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PubMed1989

Notochordal induction of cell wedging in the chick neural plate and its role in neural tube formation.

Cells in the median hinge point (MHP) of the bending chick neural plate are tightly apposed to the underlying notochord. These cells differ from those in adjacent lateral neuroepithelial areas (L) in that MHP cells are short and mainly wedge-shaped and line a furrow, whereas L cells are tall and mainly spindle-shaped and do not line a furrow. Cell generation time also differs in these regions. These consistent differences are detectable only after the notochord has formed and established contact with the neural plate; it is unclear whether they result from self-differentiation or induction. Two experiments were performed to evaluate the hypothesis that MHP characteristics develop owing to inductive interactions between the notochord and overlying neuroepithelial cells. First, notochordless chick embryos were generated to determine whether midline neuroepithelial cells still developed typical MHP characteristics. In the absence of the notochord, such characteristics did not develop. Second, isolated segments of quail notochord were transplanted subjacent to L of chick hosts to ascertain whether the notochord is capable of inducing MHP characteristics in L cells. When transplanted notochordal segments established apposition with host L cells, the apposing L cells usually developed typical MHP characteristics. Collectively, these results provide strong evidence that the notochord plays an inductive role in the formation of MHP characteristics. This investigation further revealed that bending can occur in the absence of MHP characteristics, forming a neural tube with an abnormal morphology. Thus, the formation of such characteristics, particularly cell wedging, is not required for bending but plays a major role in generating the normal cross-sectional morphology of the neural tube.

Animals↗
PubMed1992

Expansion of surface epithelium provides the major extrinsic force for bending of the neural plate.

Neurulation, formation of the neural tube, requires both intrinsic forces (i.e., those generated within the neural plate) and extrinsic forces (i.e., those generated outside the neural plate in adjacent tissues), but the precise origin of these forces is unclear. In this study, we addressed the question of which tissue produces the major extrinsic force driving bending of the neural plate. We have previously shown that 1) extrinsic forces are required for bending and 2) such forces are generated lateral to the neural plate. Three tissues flank the neural plate prior to its bending: surface epithelium, mesoderm, and endoderm. In the present study, we removed two of these layers, namely, the endoderm and mesoderm, underlying and lateral to the neural plate; bending still occurred, often with complete formation of a neural tube, although the latter usually rotated toward the side of tissue depletion. These results suggest that the surface epithelium, the only tissue remaining after microsurgery, provides the major extrinsic force for bending of the neural plate and that the mesoderm (and perhaps endoderm) stabilizes the neuraxis, maintaining its proper orientation and position on the midline.

Animals↗
PubMed2000

A region of the vertebrate neural plate in which neighbouring cells can adopt neural or epidermal fates.

Cells in the neurogenic region of the fly, Drosophila melanogaster, become either neural stem cells or epidermis and the selection of the former requires the activity of the proneural genes [1]. In contrast, it is commonly thought that all cells in the vertebrate neural plate contribute to the neural tube and that consequently there is no need for the selection of individual neural precursors (e.g., [2]). Here we present a detailed fate map of the chick caudal neural plate (CNP), a cell population that generates the posterior hindbrain and spinal cord. We show that this is a unique region of the neural plate where neighbouring cells can contribute to neural tube or epidermis. Further, neural tube precursors leave the CNP in an approximate rostro-caudal order and give rise to discrete portions of the neural tube where they or their progeny behave as neural stem cells [3]. Our data suggest that neural and epidermal cell fates are acquired on a cell-by-cell basis within the CNP and thus in a manner strikingly similar to that in the fly. Indeed, the assignment of neural cell fate in this region may prove to be mediated by the functional homologue of the fly proneural genes (chick achaete-scute homologue 4, cash4), which is expressed heterogeneously within this cell population [4].

Animals↗
PubMed1983

Studies on the development of melanophores in in vitro cultured amphibian neural plates.

