The role of vitamin A in the development of the central nervous system

We describe here the defects that arise in the central nervous system (CNS) of quail embryos when they develop in the absence of vitamin A. It has been assumed that because of the effects of excess vitamin A and its metabolites, particularly retinoic acid (RA), on the CNS they are involved in various aspects of CNS development. We show that this is indeed the case, because these deficient quail embryos have three defects in their CNS. First, the posterior hindbrain fails to develop because the cells fated to form this part of the CNS in the very early embryo die by apoptosis. Second, the neural tube fails to extend neurites into the periphery both in vivo and in vitro. Third, the neural crest cells throughout the embryo die by apoptosis. These results demonstrate a crucial requirement for vitamin A in CNS development.

Paraxial-fated mesoderm is required for neural crest induction in Xenopus embryos

Neural crest induction is thought to occur by a two-step process. Axially fated mesoderm induces neural plate, which is then recruited to neural crest by nonneural epidermal ectoderm at the neural plate border. This model suggests a rather indirect role for mesoderm in inducing neural crest. We extensively examined the role of mesoderm in neural crest induction by determining which types of mesoderm induce neural crest cells in Xenopus embryos. We found that noggin-dorsalized ventral marginal zone (VMZ) explants differentiate as melanocytes in the absence of axial mesoderm. Dorsalized VMZ is also a potent inducer of melanocytes when juxtaposed to animal cap ectoderm in recombinant explants. Dorsalized VMZ is analogous to the dorsal-lateral marginal zone (DLMZ) region of the embryo. Neural crest-inducing activities of gastrula stage DLMZ and dorsal marginal zone (DMZ) were also compared in recombinant explants. DLMZ was a stronger inducer of neural crest than was DMZ; DLMZ induced high levels of XSlug expression and melanocyte formation in recombinants, whereas DMZ weakly induced neural crest. In whole embryos lacking DLMZ, XSlug expression and melanocyte formation were significantly reduced; in contrast, no significant reduction of XSlug expression or melanocyte formation was seen in embryos lacking a DMZ. These results suggest that paraxial-fated mesoderm plays a central role in neural crest formation by inducing a novel type of lateral neural plate. This lateral neural plate is then recruited to neural crest by adjacent nonneural epidermal ectoderm.

Sox10 mutation disrupts neural crest development in Dom Hirschsprung mouse model

Hirschsprung disease (HSCR, MIM #142623) is a multigenic neurocristopathy (neural crest disorder) characterized by absence of enteric ganglia in a variable portion of the distal colon. Subsets of HSCR individuals also present with neural crest-derived melanocyte deficiencies (Hirschsprung-Waardenburg, HSCR-WS, MIM #277580). Murine models have been instrumental in the identification and analysis of HSCR disease genes. These include mice with deficiencies of endothelin B receptor (Ednrb(s-l); refs 1,2) endothelin 3 (Edn3(ls): refs 1,3) the tyrosine kinase receptor cRet and glial-derived neurotrophic factor. Another mouse model of HSCR disease, Dom, arose spontaneously at the Jackson Laboratory. While Dom/+ heterozygous mice display regional deficiencies of neural crest-derived enteric ganglia in the distal colon, Dom/Dom homozygous animals are embryonic lethal. We have determined that premature termination of Sox10, a member of the SRY-like HMG box family of transcription factors, is responsible for absence of the neural crest derivatives in Dom mice. We demonstrate expression of Sox10 in normal neural crest cells, disrupted expression of both Sox10 and the HSCR disease gene Ednrb in Dom mutant embryos, and loss of neural crest derivatives due to apoptosis. Our studies suggest that Sox10 is essential for proper peripheral nervous system development. We propose SOX10 as a candidate disease gene for individuals with HSCR whose disease does not have an identified genetic origin.

