Region Of Somites Research Articles

Pax3 functions in cell survival and in pax7 regulation.

In developing vertebrate embryos, Pax3 is expressed in the neural tube and in the paraxial mesoderm that gives rise to skeletal muscles. Pax3 mutants develop muscular and neural tube defects; furthermore, Pax3 is essential for the proper activation of the myogenic determination factor gene, MyoD, during early muscle development and PAX3 chromosomal translocations result in muscle tumors, providing evidence that Pax3 has diverse functions in myogenesis. To investigate the specific functions of Pax3 in development, we have examined cell survival and gene expression in presomitic mesoderm, somites and neural tube of developing wild-type and Pax3 mutant (Splotch) mouse embryos. Disruption of Pax3 expression by antisense oligonucleotides significantly impairs MyoD activation by signals from neural tube/notochord and surface ectoderm in cultured presomitic mesoderm (PSM), and is accompanied by a marked increase in programmed cell death. In Pax3 mutant (Splotch) embryos, MyoD is activated normally in the hypaxial somite, but MyoD-expressing cells are disorganized and apoptosis is prevalent in newly formed somites, but not in the neural tube or mature somites. In neural tube and somite regions where cell survival is maintained, the closely related Pax7 gene is upregulated, and its expression becomes expanded into the dorsal neural tube and somites, where Pax3 would normally be expressed. These results establish that Pax3 has complementary functions in MyoD activation and inhibition of apoptosis in the somitic mesoderm and in repression of Pax7 during neural tube and somite development.

Evidence that Hoxa expression domains are evolutionarily transposed in spinal ganglia, and are established by forward spreading in paraxial mesoderm

Transposition of anatomical structures along the anteroposterior axis has been a commonly used mechanism for changing body proportions during the course of evolutionary time. Earlier work (Gaunt, S.J., 1994. Conservation in the Hox code during morphological evolution. Int. J. Dev. Biol. 38, 549–552; Burke, A.C., Nelson, C.E., Morgan, B.A., Tabin, C., 1995. Hox genes and the evolution of vertebrate axial morphology. Development 121, 333–346) showed how transposition in mesodermal derivatives (vertebrae) could be attributed to transposition in the expression of Hox genes along the axial series of somites. We now show how transposition in the segmental arrangement of the spinal nerves can also be correlated with shifts in the expression domains of Hox genes. Specifically, we show how the expression domains of Hoxa-7, a-9 and a-10 in spinal ganglia correspond similarly in both mouse and chick with the positions of the brachial and lumbosacral plexuses, and that this is true even though the brachial plexus of chick is shifted posteriorly, relative to mouse, by seven segmental units. In spite of these marked species differences in the boundaries of Hoxa-7 expression, cis regulatory elements located up to 5 kb upstream of the chick Hoxa-7 gene showed much functional and structural conservation with those described in the mouse (Puschel, A.W., Balling, R., Gruss, P., 1991. Separate elements cause lineage restriction and specify boundaries of Hox-1.1 expression. Development 112, 279–287; Knittel, T., Kessel, M., Kim, M.H., Gruss, P., 1995. A conserved enhancer of the human and murine Hoxa-7 gene specifies the anterior boundary of expression during embryonal development. Development 121, 1077–1088). We also show that chick Hoxa-7 and a-10 expression domains spread forward into regions of somites that are initially negative for the expression of these genes. We discuss this as evidence that Hox expression in paraxial mesoderm spreads forward, as earlier found for neurectoderm and lateral plate mesoderm, in a process that occurs independently of cell movement.