Various parts of neural plates of Japanese newt (Cynops pyrrhogaster) neurula embryos were cultured alone in drops of culture media (Niu-Twitty's balanced salt solution or modified Leibovitz L-15 medium) with or without fetal calf serum (FCS). Although none of the parts gave rise to melanophores in a medium without FCS, some produced melanophores in a medium with FCS. The localization of melanophore-producing areas in the neural plates corresponded to that of Tada's (1944) findings. The assumption that FCS affects survival and development of melanophores is excluded, because neural fold cells do not require FCS to develop into melanophores. Therefore, there may be in FCS some factor which acts on the specialization of neural plate cells into melanophores. The results of this experiment suggest that this factor may be heat labile. The findings also indicate that FCS does not induce melanophores in gastrula ectoderm, but only affects neurula neural plate cells so as to give rise to melanophores.

Animals↗
PubMed1995

Cooperative model of epithelial shaping and bending during avian neurulation: autonomous movements of the neural plate, autonomous movements of the epidermis, and interactions in the neural plate/epidermis transition zone.

Morphogenetic movements during neurulation cause a tissue to change shape within the plane of the epithelium (e.g., conversion of the oval neural plate into the narrow spinal plate and the wide brain plate), cause bending out of the plane of the epithelium (e.g., raise the neural folds and curl the neural plate into a tube), or contribute to both phenomena. In this study, pieces that contain neural plate alone, epidermis alone, or both tissues (with or without underlying tissues) are cut from chick embryos and allowed to develop for up to 24 hr. Examination of histological sections through such isolates allows analysis of the formation of neural folds. When the neural plate/epidermis transition zone is disrupted, neural folds do not form. Conversely, when the transition zone remains intact, neural folds form. Neural folds form even when most of the medial neural plate and lateral epidermis has been removed, leaving only the isolated transition zone. These data indicate that the transition zone is both necessary and sufficient for the formation of neural folds. The transition zone may play a number of roles in epithelial bending including organizing, focussing, and redirecting movements that are autonomous to the neural plate or epidermis. Time-lapse video recording, and sequential photographs allowed the documentation of such movements. Neural plate isolates exhibit autonomous rostrocaudal lengthening and mediolateral narrowing. Isolated strips of epidermis exhibit autonomous movements which, unlike wound-healing movements, are unidirectional (medial), and region-specific (beginning and reaching their greatest extent in the cranial region). Isolated pieces of neural plate or epidermis remain flat instead of bending, providing further evidence that the transition zone is necessary for the formation of neural folds.

Animals↗
PubMed2000

Zebrafish Dkk1, induced by the pre-MBT Wnt signaling, is secreted from the prechordal plate and patterns the anterior neural plate.

mRNA injection into the ventral blastomeres of Xenopus embryos of mRNA encoding Wnt pathway genes induces a secondary axis with complete head structures. To identify target genes of the pre-MBT dorsalization pathway that might be responsible for head formation in zebrafish, we have cloned zebrafish dickkopf1 (dkk1), which is expressed in tissues implicated in head patterning. We found that dkk1 blocks the post-MBT Wnt signaling and dkk1 is a target of the pre-MBT Wnt signaling. Dkk1 overexpression in the prechordal plate suggests that Dkk1, secreted from the prechordal plate, expands the forebrain at the expense of the midbrain in the anterior neural plate. Furthermore, dkk1 acts in parallel to the homeobox gene bozozok and bozozok is required for the maintenance of dkk1 expression. The nodal gene squint is also required for the maintenance of dkk1 expression. Among the mutually dependent target genes of the pre-MBT Wnt signaling, dkk1 plays an important role in patterning the anterior head of zebrafish.

Amino Acid Sequence↗
PubMed2006

The presumptive floor plate (notoplate) induces behaviors associated with convergent extension in medial but not lateral neural plate cells of Xenopus.