A mouse mandibular culture model permits the study of neural crest cell migration and tooth development

A major issue in developmental biology is to determine how time and position-restricted instructions are signaled and received during morphogenesis of different phenotypes, of which tooth, Meckel's cartilage and tongue formation are classical examples. It is now evident that a hierarchy of growth factors and their downstream transcription factors regulate the timing, sequence and position of cells and tissues in forming different phenotypes during embryogenesis. Here we report the development of an early mandibular organ culture model. Explants of E8 and E9 first branchial arch were cultured and produced mandibular processes with cap stage tooth formation, Meckel's cartilage and tongue development. In tandem, vital dye (Dil) labeling studies confirmed that rhombomeres 1-4 give rise to craneal neural crest (CNC) cells which emigrate from the neural fold to the forming maxillary and mandibular arches. Furthermore, we have tested the feasibility of investigating the regulation of different phenotypes within the first branchial arch by a transcription factor using this early mandibular organ culture model. Lymphoid enhancing factor 1 (Lef1), a transcription factor, has been implicated to regulate tooth formation in vivo. We have analyzed the expression of Lef1 and studied the biological effects of Lef1 on E8 embryonic mouse first branchial arch explants in organ culture. Collectively, these results demonstrate that first branchial arch explant model is suitable for studies of rhombencephalic crest cell fate during mandibular morphogenesis and can be used as a model with direct access to investigate the molecular mechanism in regulating first branchial arch morphogenesis.

Hox group 3 paralogous genes act synergistically in the formation of somitic and neural crest-derived structures

Hox genes encode transcription factors that are used to regionalize the mammalian embryo. Analysis of mice carrying targeted mutations in individual and multiple Hox genes is beginning to reveal a complex network of interactions among these closely related genes which is responsible for directing the formation of spatially restricted tissues and structures. In this report we present an analysis of the genetic interactions between all members of the third paralogous group, Hoxa3, Hoxb3, and Hoxd3. Previous analysis has shown that although mice homozygous for loss-of-function mutations in either Hoxa3 or Hoxd3 have no defects in common, mice mutant for both genes demonstrate that these two genes strongly interact in a dosage-dependent manner. To complete the analysis of this paralogous gene family, mice with a targeted disruption of the Hoxb3 gene were generated. Homozygous mutants have minor defects at low penetrance in the formation of both the cervical vertebrae and the IXth cranial nerve. Analysis and comparison of all double-mutant combinations demonstrate that all three members of this paralogous group interact synergistically to affect the development of both neuronal and mesenchymal neural crest-derived structures, as well as somitic mesoderm-derived structures. Surprisingly, with respect to the formation of the cervical vertebrae, mice doubly mutant for Hoxa3 and Hoxd3 or Hoxb3 and Hoxd3 show an indistinguishable defect, loss of the entire atlas. This suggests that the identity of the specific Hox genes that are functional in a given region may not be as critical as the total number of Hox genes operating in that region.

A protein kinase inhibitor H-7 induces process extrusion in fetal rat thyroid C-cells in vitro

Calcitonin-producing cells (C-cells) are endocrine cells derived from the neural crest. We examined the effects of three types of protein kinase inhibitors on the induction of neuronal phenotypes in the rat thyroid C-cells in vitro. In a primary culture of 16-day-old fetal rat thyroid glands, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H-7, 25-75 microM) induced both process extrusion and expression of highly polysialylated neural cell adhesion molecule (NCAM) in the C-cells. These effects of H-7 were completely prevented by okadaic acid, a potent protein phosphatase inhibitor. In contrast to H-7, selective inhibitors for cyclic nucleotide-dependent protein kinases such as N-(2-guanidinoethyl)-5-isoquinolinesulfonamide hydrochloride (HA1004, 25-200 microM) and N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide (H-89, 0.25-20 microM) failed to induce process extrusion or the expression of highly polysialylated NCAM in fetal rat C-cells. In cultured C-cells of adult origin, H-7 failed to induce marked process elongation or the expression of highly polysialylated NCAM. These results suggest that the morphological plasticity of the fetal C-cells depends upon the degree of phosphorylation of some proteins, and that the plasticity of adult C-cells are more restricted than that of fetal origin.

