Pigment patterns in neural crest chimeras constructed from quail and guinea fowl embryos

The pattern of pigmentation in bird embryos is determined by the spatial organization of melanocyte differentiation. Some of the results from recent, neural crest transplantation experiments support a model based on a prepattern in the feathers; others could be interpreted in terms of a nonspecific pattern resulting from a failure of the crest cells to read the positional values in another species. To distinguish between these possibilities, the crucial test is to construct chimeras from two species with different pigment patterns. We have examined the wing plumage of quail and guinea fowl embryos. The quail has a characteristic pattern of pigmented and unpigmented feather papillae, whereas the guinea fowl shows uniform pigmentation. Chimeras were constructed by grafting wing buds isotopically between embryos. The wing buds were transplanted before they had become invaded by neural crest cells. Quail wing buds grafted to the guinea fowl developed, in most cases, a pigment pattern resembling that of the quail and not that of the guinea fowl. A few cases became uniformly pigmented and appeared to represent nonspecific patterns. The reciprocal grafts (guinea fowl wing buds grafted to the quail) became pigmented all over. We found evidence that the timing of melanocyte differentiation is controlled by cues in the feather papillae. Some cases developed a severe inflammatory response. The model which best accounts for these findings--and which can account for inconsistencies in previous reports--is the following. A prepattern is present in the feathers and this can control the differentiation of melanoblasts, even if they come from a different species. The local cues which constitute the prepattern are not positional values. In some chimeras melanoblasts fail to respond to the prepattern and so a nonspecific pattern of uniform pigmentation is produced.     

Plumage pigmentation and expression of its regulatory genes during quail development--histochemical analysis using Bh (black at hatch) mutants

The plumage on the dorsal trunk of normal quail embryos exhibits longitudinal black and brown stripes of pigments produced by melanocytes. However, this pigmentation pattern disappeared in Bh (black at hatch) heterozygous and homozygous embryos because of overall black and brown pigmentation of plumages, respectively. To investigate the mechanisms of the pigment pattern formation of plumage and clarify the roles of the Bh locus in the pattern formation, we examined the expression pattern of genes relating to melanocyte development (Mitf, MelEM antigen, Kitl, Kit and EdnrB2) and melanin pigment production (Dct, Tyrp1, Tyr and Mmp115) in Bh mutant and wild-type embryos throughout development. As a result, we found that MelEM antigen was expressed in melanoblasts committed to produce black pigment before apparent melanogenic gene expression, and that Bh heterozygotes and homozygotes showed abnormal expression patterns of the MelEM antigen. These results indicate that MelEM antigen is a good marker for melanoblasts committed to produce black pigment, and suggests that the Bh locus directs melanocytes to produce eumelanin in proper positions.     

Bh (black at hatch) gene appears to be expressed in melanocytes of feather germs in the Japanese quail (Coturnix coturnix japonica)

The Bh (black at hatch) gene was examined to determine whether it is expressed in plumage melanocytes by analyzing pigmentation patterns of Bh melanocytes placed in the micro-environment of the feather germs of quail embryos with pink eyes. These host quails genetically lack a large part of plumage melanin. The Bh locus in these almost white quails is wild-type. When Bh neural crest cells were transplanted orthotopically into the host embryos, wild-type and Bh /+ melanocytes, which differentiated from the transplanted neural crest cells, formed plumage pigmentation patterns characteristic of each genotype in the micro-environment of the host feather germs. Brown plumage pigmentation, which was very similar to that of 10-day Bh / Bh embryos, was also observed in the feather germs of host embryos that received Bh neural crest cells, although the genotype of the donors could not be determined. These donors died before pigmentation of their feather germs occurred. The results demonstrate that pigmentation patterns of Bh menalocytes are not altered in the micro-environment of the host germs, suggesting that the Bh gene is autonomous in Bh melanocytes and is expressed in melanocytes of both Bh and the host feather germs, and that it causes the normal pigmentation pattern to be altered.     

Chemical analysis of melanin pigments in feather germs of Japanese quail Bh (black at hatch) mutants.