F-Spondin, Expressed in Somite Regions Avoided by Neural Crest Cells, Mediates Inhibition of Distinct Somite Domains to Neural Crest Migration

Neural crest (NC) cells migrate exclusively into the rostral half of each sclerotome, where they avoid the dermomyotome and the paranotochordal sclerotome. F-spondin is expressed in these inhibitory regions and throughout the caudal halves. In vitro bioassays of NC spreading on substrates of rostral or caudal epithelial–half somites (RS or CS, respectively) revealed that NC cells adopt on RS a fibroblastic morphology, whereas on CS they fail to flatten. F-spondin inhibited flattening of NC cells on RS. Conversely, F-spondin antibodies prevented rounding up of NC cells on CS. Addition of F-spondin to trunk explants inhibited NC migration into the sclerotome, and treatment of embryos with anti-F-spondin antibodies yielded migration into otherwise inhibitory sites. Thus, somite-derived F-spondin is an inhibitory signal involved in patterning the segmental migration of NC cells and their topographical segregation within the RS.

Spatial pattern of constitutive and heat shock-induced expression of the small heat shock protein gene family, Hsp30, in Xenopus laevis tailbud embryos.

We employed whole-mount in situ hybridization and immunohistochemistry to study the spatial pattern of hsp30 gene expression in normal and heatshocked embryos during Xenopus laevis development. Our findings revealed that hsp30 mRNA accumulation was present constitutively only in the cement gland of early and midtailbud embryos, while hsp30 protein was detected until at least the early tadpole stage. Heat shock-induced accumulation of hsp30 mRNA and protein was first observed in early and midtailbud embryos with preferential enrichment in the cement gland, somitic region, lens placode, and proctodeum. In contrast, cytoskeletal actin mRNA displayed a more generalized pattern of accumulation which did not change following heat shock. In heat shocked midtailbud embryos the enrichment of hsp30 mRNA in lens placode and somitic region was first detectable after 15 min of a 33 degrees C heatshock. The lowest temperature capable of inducing this pattern was 30 degrees C. Placement of embryos at 22 degrees C following a 1-h 33 degrees C heat shock resulted in decreased hsp30 mRNA in all regions with time, although enhanced hsp30 mRNA accumulation still persisted in the cement gland after 11 h compared to control. In late tailbud embryos the basic midtailbud pattern of hsp30 mRNA accumulation was enhanced with additional localization to the spinal cord as well as enrichment across the embryo surface. These studies demonstrate that hsp30 gene expression can be detected constitutively in the cement gland of tailbud embryos and that heat shock results in a preferential accumulation of hsp30 mRNA and protein in certain tissues.

N-cadherin/catenin-mediated morphoregulation of somite formation.

Somitogenesis during early stages in the chick and mouse embryo was examined in relation to N-cadherin-mediated adhesion. Previous studies indicated that N-cadherin localizes to the somite regions during their formation. Those observations were extended to include a spatiotemporal immunohistochemical analyses of beta-catenin and alpha-catenin, as well as a more detailed study of N-cadherin, during segmentation, compaction, and compartmentalization of the somite. N-cadherin and the catenins appear early within the segmental plate and are expressed as small patch-like foci throughout this tissue. The small foci of immunostaining coalesce into larger clusters of N-cadherin/catenin-expressing regions. The clusters subsequently coalesce into a region of centrally localized cells that express N-cadherin/catenins at their apical surfaces. The multiple clusters are spaced wide apart in the anterior segmental plates that form the first 6 somite pairs, as contrasted to segmental plates that form somites 7 and beyond. To examine the functional significance of N-cadherin, segmental plates were exposed to antibodies that perturb N-cadherin-mediated adhesion in the chick embryo. The multiple, anomalous somites that result in these experiments indicate that each N-cadherin/catenin-expressing cluster can give rise to a somitic structure. beta-Catenin involvement in somitogenesis suggests a role for Wnt-mediated signaling. Embryos treated with LiCl also show induction of similar anomalous somites indicating further the possibility that Wnt-mediated signaling may be involved in the clustering event. It is suggested that beta-catenin serves to initiate the adhesion process which is spread then by N-cadherin. Later during compartmentalization, N-cadherin/catenins remain expressed by the myotome compartment. Taken together, these results suggest that the Ca2+-dependent cell adhesion molecule N-cadherin and the intracellular catenins are important in segmentation and formation of the somite and myotome compartment. It is proposed that the N-cadherin-mediated adhesion process may serve as a common, evolutionarily conserved, link in the differentiation pathways of skeletal and cardiac muscle.