In previous work (Elul, T., Keller, R., 2000. Monopolar protrusive activity: a new morphogenic cell behavior in the neural plate dependent on vertical interactions with the mesoderm in Xenopus. Dev. Biol. 224, 3-19; Ezin, A.M., Skoglund, P. Keller, R. 2003. The midline (notochord and notoplate) patterns the cell motility underlying convergence and extension of the Xenopus neural plate. Dev. Biol. 256, 100-114), the midline tissues of notochord and overlying notoplate were found to induce the monopolar, medially directed protrusive activity of deep neural cells. This behavior is thought to drive the mediolateral intercalation and convergent extension of the neural plate in Xenopus. Here we address the issue of whether the notochord, the notoplate, or both is essential for this induction. Our strategy was to remove the notochord, leaving the overlying notoplate intact, and determine whether it alone can induce the monopolar, medially directed cell behavior. We first establish that the notoplate (presumptive floor plate), when separated from the underlying notochord in the early neurula (stages 13-14), will independently mature into a floor plate as assayed three criteria: (1) continued expression of an early marker, sonic hedgehog, and a later, marker, F-spondin; (2) the display of the notoplate/floor plate-specific randomly oriented protrusive activity; (3) the characteristic lack of mixing of cells between the notoplate and lateral neural plate. Under these conditions, in the presence of a mature notoplate/floor plate and in the absence of the notochord, the characteristic monopolar, medially directed behavior occurred, but only locally near the midline. These results show that the notoplate/floor plate capacity to induce the medially directed motility is limited in range, and they suggest that the notochord is necessary for the normally observed longer range induction in lateral neural plate cells. This work helps to further the understanding of molecular and tissue interactions required for convergent extension.

Animals↗
PubMed1984

Differentiation and transformation of neural plate cells.

The developmental potential of presumptive neural plate cells of prestreak chick embryos (stage 1) and neural plate cells from definitive streak chick embryos (stage 4) has been examined in cell culture using specific markers that identify the major cell types in the vertebrate central nervous system. The prestreak presumptive neural plate (PSPNP) cells, stage 1, assume an epithelial appearance in vitro and synthesize cellular fibronectin, but do not express markers for the neuronal, astrocytic, melanocytic, or oligodendrocytic lineages. Conversely, definitive-streak neural plate (DSNP) cells contain precursors which express cell-type-specific markers for terminally differentiated neurons, astrocytes, and melanocytes, and synthesize an extracellular matrix of cellular fibronectin. Differentiation of DSNP cells in vitro can be prevented by infection with a temperature-sensitive mutant of Rous sarcoma virus (RSV), tsNY68. Differentiation of DSNP cell transformants can be resumed by a temperature shift to the nonpermissive temperature. The morphological and biochemical changes associated with tsNY68 transformation are accompanied by alterations in pp60src kinase activity in the transformed cells.

Animals↗
PubMed1992

Morphological and mapping studies of the paranodal and postnodal levels of the neural plate during chick neurulation.

The morphology of the paranodal and postnodal levels of the neural plate as well as the fate of its cells was examined in chick embryos at stages 3-11. The morphology of the paranodal and postnodal levels of the neural plate closely resembles that of the prenodal neural plate. Furthermore, during shaping and bending of the neural plate, these levels undergo changes similar to those of the prenodal level. In short, the paranodal and postnodal levels of the neural plate consist of a pseudostratified columnar epithelium that thickens dorsoventrally and narrows mediolaterally and then undergoes localized furrowing and folding. Fate mapping revealed that at mid-neurula stages, the prospective hindbrain and spinal cord levels of the neuraxis flank the primitive streak. Hensen's node moves caudally with respect to these future neuraxial levels as it regresses during the latter stages of gastrulation. Cells of the medullary cord, the rudiment of the secondary portion of the neural tube, arise in the vicinity of the cranial portion of the primitive streak, near the caudal end of the postnodal levels of the neural plate. Thus, during stages of gastrulation and primary neurulation, the precursor cells of the primary and secondary portions of the neural tube (spinal cord) lie in close proximity to one another. This study provides new information on the morphology and extent of the paranodal and postnodal levels of the neural plate, the changes these areas undergo during shaping and bending of the neural plate, and the contributions of its cells to the primary and secondary levels of the neural tube, increasing our understanding of the complex events underlying avian gastrulation and neurulation.