Bh (black at hatch) is a mutation of Japanese quails which causes darkening or lightening of the plumage in heterozygotes or homozygotes, respectively. We chemically analyzed melanin pigments in feather germs of Bh mutant embryos and in feathers of adult animals. Dark brown dorsal feathers of wild-type adult animals had white barrings, but heterozygous ones lacked clear barrings. The feathers of wild-type and heterozygote animals contained both eumelanins and pheomelanins, the latter being more pheomelanic. On the dorsal skin of 10-day old wild-type embryos, longitudinal stripes from black and yellow rows of feather germs developed; two or three longitudinal rows of black feather germs and then two or three rows of yellow feather germs next to the short central feather germs. Heterozygous embryos appeared black in plumage pigmentation, due to the presence of 'gray' feather germs in rows of dorsal feather germs that corresponded to yellow rows in wild-type embryos. Homozygous dorsal feather germs did not develop the black and yellow longitudinal stripes, but were brown. Chemical analysis showed that embryos of each genotype contained both eumelanins and pheomelanins in the feather germs; however, the eumelanin content in homozygous feather germs was very low. These results suggest that the Bh mutation causes pheomelanic changes in feathers of quails.     

Chemical Analysis of Melanin Pigments in Feather Germs of Japanese Quail Bh (Black at Hatch) Mutants

Bh (black at hatch) is a mutation of Japanese quails which causes darkening or lightening of the plumage in heterozygotes or homozygotes, respectively. We chemically analyzed melanin pigments in feather germs of Bh mutant embryos and in feathers of adult animals. Dark brown dorsal feathers of wild-type adult animals had white barrings, but heterozygous ones lacked clear barrings. The feathers of wild-type and heterozygote animals contained both eumelanins and pheomelanins, the latter being more pheomelanic. On the dorsal skin of 10-day old wild-type embryos, longitudinal stripes from black and yellow rows of feather germs developed; two or three longitudinal rows of black feather germs and then two or three rows of yellow feather germs next to the short central feather germs. Heterozygous embryos appeared black in plumage pigmentation, due to the presence of‘gray’feather germs in rows of dorsal feather germs that corresponded to yellow rows in wild-type embryos. Homozygous dorsal feather germs did not develop the black and yellow longitudinal stripes, but were brown. Chemical analysis showed that embryos of each genotype contained both eumelanins and pheomelanins in the feather germs; however, the eumelanin content in ho-mozygous feather germs was very low. These results suggest that the Bh mutation causes pheomelanic changes in feathers of quails.     

Wnt and BMP signaling govern lineage segregation of melanocytes in the avian embryo

Recent studies show that specification of some neural crest lineages occurs prior to or at the time of migration from the neural tube. We investigated what signaling events establish the melanocyte lineage, which has been shown to migrate from the trunk neural tube after the neuronal and glial lineages. Using in situ hybridization, we find that, although Wnts are expressed in the dorsal neural tube throughout the time when neural crest cells are migrating, the Wnt inhibitor cfrzb-1 is expressed in the neuronal and glial precursors and not in melanoblasts. This expression pattern suggests that Wnt signaling may be involved in specifying the melanocyte lineage. We further report that Wnt-3a-conditioned medium dramatically increases the number of pigment cells in quail neural crest cultures while decreasing the number of neurons and glial cells, without affecting proliferation. Conversely, BMP-4 is expressed in the dorsal neural tube throughout the time when neural crest cells are migrating, but is decreased coincident with the timing of melanoblast migration. This expression pattern suggests that BMP signaling may be involved in neural and glial cell differentiation or repression of melanogenesis. Purified BMP-4 reduces the number of pigment cells in culture while increasing the number of neurons and glial cells, also without affecting proliferation. Our data suggest that Wnt signaling specifies melanocytes at the expense of the neuronal and glial lineages, and further, that Wnt and BMP signaling have antagonistic functions in the specification of the trunk neural crest.     