Murine Cerberus Homologue mCer-1: A Candidate Anterior Patterning Molecule

Xenopuscerberus (Xcer) is a cytokine expressed in anterior mesendoderm overlapping and surrounding Spemann's gastrula organiser. When misexpressed in blastomeres, Xcer can induce ectopic heads with well-defined brain, cement gland, olfactory placodes, cyclopic eye, and occasionally liver and heart. We report here the identification ofmCer-1,a murine gene related tocerberus.Both mCer-1 and Xcer appear to belong to the cystine knot superfamily, which includes TGFβs and BMPs. InXenopusanimal cap assays, mCer-1 and Xcer induced cement glands and markers of anterior neural tissue and endoderm, characteristic of BMP inhibition. Furthermore, both antagonised the ventrolateral mesoderm-inducing activity of coexpressed BMP4. In mouse embryos,mCer-1was expressed at early gastrulation in a stripe of primitive endoderm along the future anterior side of the egg cylinder, a region essential for anterior patterning. A second phase of expression was detected in anterior embryonic mesendoderm, and by late-streak stages most of the anterior half of the embryo was positive, except for the node and cardiac progenitors. Expression was later seen in the cranial portion of the two most-recently formed somites and in two stripes within presomitic mesoderm. In embryos lackingOtx2,a homeogene with a demonstrated role in anterior patterning,mCer-1was still expressed in an anterior zone, although often abnormally. The data suggest that mCer-1 shares structural, functional, and expression characteristics with Xcer and may participate in patterning the anterior of the embryo and nascent somite region, in part, through a BMP-inhibitory mechanism.

Positional apoptosis during vertebrate CNS development in the absence of endogenous retinoids.

We have previously shown that quail embryos that develop in the absence of vitamin A have severe defects in their central nervous system. One defect is a completely missing posterior hindbrain. Here we have studied how this comes about by examining cell death using a wholemount technique. In these A- embryos we observe two narrow bands of ectopic apoptosis. One is in the mesenchyme in the region of the first somite and occurs at the 4-6 somite stage, before neural tube closure. The second band follows immediately afterwards and occurs in the neuroepithelium of the presumptive posterior hindbrain at the 6-8 somite stage. Electron microscopy shows that the dying neuroepithelial cells exhibit the characteristics of apoptosis. Rescuing the embryos by injecting retinol before gastrulation completely prevents these apoptotic events. In an effort to identify some of the genes that may be involved in the apoptotic pathway we show that Msx-2 is upregulated in the apoptotic neuroepithelium and thus may be involved, whereas Bmp-4 is not altered and thus presumably not involved. Since these apoptotic event take place at the time of specification of axial identity and segmentation in the mesenchyme and neuroepithelium we conclude that these cells die because they are wrongly specified in terms of their rostrocaudal position, a novel phenomenon which we refer to as positional apoptosis.

Severe defects in the formation of epaxial musculature in open brain (opb) mutant mouse embryos

The differentiation of somite derivatives is dependent on signals from neighboring axial structures. While ventral signals have been described extensively, little is known about dorsal influences, especially those from the dorsal half of the neural tube. Here, we describe severe phenotypic alterations in dorsal somite derivatives of homozygous open brain (opb) mutant mouse embryos which suggest crucial interactions between dorsal neural tube and dorsal somite regions. At Theiler stage 17 (day 10.5 post coitum) of development, strongly altered expression patterns of Pax3 and Myf5 were observed in dorsal somite regions indicating that the dorsal myotome and dermomyotome were not differentiating properly. These abnormalities were later followed by the absence of epaxial (dorsal) musculature; whereas, body wall and limb musculature formed normally. Analysis of Mox1 and Pax1 expression in opb embryos revealed additional defects in the differentiation of the dorsal sclerotome. The observed abnormalities coincided with defects in differentiation of dorsal neural tube regions. The implications of our findings for interactions between dorsal neural tube, surface ectoderm and dorsomedial somite regions in specifying epaxial musculature are discussed.