Animals↗
PubMed1987

The mouse neural plate as starting material for studying neuronal differentiation in vitro.

Tissue from the mouse neural plate and neural tube was studied, by light and electron microscopy, as starting material for tissue culture. In vivo, up to embryonic day 9 (E 9, stage Th 14; Theiler 1972) all neuroepithelial cells of the neural plate were mitotically active. As judged from their light microscopic or ultrastructural appearance, they could hardly be distinguished from one another or from neuroepithelial cells of more mature embryos. The earliest few immature neurons in the mesencephalic anlage were discernible on day 9 1/2 (stage Th 15) in the prospective intermediate layer of the neural tube, concomitantly with the development of processes containing neurotubules and vesicles which were oriented in parallel to the basal lamina. For tissue culture, explants of the mesencephalic anlage of embryonic days 8 (Th 12/13), 9 1/2 (Th 15), and 11 (Th 18) were kept in vitro and their development was compared with each other and with the corresponding developmental stage in vivo in the initial phase of culture (e.g., E 8, day of explanation, kept in vitro for 2 days, E 10 in vivo being the stage for comparison). The study demonstrated that further in vitro development proceeded in an accelerated manner, independent of the developmental stage of the embryo from which the tissue was explanted. In vitro, proliferation of the explanted neuronal progenitor cells stopped in all explants within 24 h of culture as revealed by autoradiographic and electron microscopic techniques. Cytoplasmic transformation was observed corresponding to that found in vivo, but always greatly accelerated. Earliest axons had formed after 24 h in vitro; synapses with clear vesicles and dense core vesicles were observed after at least 3 days in culture in all explants regardless of age at the time of explantation (E 8 or E 11). The present ultrastructural results indicate that prospective neurons within the neuroepithelium of the neural plate and early neural tube were immediately able to develop into neurons without the complete sequence of mitotic events normally occurring under in vivo conditions.

Animals↗
PubMed2003

Dlx proteins position the neural plate border and determine adjacent cell fates.

The lateral border of the neural plate is a major source of signals that induce primary neurons, neural crest cells and cranial placodes as well as provide patterning cues to mesodermal structures such as somites and heart. Whereas secreted BMP, FGF and Wnt proteins influence the differentiation of neural and non-neural ectoderm, we show here that members of the Dlx family of transcription factors position the border between neural and non-neural ectoderm and are required for the specification of adjacent cell fates. Inhibition of endogenous Dlx activity in Xenopus embryos with an EnR-Dlx homeodomain fusion protein expands the neural plate into non-neural ectoderm tissue whereas ectopic activation of Dlx target genes inhibits neural plate differentiation. Importantly, the stereotypic pattern of border cell fates in the adjacent ectoderm is re-established only under conditions where the expanded neural plate abuts Dlx-positive non-neural ectoderm. Experiments in which presumptive neural plate was grafted to ventral ectoderm reiterate induction of neural crest and placodal lineages and also demonstrate that Dlx activity is required in non-neural ectoderm for the production of signals needed for induction of these cells. We propose that Dlx proteins regulate intercellular signaling across the interface between neural and non-neural ectoderm that is critical for inducing and patterning adjacent cell fates.

Animals↗
PubMed1989

Neural fold formation at newly created boundaries between neural plate and epidermis in the axolotl.

