tube, and that the response is dependent on the competence of the neural tissue.

Neural induction and regionalisation by different subpopulations of cells in Hensen's node.

Cell lineage analysis has revealed that the amniote organizer, Hensen's node, is subdivided into distinct regions, each containing a characteristic subpopulation of cells with defined fates. Here, we address the question of whether the inducing and regionalising ability of Hensen's node is associated with a specific subpopulation. Quail explants from Hensen's node are grafted into an extraembryonic site in a host chick embryo allowing host- and donor-derived cells to be distinguished. Cell-type- and region-specific markers are used to assess the fates of the mesodermal and neural cells that develop. We find that neural inducing ability is localised in the epiblast layer and the mesendoderm (deep portion) of the medial sector of the node. The deep portion of the posterolateral part of the node does not have neural inducing ability. Neural induction also correlates with the presence of particular prospective cell types in our grafts: chordamesoderm (notochord/head process), definitive (gut) endoderm or neural tissue. However, only grafts that include the epiblast layer of the node induce neural tissue expressing a complete range of anteroposterior characteristics, although prospective prechordal plate cells may also play a role in specification of the forebrain.

Origins of the avian neural crest: the role of neural plate-epidermal interactions.

We have investigated the lineage and tissue interactions that result in avian neural crest cell formation from the ectoderm. Presumptive neural plate was grafted adjacent to non-neural ectoderm in whole embryo culture to examine the role of tissue interactions in ontogeny of the neural crest. Our results show that juxtaposition of non-neural ectoderm and presumptive neural plate induces the formation of neural crest cells. Quail/chick recombinations demonstrate that both the prospective neural plate and the prospective epidermis can contribute to the neural crest. When similar neural plate/epidermal confrontations are performed in tissue culture to look at the formation of neural crest derivatives, juxtaposition of epidermis with either early (stages 4-5) or later (stages 6-10) neural plate results in the generation of both melanocytes and sympathoadrenal cells. Interestingly, neural plates isolated from early stages form no neural crest cells, whereas those isolated later give rise to melanocytes but not crest-derived sympathoadrenal cells. Single cell lineage analysis was performed to determine the time at which the neural crest lineage diverges from the epidermal lineage and to elucidate the timing of neural plate/epidermis interactions during normal development. Our results from stage 8 to 10+ embryos show that the neural plate/neural crest lineage segregates from the epidermis around the time of neural tube closure, suggesting that neural induction is still underway at open neural plate stages.

Origins of neural crest cell diversity.

The neural crest is a population of migratory cells, arising from the ectoderm, that invades many sites within the embryo and differentiate into a variety of diverse cell types. Pigment cells, most cells of the peripheral nervous system, adrenal medullary cells, and some cranial cartilage are derived from the neural crest. Despite a wealth of knowledge concerning their pathways of migration and vast array of derivatives, little is known about the formation of neural crest cells or their acquisition of positional identity. This review focuses on the origin of neural crest cells from the ectoderm and the generation of differences in neural crest cell fates along the rostrocaudal axis. In addition, we consider the role of temporal restriction in the developmental potential of premigratory neural crest cells. While evidence for the existence of multipotent stem cells is strong, some experiments also suggest that there may be heterogeneity among neural crest cell precursors, perhaps due to differences in origin, that might explain commitment events occurring early in neural crest development.

Relationships between mesoderm induction and the embryonic axes in chick and frog embryos.

The hypoblast is generally thought to be responsible for inducing the mesoderm in the chick embryo because the primitive streak, and subsequently the embryonic axis, form according to the orientation of the hypoblast. However, some cells become specified as embryonic mesoderm very late in development, towards the end of the gastrulation period and long after the hypoblast has left the embryonic region. We argue that induction of embryonic mesoderm and of the embryonic axis are different and separable events, both in amniotes and in amphibians. We also consider the relationships between the dorsoventral and anteroposterior axes in both groups of vertebrates.

Fate mapping and cell lineage analysis of Hensen's node in the chick embryo.

Fate maps of chick Hensen's node were generated using DiI and the lineage of individual cells studied by intracellular injection of lysine-rhodamine-dextran (LRD). The cell types contained within the node are organized both spatially and temporally. At the definitive primitive streak stage (Hamburger and Hamilton stage 4), Hensen's node contains presumptive notochord cells mainly in its anterior midline and presumptive somite cells in more lateral regions. Early in development it also contains presumptive endoderm cells. At all stages studied (stages 3-9), some individual cells contribute progeny to more than one of these tissues. The somitic precursors in Hensen's node only contribute to the medial halves of the somites. The lateral halves of the somites are derived from a separate region in the primitive streak, caudal to Hensen's node.