Mice lacking the ski proto-oncogene have defects in neurulation, craniofacial, patterning, and skeletal muscle development

The c-ski proto-oncogene has been implicated in the control of cell growth and skeletal muscle differentiation. To determine its normal functions in vivo, we have disrupted the mouse c-ski gene. Our results show a novel role for ski in the morphogenesis of craniofacial structures and the central nervous system, and confirm its proposed function as a player in skeletal muscle development. Homozygous mutant mice show perinatal lethality resulting from exencephaly, a defect caused by failed closure of the cranial neural tube during neurulation. The timing of the neural tube defect in ski -/- embryos coincides with excessive apoptosis in the cranial neuroepithelium, as well as in the cranial mesenchyme. Homozygous ski mutants also exhibit a dramatic reduction in skeletal muscle mass, consistent with a defect in expansion of a myogenic precursor population. Nestin is an intermediate filament expressed in highly proliferative neuroepithelial stem cells and in myogenic precursors. Interestingly, we find decreased nestin expression in both the cranial neural tube and the somites of ski -/- embryos, compared with their normal littermates, but no reduction of nestin in the caudal neural tube. These results are consistent with a model in which ski activities are required for the successful expansion of a subset of precursors in the neuroepithelial or skeletal muscle lineages.

Absence of neural crest cell regeneration from the postotic neural tube

The preotic neural tube has a variable ability for regeneration of neural crest depending on the neuraxial level. There is robust regeneration of neural crest in the caudal midbrain/rostral hindbrain. In contrast, removal of the cardiac neural crest results in cardiovascular abnormalities suggesting the lack of regeneration in this area, although the regenerative capacity of the cardiac crest region has never been tested directly. Premigratory cardiac neural crest was ablated bilaterally using laser irradiation or extirpation by tungsten needle, and the remaining ventral neural tube was labeled with DiI to examine any neural crest regeneration from the neural tube. The results indicate that there is very little regeneration of crest cells from the cardiac region of the neural tube if the ablation is done prior to the 5-somite stage and no regeneration after the 6-somite stage with either ablation procedure. Furthermore no compensatory response occurs from the adjacent regions of the neural crest. By contrast, we were able to confirm that regeneration of neural crest occurs in the preotic rhombencephalic neural tube even after laser irradiation. An analysis in the trunk region suggests that the trunk neural tube is similar to the cardiac region in that it does not regenerate crest cells in the ventral migratory pathway after ablation. However, melanocytes generated cranial and caudal to the ablated region migrate radially and fill in the ablated region so that there is no interruption of the normal pigment pattern. This study indicates that even though there is a variable capacity for crest regeneration in the preotic neural tube, the postotic neural tube does not have such regenerative ability.

When neural crest and placodes collide: interactions between melanophores and the lateral lines that generate stripes in the salamander Ambystoma tigrinum tigrinum (Ambystomatidae)

A prominent element of the early larval pigment pattern in the salamander Ambystoma tigrinum tigrinum (family Ambystomatidae) is a horizontal stripe over the lateral surface of the myotomes where otherwise abundant, neural crest-derived melanophores are not found. This study examines the formation of this "melanophore-free region". When the trunk lateral lines were ablated (by removing cranial lateral line placodes), the melanophore-free region did not form; instead, melanophores populated the middle of the flank and the distribution of yellow, neural chest-derived zanthophores was perturbed. Time-lapse videomicrography demonstrated that during normal development, the melanophore-free region is established because melanophores retreat from the midbody lateral line primordium as it migrates caudally along the inner side of the epidermis. Melanophores do not repopulate the middle of the flank after primordium migration and heterochronic grafting experiments suggest that extracellular factors contribute to maintaining the melanophore-free region during these later stages. Finally, photographic series, microsurgical manipulations, electron microscopy, and staining for molecules of the extracellular matrix (peanut agglutinin-binding components, tenascin, chondroitin sulfate proteoglycans, fibronectin, laminin) suggest that several factors contribute to establishing and maintaining the melanophore-free region, including steric effects of the lateral lines, interactions between melanophores and xanthophores, lateral line-dependent alterations of the subepidermal basement membrane, and a general elaboration of the extracellular matrix. Lateral line effects on melanophores are inferred to be a shared, ancestral feature of pigment pattern development for the families Ambystomatidae and Salamandridae (D.M. Parichy, Dev. Biol. 174, 265-282. 1996). The results of this study thus provide insights into a phylogenetically primitive mechanism for stripe formation, and a context for interpreting evolutionary innovations in pattern-forming mechanisms.