Embryonic origin of skeletal muscle cells in the iris of the duck and quail

Summary The origin of skeletal muscle cells in avian iris muscle was investigated by quantitative analysis of heterochromatin profiles at the electron-microscopic level in irides of six types of quail-duck chimeras. Each of the following tissues was transplanted into the head region from quail to duck between stages 9 and 10: cranial neural crest; trunk neural crest; midbrain and adjacent mesoderm; forebrain; forebrain without neural crest; and forebrain without neural crest and mesoderm. The average ratio of heterochromatin profile to nucleus profile in iris skeletal muscle cells was high (quail type) in the dorsal iris, but low (duck type) in the ventral iris of the chimeras resulting from isotopic transplantation of cranial neural crest. Heterotopic transplantation of trunk neural crest to cranial position resulted in failure of development of skeletal muscle cells in the dorsal iris, but not in the appearance of skeletal muscle cells in the ventral iris. The average ratio of heterochromatin profile to nucleus profile in iris skeletal muscle cells was high in the chimeras resulting from transplantation of midbrain region and the chimeras resulting from transplantation of forebrain region, intermediate in the chimeras resulting from transplantation of forebrain region without neural crest, and low in the chimeras resulting from transplantation of forebrain region without neural crest and mesoderm. These results indicate that the skeletal muscle cells in the dorsal iris are of cranial neural crest origin while those in the ventral iris are not, and could possibly arise from cranial mesoderm.     

Mutational analysis of endothelin receptor b1 (rose) during neural crest and pigment pattern development in the zebrafish Danio rerio

Pigment patterns of fishes are a tractable system for studying the genetic and cellular bases for postembryonic phenotypes. In the zebrafish Danio rerio, neural crest-derived pigment cells generate different pigment patterns during different phases of the life cycle. Whereas early larvae exhibit simple stripes of melanocytes and silver iridophores in a background of yellow xanthophores, this pigment pattern is transformed at metamorphosis into that of the adult, comprising a series of dark melanocyte and iridophore stripes, alternating with light stripes of iridophores and xanthophores. Although several genes have been identified in D. rerio that contribute to the development of both early larval and adult pigment patterns, comparatively little is known about genes that are essential for pattern formation during just one or the other life cycle phase. In this study, we identify the gene responsible for the rose mutant phenotype in D. rerio. rose mutants have wild-type early larval pigment patterns, but fail to develop normal numbers of melanocytes and iridophores during pigment pattern metamorphosis and exhibit a disrupted pattern of these cells. We show that rose corresponds to endothelin receptor b1 (ednrb1), an orthologue of amniote Ednrb genes that have long been studied for their roles in neural crest and pigment cell development. Furthermore, we demonstrate that D. rerio ednrb1 is expressed both during pigment pattern metamorphosis and during embryogenesis, and cells of melanocyte, iridophore, and xanthophore lineages all express this gene. These analyses suggest a phylogenetic conservation of roles for Ednrb signaling in the development of amniote and teleost pigment cell precursors. As murine Ednrb is essential for the development of all neural crest derived melanocytes, and D. rerio ednrb1 is required only by a subset of adult melanocytes and iridophores, these analyses also reveal variation among vertebrates in the cellular requirements for Ednrb signaling, and suggest alternative models for the cellular and genetic bases of pigment pattern metamorphosis in D. rerio.     

Embryonic requirements for ErbB signaling in neural crest development and adult pigment pattern formation

Vertebrate pigment cells are derived from neural crest cells and are a useful system for studying neural crest-derived traits during post-embryonic development. In zebrafish, neural crest-derived melanophores differentiate during embryogenesis to produce stripes in the early larva. Dramatic changes to the pigment pattern occur subsequently during the larva-to-adult transformation, or metamorphosis. At this time, embryonic melanophores are replaced by newly differentiating metamorphic melanophores that form the adult stripes. Mutants with normal embryonic/early larval pigment patterns but defective adult patterns identify factors required uniquely to establish, maintain or recruit the latent precursors to metamorphic melanophores. We show that one such mutant, picasso, lacks most metamorphic melanophores and results from mutations in the ErbB gene erbb3b, which encodes an EGFR-like receptor tyrosine kinase. To identify critical periods for ErbB activities, we treated fish with pharmacological ErbB inhibitors and also knocked down erbb3b by morpholino injection. These analyses reveal an embryonic critical period for ErbB signaling in promoting later pigment pattern metamorphosis, despite the normal patterning of embryonic/early larval melanophores. We further demonstrate a peak requirement during neural crest migration that correlates with early defects in neural crest pathfinding and peripheral ganglion formation. Finally, we show that erbb3b activities are both autonomous and non-autonomous to the metamorphic melanophore lineage. These data identify a very early, embryonic, requirement for erbb3b in the development of much later metamorphic melanophores, and suggest complex modes by which ErbB signals promote adult pigment pattern development.     