According to a recent model, the cortical tractor model, neural fold and neural crest formation occurs at the boundary between neural plate and epidermis because random cell movements become organized at this site. If this is correct, then a fold should form at any boundary between epidermis and neural plate. To test that proposition, we created new boundaries in axolotl embryos by juxtaposing pieces of neural plate and epidermis that would not normally participate in fold formation. These boundaries were examined superficially and histologically for the presence of folds, permitting the following observations. Folds form at each newly created boundary, and as many folds form as there are boundaries. When two folds meet they fuse into a hollow "tube" of neural tissue covered by epidermis. Sections reveal that these ectopic folds and "tubes" are morphologically similar to their natural counterparts. Transplanting neural plate into epidermis produces nodules of neural tissue with central lumens and peripheral nerve fibers, and transplanting epidermis into neural plate causes the neural tube and the dorsal fin to bifurcate in the region of the graft. Tissue transplanted homotypically as a control integrates into the host tissue without forming folds. When tissue from a pigmented embryo is transplanted into an albino host, the presence of pigment allows the donor cells to be distinguished from those of the host. Mesenchymal cells and melanocytes originating from neural plate transplants indicate that neural crest cells form at these new boundaries. Thus, any boundary between neural plate and epidermis denotes the site of a neural fold, and the behavior of cells at this boundary appears to help fold the epithelium. Since folds can form in ectopic locations on an embryo, local interactions rather than classical neural induction appear to be responsible for the formation of neural folds and neural crest.

Ambystoma mexicanum↗
PubMed1997

Epidermal ectoderm is required for full elevation and for convergence during bending of the avian neural plate.

Previous studies suggest that bending of the neural plate requires the juxtaposition of neural plate and non-neuroepithelial tissues. The current study examines the role of one of these tissues, the epidermal ectoderm, in bending. Chick blastoderms were harvested from fertile eggs incubated for 24 hr and cultured dorsal-side-up on agar-albumen substrates. In one experiment, a rectangular flap of epidermal ectoderm on one side of each blastoderm was separated from underlying layers and gently reflected onto the area opaca; a fragment of tungsten wire was placed on top of the flap to hold it down and to prevent healing. Embryos were then allowed to develop in a humidified incubator for 2-18 hr. Asymmetric neurulation was observed between the operated and control sides as early as 2 hr after surgery. The amount of asymmetry was quantified in serial transverse sections from embryos collected 8 hr after surgery. Elevation of the lateral edge of the neural plate on the operated side averaged one half to two thirds of that on the control side, and convergence of the operated side around the dorsolateral hinge point toward the dorsal midline did not occur. These results demonstrate that epidermal ectoderm is required for full elevation and for convergence during bending. In another experiment, lateral epidermal ectoderm was removed, leaving only a medial strip consisting of both the epidermal component of the future neural fold and flanking future epidermis. This experiment revealed that although epidermal ectoderm is necessary for full elevation and for convergence of the neural folds, a medial strip of epidermal ectoderm is sufficient to drive bending. Collectively, these results further support the idea that neurulation is a multifactorial process driven by both intrinsic and extrinsic factors acting in concert.

Animals↗
PubMed1999

Neurotrophin-3 increases neurite outgrowth and apoptosis in explants of the chicken neural plate.

This study examined the effect of neurotrophin-3 (NT-3) on neurite outgrowth and apoptosis of chicken neural plate explants prior to neural tube formation. In situ hybridization revealed that mRNA for the full-length (catalytic) NT-3 receptor, TrkC, was present in, and limited to, the neural plate (including the neural folds) coincident with its formation. Neural plate explants were maintained in vitro on a collagen gel under serum-free conditions in the presence or absence of exogenous NT-3 and/or an antibody to NT-3. In the absence of exogenous NT-3, explants exhibited neurite outgrowth after several days in vitro; apoptotic cells were also seen after 2 days in vitro. This does not appear to be due to endogenous NT-3, since the total number of neurites or apoptotic cells was unchanged if explants were exposed to an NT-3 antibody for the entire culture period. In the presence of exogenous NT-3, neural plate explants exhibited a dose-dependent statistically significant increase in the total number of neurites as compared to explants maintained under control conditions, as well as a statistically significant increase in apoptosis. These NT-3 effects were blocked by an NT-3 antibody. In contrast, NT-3 had no effect on the length of neurites. These findings suggest that NT-3 may play a role during early neural development in vivo.