Mutation of the endothelin-3 gene in the Waardenburg-Hirschsprung disease (Shah-Waardenburg syndrome)

Hirschsprung disease (HSCR) and Waardenburg sundrome (WS) are congenital malformations regarded as neurocristopathies since both disorders involve neural crest-derived cells. The WS-HSCR association (Shah-Waardenburg syndrome) is a rare autosomal recessive condition that occasionally has been ascribed to mutations of the endothelin-receptor B (EDNRB) gene. WS-HSCR mimicks the megacolon and white coat-spotting observed in Ednrb mouse mutants. Since mouse mutants for the EDNRB ligand, endothelin-3 (EDN3), displayed a similar phenotype, the EDN3 gene was regarded as an alternative candidate gene in WS-HSCR. Here, we report a homozygous substitution/deletion mutation of the EDN3 gene in a WS-HSCR patient. EDN3 thus becomes the third known gene (after RET and EDNRB) predisposing to HSCR, supporting the view that the endothelin-signaling pathways play a major role in the development of neural crests.

The role of neurotrophins in development of neural-crest cells that become sensory ganglia

A fundamental issue of neural-crest ontogeny is understanding how different types of cells are created at the right time and in the correct numbers. Sensory ganglia are among the many derivatives of the vertebrate neural crest. Their proper formation requires the regulation of several processes such as cell fate specification, proliferation, survival, and terminal differentiation. The timescale of the occurrence of processes involved in the regulation of cell number and identity, coincides with key morphogenetic events such as cell migration, homing and gangliogenesis. To gain insight into these processes, we characterized the cellular basis of metameric migration of neural-crest cells and of consequent ganglion organization, which are imposed by intrinsic differences within rostral and caudal sclerotomal compartments. We also established a transient requirement for neural tube-derived factors in regulating the proliferation, survival and differentiation of prospective DRG cells. Additionally, we showed that cooperation between the mesodermal cells and the neural tube is necessary for modulating cell number in the nascent ganglia. BDNF, NT-3 and basic FGF were found to mediate this environmental signalling. All the above factors display neurogenic activity for a subset of early-committed sensory neuron progenitors. This observation raises the possibility of an early redundancy in the response of individual neural-crest progenitors to distinct factors. This overlap in responsiveness progressively disappears upon the colonization of specific ganglionic sites and the subsequent establishment of selective innervation patterns by post-mitotic sensory neurons.

Cardiac neural crest is essential for the persistence rather than the formation of an arch artery

Double-label immunohistochemistry was used to compare early aortic arch artery development in cardiac neural crest-ablated and sham-operated quail embryos ranging from stage 13 to stage 18. The monoclonal antibody QH-1 labeled endothelial cells and their precursors, and HNK-1 labeled migrating neural crest cells. In the sham-operated embryos, the third aortic arch artery developed from a lumenizing strand of endothelial precursors that became separated from the pharyngeal endoderm by migrating cardiac neural crest cells as they ensheathed the artery. The arch artery of the neural crest-ablated embryos lumenized but failed to become separated from the pharyngeal endoderm, indicating that neural crest is unnecessary for the early formation of the aortic arch artery. However, once blood flow was initiated through the third arch artery of crest-ablated embryos at stage 16, the artery became misshapen and sinusoidal. By embryonic day 3, abnormal connections to the dorsal aorta occurred and bilateral symmetry was lost, suggesting that the loss of neural crest-derived ectomesenchyme destabilizes the nascent artery. Although here we show no loss of the third arch artery, past studies have reported hypoplasia or missing carotids in older neural crest-ablated embryos (Bockman et al. [1987] Am. J. Anat. 180:332-341; Bockman et al. [1989] Anat. Rec. 225:209-217; Nishibatake et al. [1987] Circulation 75:255-264; Tomita et al. [1991] Circulation 84:1289-1295). We suggest that the cardiac neural crest is essential for the persistence of an arch artery, but not its formation. Furthermore, since changes in the development of the arch artery are seen prior to the formation of the tunica media, it is suggested that a critical period is reached in the development of the arch artery, after lumenization, but prior to the formation of the tunica media, which necessitates the presence of the cardiac neural crest.