Novel expression patterns of Pax3/Pax7 in early trunk neural crest and its melanocyte and non‐melanocyte lineages in amniote embryos

Neural crest cells are considered a key vertebrate feature that is studied intensively because of their relevance to development and evolution. Here we report the expression of Pax7 in the dorsal non‐neural ectoderm and in the trunk neural crest of the early chick embryo. Pax7 is expressed in the trunk neural crest migrating along the ventral and dorsolateral routes. Pax7 is first downregulated in the neural crest‐derived neuronal precursors, secondly in the glial, and finally in the melanocyte precursors. Conserved developmental expression in the melanocyte lineage of both Pax3 and Pax7 was evidenced in chick and quail, but only Pax3 in mouse and rat.     

家蚕两种新的类鹑斑突变体的遗传分析和SSR标记定位

在家蚕品种选育过程中发现了两种斑纹突变体, 与普通斑相比, 其幼虫眼状纹不明显, 而半月纹和星状纹正常, 其间有点和线构成鹑状斑纹, 第6、7腹节背面布有纵向波纹状斑纹, 整体斑纹与鹑斑(quail,q)极其相似, 暂且命名为类鹑斑(quail-like, q-l)。其中一种突变体稚蚕期体色呈褐色, 蚕体发育正常, 蚕茧大小一致, 茧型正常, 称为褐色类鹑斑(brown quail-like, q-lb); 另一种突变体幼虫体色为浅粉紫色, 幼虫食桑量少, 发育缓慢, 体质较弱, 体型较小, 茧型偏小, 称为紫色类鹑斑(purple quail-like, q-lp)。遗传分析表明, 两个类鹑斑基因均为隐性基因; 褐色类鹑斑(q-lb)与紫色类鹑斑(q-lp) 为等位基因, 紫色类鹑斑(q-lp)对褐色类鹑斑(q-lb)为隐性。经与形态标记P3(2)、p(2)、Ze(3)、L(4)、re(5)、E(6)、q(7)、I-a(9)、ms(12)、ch(13)、oa(14)、cts(16)、mln(18)、 msn(19)、rb(21)、so(26)测验和SSR分子标记多态性分析, 新发现的两种类鹑斑不同于鹑斑(q), 其基因座位于第8连锁群。     

Novel expression patterns of Pax3/Pax7 in early trunk neural crest and its melanocyte and non-melanocyte lineages in amniote embryos

Neural crest cells are considered a key vertebrate feature that is studied intensively because of their relevance to development and evolution. Here we report the expression of Pax7 in the dorsal non-neural ectoderm and in the trunk neural crest of the early chick embryo. Pax7 is expressed in the trunk neural crest migrating along the ventral and dorsolateral routes. Pax7 is first downregulated in the neural crest-derived neuronal precursors, secondly in the glial, and finally in the melanocyte precursors. Conserved developmental expression in the melanocyte lineage of both Pax3 and Pax7 was evidenced in chick and quail, but only Pax3 in mouse and rat.     

Testing homology of loci for two plumage colors, "lavender" and "recessive white," with chicken and Japanese quail hybrids

Homology for two plumage color loci was studied by hybridization between chickens and Japanese quail. First, chicken-quail hybrids were produced from homozygous "lavender" chicken cocks and "bleu" Japanese quail, and all 30 hybrids had the same parental slate blue plumage color. On the other hand, no hybrids with this plumage were obtained out of 18 progeny from the same cocks and wild-type quail. These results show that the slate blue plumage color is determined by homologous loci in Japanese quail and chickens. Second, all (n = 25) chicken-quail hybrids hatched from homozygous "recessive white" cocks and "recessive white" (n = 8) or "wild-type" (n = 17) quail had the same pattern of plumage color, with white feathers on the ventral face and colored feathers elsewhere. These results indicate that the recessive white mutations are not homologous in Japanese quail and chickens.     

Generation of pigmented stripes in albino mice by retroviral marking of neural crest melanoblasts.