A Chicken WNT Gene, WNT-11, Is Involved in Dermal Development

We have isolated a new member of the Wnt gene family, Wnt-11, from chick embryo cDNA library and examined the expression pattern during embryogenesis by in situ hybridization. The Wnt-11 gene encodes Cys-rich secretory protein distantly related to Wnt-1 through Wnt-8 from the mouse and Xenopus. Expression of the Wnt-11 gene became evident at stage 14 in the dorsolateral region of somites and gradually restricted to the dermatome at stage 19 and later. In contrast to the other Wnt genes, Wnt-11 was not expressed in the neuroepithelium throughout stages 14-26. At stage 24 and later, Wnt-11 was expressed in the subectodermal mesenchyme of the limb and feather buds. The unique expression pattern of Wnt-11 in the paraxial mesoderm and dermatome suggests that Wnt-11 may play an important role in dermal development.

Superficial Cells in the Early Gastrula of Rana pipiens Contribute to Mesodermal Derivatives

We have studied the fate of presumptive mesodermal cells in the early Rana pipiens gastrula. We labeled superficial cells of the early gastrula with 125I. We also labeled all cells in a gastrula with rhodamine-lysine-dextran cell lineage tracer and superficial cells with 125 I and then grafted small pieces of the marginal zone orthotopically into unlabeled host embryos. Labeled progeny were identified in sectioned embryos at the tail bud stage. The use of double-labeled grafts allowed us to study the relative contributions by superficial and deep cells to different derivatives. We found that the notochord and somite regions are both derived from the superficial and deep portions of circumblastoporal regions. In contrast, pronephros, lateral plate, cardiac anlagen, and blood cells only arise from deep cells in circumblastoporal regions. Such observations indicate that the fate map for R. pipiens is different from that of Xenopus laevis, where mesodermal derivatives appear to be restricted to the deep layers, and from that of Pleurodeles waltl, where all mesodermal derivatives are formed both from superficial and deep layers. We also have shown that from the neurula stage there is substantial mixing between cells due to the ingression of cells in the dorsal region so that superficial labeled cells, initially located in the roof of the archenteron, contribute to ventral regions of both the somites and notochord.

Myocardial enlargement in defective heart development

The cardiac neural crest (neural crest extending from the mid-otic placode to the caudal region of somite 3) provides ectomesenchymal cells that contribute to aortic arch development and are essential for aortico-pulmonary septation of the outflow tract. Bilateral ablation of the cardiac neural crest in the chick embryo, prior to migration, leads to aortic arch anomalies and failure of septation of the cardiac outflow tract, which produces a severe defect known as persistent truncus arteriosus (PTA). Altered hemodynamics resulting from abnormal aortic arch artery development and PTA and other unknown factors related to the absence of neural crest, are likely to alter the developmental history of the myocardium. In this study the wet and dry weights of ventricles and whole embryos, the total number of myocytes per ventricle and the myocyte density (number of myocytes per unit volume of ventricular myocardium) were compared in control (unwindowed eggs), sham-operated and cardiac neural crest ablated chick embryos at day 11 of incubation. We found that the wet and dry weights of ventricles from hearts with PTA were not different from normal hearts in control and sham-operated embryos. However, the embryos with PTA weighed less than embryos with normal hearts. Thus, the ventricle to embryo weight ratios were greater in embryos with PTA compared to control and sham-operated embryos for both wet (14 and 20%, respectively) and dry (30 and 59%) weights. The data further implied that more water was present with respect to body weight in comparison with sham-operated and control embryos which indicated that the embryos with PTA were edematous. The total number of myocytes and the number of myocytes per unit volume were not different when comparing sham-operated with PTA. Further, there was no indication that the myocardium from hearts with PTA was abnormal despite the small size and edema of the embryos. It appears that hemodynamic stresses, resulting from the structural defects produced by neural crest ablation, are insufficient to increase heart growth, although cardiac function is depressed as evidenced by edema and failure of the embryo to thrive.

Pax-3 is required for the development of limb muscles: a possible role for the migration of dermomyotomal muscle progenitor cells.