Targeted and natural (piebald-lethal) mutations of endothelin-B receptor gene produce megacolon associated with spotted coat color in mice

Endothelins act on two subtypes of G protein-coupled receptors, termed endothelin-A and endothelin-B receptors. We report a targeted disruption of the mouse endothelin-B receptor (EDNRB) gene that results in aganglionic megacolon associated with coat color spotting, resembling a hereditary syndrome of mice, humans, and other mammalian species. Piebald-lethal (sl) mice exhibit a recessive phenotype identical to that of the EDNRB knockout mice. In crossbreeding studies, the two mutations show no complementation. Southern blotting revealed a deletion encompassing the entire EDNRB gene in the sl chromosome. A milder allele, piebald (s), which produces coat color spotting only, expresses low levels of structurally intact EDNRB mRNA and protein. These findings indicate an essential role for EDNRB in the development of two neural crest-derived cell lineages, myenteric ganglion neurons and epidermal melanocytes. We postulate that defects in the human EDNRB gene cause a hereditary form of Hirschsprung's disease that has recently been mapped to human chromosome 13, in which EDNRB is located.

Neural development. Fate diverted

Signals that alter cell fate are probably crucial in metazoan development. Glial growth factor may play such a role in the mammalian neural crest, by regulating the generation of neurons and Schwann cells.

Pathology of neuroblastic tumors

Histogenesis, basic histologic features, nomenclature, and criteria for diagnosis and prognostic classifications based on morphological features of neuroblastic tumors (NTs) are described. NTs that arise from neuroectodermal cells of the adrenal medulla and sympathetic ganglia recapitulate the development of sympathetic ganglion. The following three basic types of NTs are recognized: neuroblastoma (NBs) and ganglioneuroblastomas (GNBs) and ganglioneuromas (GNs). NBs can be undifferentiated, poorly differentiated or differentiating; GNBs have the following three subtypes. (1) nodular, (2) intermixed, and (3) borderline. The conventional and recommended terminology and Shimada terminology of NTs are described (Tables 1 and 2). There are three basic pathologic components of NTs, neuroblastomatous, ganglioneuromatous, and intermediate components (Figs 3, 5, and 7). There are two major prognostic classifications based on morphological features of NTs, Shimada classification and histologic grading (Tables 3 and 4).

Molecular regulation of neural crest development

The neural crest is a transient embryonic structure that gives rise to a multitude of different cell types in the vertebrate. As such, it is an ideal model to study the processes of vertebrate differentiation and development. This review focuses on two major questions related to neural crest development. The first question concerns the degree and time of commitment of the neural crest cells to different cell lineages and the emerging role of the homeobox containing genes in regulating this process. Evidence from the cephalic crest suggests that the commitment process does start before the neural crest cells migrate away from the neural tube and gene ablation experiments suggest that different homeobox genes are required for the development of neural and mesenchymal tissue derivatives. However, clonal analysis of neural crest cells before migration suggests that many of the cells remain multi-potential indicating that the final determinative steps occur progressively during migration and in association with environmental influences. The second question concerns the nature of the environmental factors that determine the differentiation of neural crest cells into discrete lineages. Evidence is provided, mainly from in vitro experiments, that purified growth factors selectively promote the differentiation of neural crest cells down either sympathetic, adrenal, sensory, or melanocytic cell lineages.

Differentiation of neurogenic precursors within the neural crest cell lineage

It has been suggested that many, if not all crest-derived neurons develop from a limited subpopulation of neurogenic precursors. To develop cell-type specific markers that identify these precursors directly we have used differential screening of crest-derived cell populations known to have, or not to have, neurogenic ability. We have determined that the neuron-specific human auto-antibodies designated Anti-Hu bind to cytoplasmic and nuclear determinants not only in mature avian neurons and neuroendocrine cells but also in subpopulations of morphologically non-neuronal avian crest-derived cells. Significantly, these Anti-Hu+ non-neuronal crest-derived cells are present only in populations that have neurogenic ability and are absent from populations that lack neurogenic ability. Moreover, following additional development in vivo or in vitro, Anti-Hu+ non-neuronal crest-derived cells appear to express other neuronal traits. These results suggest that Anti-Hu-immunoreactivity is an early indicator of neurogenesis among crest-derived cells, and that Anti-Hu+ non-neuronal cells are either neurogenic precursors or immature neurons. Similarly, using the same differential screening paradigm, we have identified two monoclonal antibodies, designated 12E10 and 17F5, which also label both neurons and some apparently nonneuronal cells in neurogenic populations of neural crest cells. Anti-Hu-IR appears to precede expression of either of these two markers.