The pigment cells of the skin are derived from melanoblasts which originate in the neural crest. The dorsoventral migration of melanoblasts has been visualized in pigment stripes seen in aggregation chimeras, and the width of these bands has suggested that the entire pigmentation of the coat is derived from a small number of founder cells. We have generated mosaic mice by marking single melanoblasts in utero to gain information on the clonal history of pigment-forming cells. A retroviral vector carrying the human tyrosinase gene was constructed and microinjected into neurulating albino mouse embryos. Albino mice are devoid of pigmentation due to deficiency of tyrosinase. Thus, transduction of the wild-type gene into the otherwise normal melanoblasts should rescue the mutant phenotype, giving rise to patches of pigmentation, which correspond to the area colonized by the mitotic progeny of a marked clone. Mosaic animals derived from the injected embryos indeed showed pigmented bands with a width strikingly similar to the 'standard' stripes seen in aggregation chimeras. These results are consistent with the notion that the unit width bands seen in aggregation chimeras represent the clonal progeny of a single melanoblast and verify Mintz's (1967) conclusion that a few founder melanoblasts give rise to coat pigmentation. The pigment cells of the eye are of dual origin: the melanocytes in choroid and outer layer of the iris are derived from the neural crest and those in the pigment layer of the retina from the neuroepithelium of the optic cup. Marked clones in both lineages were observed in the eyes of many mosaic animals.     

Pax3-expressing trigeminal placode cells can localize to trunk neural crest sites but are committed to a cutaneous sensory neuron fate

The cutaneous sensory neurons of the ophthalmic lobe of the trigeminal ganglion are derived from two embryonic cell populations, the neural crest and the paired ophthalmic trigeminal (opV) placodes. Pax3 is the earliest known marker of opV placode ectoderm in the chick. Pax3 is also expressed transiently by neural crest cells as they emigrate from the neural tube, and it is reexpressed in neural crest cells as they condense to form dorsal root ganglia and certain cranial ganglia, including the trigeminal ganglion. Here, we examined whether Pax3+ opV placode-derived cells behave like Pax3+ neural crest cells when they are grafted into the trunk. Pax3+ quail opV ectoderm cells associate with host neural crest migratory streams and form Pax3+ neurons that populate the dorsal root and sympathetic ganglia and several ectopic sites, including the ventral root. Pax3 expression is subsequently downregulated, and at E8, all opV ectoderm-derived neurons in all locations are large in diameter, and virtually all express TrkB. At least some of these neurons project to the lateral region of the dorsal horn, and peripheral quail neurites are seen in the dermis, suggesting that they are cutaneous sensory neurons. Hence, although they are able to incorporate into neural crest-derived ganglia in the trunk, Pax3+ opV ectoderm cells are committed to forming cutaneous sensory neurons, their normal fate in the trigeminal ganglion. In contrast, Pax3 is not expressed in neural crest-derived neurons in the dorsal root and trigeminal ganglia at any stage, suggesting either that Pax3 is expressed in glial cells or that it is completely downregulated before neuronal differentiation. Since Pax3 is maintained in opV placode-derived neurons for some considerable time after neuronal differentiation, these data suggest that Pax3 may play different roles in opV placode cells and neural crest cells.     

Relations between pigmentation of the cornea and the number of epidermal melanophores in Rana temporaria L. larvae

The dynamics of the external cornea pigmentation in Rana temporaria L. larvae at the 22d developmental stage have been studied under conditions favourable for various course of certain morphological reactions in the pigment system. The cornea together with the surrounding skin is transferred on the dorsal surface of the larva body, and the piece of the dorsal surface skin is put instead of the cornea removed. When using the reciprocal transplantation method and preserving the organism's integrity (without disturbing melanocyte-stimulating source--namely, the hypophysis, and melatonine sources--namely, the pineal gland and the lateral eyes) the corneal pigmentation is observed on the background of perfect morphological reactions in the pigment system, while the larvae are maintained on the dark and light substrates, that is at various density of the pigment cells (120 larvae have been used). The pigmentation dynamics have been studied from the 6th up to the 20th day in total preparations. The epidermal melanophores density is estimated in 4 areas of each preparation. The melanin amount is estimated by means of the electron paramagnetic resonance-spectrometry according to the contents of free radicals expressed in relative units. A direct proportional dependence between the significantly higher melanin contents (1.5-fold) and a significantly quicker (1.5-fold) process of the corneal pigmentation is revealed, that agrees with an increasing number of the pigment cells per one unit of the body surface in the larvae maintained on the dark substrate. In the larvae maintained on the light substrate, the dependence is of a reverse character. It is probable that the factors forcing the pigmented cells, at cultivation the neural crest cells in vitro to reject from each other, affect the pigmentation of the larval cornea in vivo. If it is the case, the processes specific for the embryonal period, transgress during the cornea pigmentation at the larval stages of development.     