Animals↗
PubMed2003

The midline (notochord and notoplate) patterns the cell motility underlying convergence and extension of the Xenopus neural plate.

We investigated the role of the dorsal midline structures, the notochord and notoplate, in patterning the cell motilities that underlie convergent extension of the Xenopus neural plate. In explants of deep neural plate with underlying dorsal mesoderm, lateral neural plate cells show a monopolar, medially directed protrusive activity. In contrast, neural plate explants lacking the underlying dorsal mesoderm show a bipolar, mediolaterally directed protrusive activity. Here, we report that "midlineless" explants consisting of the deep neural plate and underlying somitic mesoderm, but lacking a midline, show bipolar, mediolaterally oriented protrusive activity. Adding an ectopic midline to the lateral edge of these explants restores the monopolar protrusive activity over the entire extent of the midlineless explant. Monopolarized cells near the ectopic midline orient toward it, whereas those located near the original, removed midline orient toward this midline. This behavior can be explained by two signals emanating from the midline. We postulate that one signal polarizes neural plate deep cells and is labile and short-lived and that the second signal orients any polarized cells toward the midline and is persistent.

Animals↗
PubMed1987

Ventricular cells from the mouse neural plate, stage Theiler 12, transform into different neuronal cell classes in vitro.

The rostral parts of the cephalic neural plate and neural crest of mice, stage Theiler 12, were prepared and cultured. At that stage of development they exclusively consist of proliferative ventricular cells, which do not yet display vimentin and neurofilament immunoreactivity. 3H-thymidine autoradiography showed that the progenitor cells of neurons became postmitotic as soon as they were taken into culture. The neurofilament protein (kD 68) was immunocytochemically demonstrable from day 2 in culture, while immunoreactivity to vimentin was never observed. The neurons, prematurely developed from the neuroepithelium of stage Theiler 12-embryos, were identified by their histological and immunocytochemical properties. They gave distinct patterns of immunoreactivity to neuropeptides and anti-serotonin antibodies. Anti-serotonin and anti-somatostatin antibodies reacted from the 3rd day of culture. Antibodies against ACTH, luliberin, substance P and vasopressin gave positive reactions at day 7. Two classes of neurons, the serotonin and the large substance P-immunoreactive ones, were recognized by both immunoreactivity and morphology. The serotonin immunoreactive neurons usually were of a multipolar shape and had a long, varicose axon that was heavily stained, particularly at its distal third. The perikarya appeared in limited areas of the cultured tissue. They grew in the vicinity of each other, but never in densely packed aggregates. The large neurons, reacting heavily with antibodies against substance P and faintly with all the other neuropeptide antibodies applied, were up to 50 micron in diameter and usually occurred in 20-40 cells per preparation of half a neural plate. The results suggest that at least some classes of neurons can develop from the cultured neural plates of stage Th12.

Adrenocorticotropic Hormone↗
PubMed1994

Pattern formation in the vertebrate neural plate.

Recent advances have been made in the understanding of the cellular and molecular mechanisms involved in the formation and patterning of the neural plate of vertebrate embryos. Both planar and vertical signaling pathways appear to be involved in the neural induction and axial patterning of the neural plate. The neural plate, behaving as a developmental field, might be patterned by signals emanating from boundary regions: the organizer region and the midline and edges of the neural plate. Here, A. Ruiz i Altaba describes a possible model for anteroposterior patterning involving ;lanar signals for amphibian, avian and mammalian embryos, compares the axial patterning of the neural plate with the patterning of insect epithelia, and discussed possible roles of noggin, follistatin and hedgehog-related genes in neural induction and patterning.

Animals↗