Alpha 1 beta 1 integrin on neural crest cells recognizes some laminin substrata in a Ca(2+)-independent manner

Neural crest cells migrate along pathways containing laminin and other extracellular matrix molecules. In the present study, we functionally and biochemically identify an alpha 1 beta 1 integrin heterodimer which bears the HNK-1 epitope on neural crest cells. Using a quantitative cell adhesion assay, we find that this heterodimer mediates attachment to laminin substrata prepared in the presence of Ca2+. Interestingly, neural crest cells bind to laminin-Ca2+ substrata in the presence or absence of divalent cations in the cell attachment medium. In contrast, the attachment of neural crest cells to laminin substrata prepared in the presence of EDTA, heparin, Mg2+, or Mn2+ requires divalent cations. Interactions with these laminin substrata are mediated by a different integrin heterodimer, since antibodies against beta 1 but not alpha 1 integrins inhibit neural crest cell attachment. Thus, the type of laminin substratum appears to dictate the choice of laminin receptor used by neural crest cells. The laminin conformation is determined by the ratio of laminin to Ca2+, though incorporation of heparin during substratum polymerization alters the conformation even in the presence of Ca2+. Once polymerized, the substratum appears stable, not being altered by soaking in either EDTA or divalent cations. Our findings demonstrate: (a) that the alpha 1 beta 1 integrin can bind to some forms of laminin in the absence of soluble divalent cations; (b) that substratum preparation conditions alter the conformation of laminin such that plating laminin in the presence of Ca2+ and/or heparin modulates its configuration; and (c) that neural crest cells utilize different integrins to recognize different laminin conformations.

Developmental changes of fucosylated glycoconjugates in rabbit dorsal root ganglia

Developmental changes of the fucosylated glycoconjugates in the dorsal root ganglia (DRG) of the rabbit were investigated histochemically using anti-fucosyl GM1 antibody and Ulex europaeus agglutinin 1 (UEA-1) lectin. Neither anti-fucosyl GM1 antibody nor UEA-1 lectin bound to the neural tubes or to the neural crest on embryonic day 14 (E14). Anti-fucosyl GM1 antibody binds diffusely to the DRG of E25. Large neurons unreactive with anti-fucosyl GM1 antibody appeared at 1 month and increased within 6 months after birth. Schwann cells immunoreactive with anti-fucosyl GM1 antibody came to be limited to the satellite cells surrounding the positive neurons. No staining with UEA-1 lectin was observed in the DRG of E25. Some small neurons became reactive with UEA-1 lectin within 1 month and remained to be so at 6 months after birth. Schwann cells including satellite cells were unreactive with this lectin. Since fucosyl GM1 was detected in the lipid fraction of DRGs from 1-month-old and 6-month-old rabbits, fucosyl GM1 itself should be the antigen molecule recognized by the anti-fucosyl GM1 antibody. Further study is necessary to elucidate the association between these developmental changes of the fucosylated glycoconjugates in DRG and their possible functional roles.

Effect of insulin and insulin-like growth factor I on the expression of the catecholaminergic phenotype by neural crest cells

Neural crest-derived catecholaminergic precursors have been used as a model to study the role of signals supplied by the environment during avian neurogenesis. A new culture system consisting of dissociated sclerotomes or somites isolated at embryonic day 3 (E3) or 2.5 (E2.5) has been established, which allows quantitative comparison of various culture conditions. As a first step of this study, the role of insulin and insulin-like growth factor I (IGF-I) in catecholaminergic differentiation has been investigated. Our results show that both factors are able to increase by a factor 2 to 3 the number of catecholaminergic cells present in the culture of sclerotomes after 24 h of culture. The effect is dose-dependent and the half-maximal effect is obtained with low concentrations of each peptide. Since insulin, IGF-I and their respective receptors are present at this stage of development in avian embryo, our observations suggest that an early step in catecholaminergic differentiation could be under at least the partial control of insulin and insulin-related peptides. On the other hand, neural crest precursors isolated at E2.5 are not able to generate catecholaminergic cells and to respond to insulin when cultivated for one day, indicating that these precursors are subjected in vivo to a maturation step, within the somite/sclerotome between E2.5 and E3; this step could be obtained in vitro by cultivating the precursors for 1 day, which resulted in the development of insulin responsiveness by catecholaminergic precursors.