PTERYLOGRAPHY, PLUMAGE DEVELOPMENT AND MOULT OF JAPANESE QUAIL COTURNIX C. JAPONICA IN CAPTIVITY

Japanese Quail were kept in small cages under controlled conditions of temperature and light, and their pterylography and moult are described. There are 10 primaries, 14 secondaries and corresponding numbers of greater upper and lower wing coverts. The alula has four feathers and the tail from five to six pairs of feathers. There is an apterium in the dorso-pelvic tract similar to that in other quail genera. The arrangement of feathers in the ventral and cervical tracts appears to differ from that described for some North American quail.
The chicks hatch with a covering of natal down. Pre-juvenile moult can be seen when the chicks are three days old. Juvenile body plumage is complete in about 30 days; the sides of the face, around the eyes, are the last places to acquire feathers. The tenth and last juvenile primary to grow is mature when the chicks are 41 days old.
The moult in which the juvenile plumage is replaced overlaps the post-natal moult and in part of the ventral tract natal down is replaced by the first adult feathers. This makes it possible to sex the quail at 14 days old. The first adult moult is complete, in the body tracts, by the time the birds are five to six weeks old. The dropping of juvenile primaries commences at about three weeks old and ceases when about eight weeks old. Only from three to six primaries are replaced; most birds studied replaced five. The significance of this difference from other Galliformes is discussed; it is thought to be associated with the species' migratory behaviour. Quail which remained in the controlled laboratory environment did not undergo any further moult. All birds moulted when both temperature and light period were reduced and most birds moulted when the light period alone was reduced. Adult birds housed in small cages in an unheated, unlit shed underwent a complete moult between August and December in which all primaries were replaced. This moult took 8–14 weeks to complete.     

Neural crest cell behavior in white and dark larvae of Ambystoma mexicanum: differences in cell morphology, arrangement, and extracellular matrix as related to migration

Melanocytes of white (d/d) larvae of the Mexican axolotl (Ambystoma mexicanum) are confined to the dorsal midline of the trunk region, whereas in dark (D/-) larvae they are spread laterally on the flank as well, where they contribute to the normal pigment pattern of the trunk. Pigment cell migration in the subepidermal space of white larvae is inhibited by the white epidermis (Dalton '50; Keller et al., '82). The present scanning electron microscopic study describes a well-defined sequence of changes in shape and arrangement of neural crest cells during and after their segregation from the neural tube in both dark and white axolotls. The morphology of the neural crest cells migrating in the subepidermal pathway of dark larvae is correlated with their motile behavior and pattern of migration in vivo, as described by time-lapse cinemicrography (Keller and Spieth, '83). Also, the structures of the matrix material in the subepidermal space of dark and white axolotls differ in ways that may be related to the epidermal inhibition of migration in the latter. Numerous possibilities for contact guidance offered by the structure and topography of the substrata, neighboring cells, and the extracellular matrix in the migration path are described and discussed.     

Dorsal patterning defects in the hindbrain, roof plate and skeleton in the dreher (dr(J)) mouse mutant

dreher is a spontaneous mouse mutation in which adult animals display a complex phenotype associated with hearing loss, neurological, pigmentation and skeletal abnormalities. During early embryogenesis, the neural tube of dreher mutants is abnormally shaped in the region of the rhomboencephalon, due to problems in the formation of a proper roof plate over the otic hindbrain. We have studied the expression of Hox/lacZ transgenic mouse strains in the dreher background and shown that primary segmentation of the neural tube is not altered in these mutants, although correct morphogenesis is affected resulting in misshapen rhombomeres. Neural crest derivatives from rhombomere 6, such as the glossopharyngeal ganglion, are defective, and the dorsal neural tube marker Wnt1 is absent from this segment. Selected trunk neural crest populations are also altered, as there is a lack of pigmentation in the thoracic region of mutant mice. Skeletal defects include abnormal cranial bones of neural crest origin, and improper fusion of the dorsal aspects of cervical and thoracic vertebrae. Taken together, the gene affected in the dreher mutant is responsible for correct patterning of the dorsal-most cell types of the neural tube, that is, the neural crest and the roof plate, in the hindbrain region. Axial skeletal defects could reflect inductive influence of the dorsal neural tube on proper fusion of the neural arches. It is possible that a common precursor population for both neural crest and roof plate is the cellular target of the dreher mutation.