Limb muscles in vertebrates originate from dermomyotomal cells, which during early development migrate from the ventrolateral region of somites into the limb buds. These progenitor cells do not express any muscle-specific marker genes or myogenic transcription factors until they reach their destination in the limbs. Here, we demonstrate by in situ hybridization that myogenic cells in somites and a population of presumably migratory muscle precursor cells in somatopleural tissue as well as myoblasts in the developing limbs express Pax-3. Significantly, in homozygous splotch mutant mice, which synthesize altered Pax-3 mRNA but make no normal protein, no cells positive for Pax-3 transcripts can be detected in the region of migrating limb muscle precursors or in the limb itself. In contrast, myotomal precursor cells and axial skeletal muscles contain Pax-3 transcripts also in the mutant. Interestingly, these animals fail to develop limb musculature as demonstrated by the lack of hybridization with various probes for myogenic transcription factors (Myf-5, myogenin, MyoD) but make apparently normal axial muscles. These observations suggest that Pax-3 is necessary for the formation of limb muscles, affecting either the generation of myogenic precursors in the somitic dermomyotome or the migration of these cells into the limb field.

Developmental Plasticity of the Prospective Dermatome and the Prospective Sclerotome Region of an Avian Somite: (somite/dermomyotome/sclerotome/determination/quail-chick chimera).

The ventro-medial wall of a somite gives rise to the sclerotome and then to cartilaginous axial skeleton, while the dorso-lateral wall differentiates into the dermomyotome to form dermal mesenchyme and muscle. Although previous studies suggested pluri-potency of somite cell differentiation, apparent pluri-potency may be the result of migration of predetermined cells. To investigate whether the developmental fate of any region is determined, I isolated fragments of a region of a quail somite and transplanted them into chick embryos. When a fragment of the ventral wall of a quail somite, the prospective sclerotome, was transplanted into a chick embryo between the ectoderm and a newly formed somite, the transplanted quail cells were shown to form myotome and mesenchyme in 4-day chimera embryos and to form muscle and dermal tissue in 9-day chimeras. On the other hand, when a fragment of the dorsal wall of a quail somite, the prospective dermomyotome, was transplanted into a chick embryo between the neural tube and a newly formed somite, the graft gave rise to mesenchyme around the neural tube and notochord and then to vertebral cartilage. Thus the developmental fate of a region of a somite was shown not to be determined at the time of somite segmentation, confirming previous observations.

The expression of the regulatory myosin light chain 2 gene during mouse embryogenesis

The fast skeletal muscle myosin light chain 2 (MLC2) gene is expressed specifically in skeletal muscles of newborn and adult mice, and has no detectable sequence homology with any of the other MLC genes including the slow cardiac MLC2 gene. The expression of the fast skeletal muscle MLC2 gene during early mouse embryogenesis was studied by in situ hybridization. Serial sections of embryos from 8.5 to 12.5 days post coitum (d.p.c.) were hybridized to MLC2 cRNA and to probes for the myogenic regulatory genes MyoD1 and myogenin. The results revealed different temporal and spatial patterns of hybridization for different muscle groups. MLC2 transcripts were first detected 9.5 d.p.c. in the myotomal regions of rostral somites, already expressing myogenin. Surprisingly, at the same stage, a weak MLC2 signal was also detected in the cardiomyocytes. The cardiac expression was transient and could not be detected at later stages while the myotomal signal persisted and spread to the more caudal somites, very similar to the expression of myogenin. Beginning from 10.5 d.p.c., several extramyotomal premuscle cells masses have been demarcated by MyoD1 expression. MLC2 transcripts were detected in only one of these cell masses. Although, transcripts of myogenin were detected in all these cell masses, the number of expressing cells was significantly lower than that observed for MyoD1. By 11.5 d.p.c., all three hybridization signals colocalized in most extramyotomal muscle-forming regions, with the exception of the diaphragm and the hindlimb buds, where only few cells expressed MLC2 and more cells expressed MyoD1 than myogenin. At 12.5 d.p.c., all three studied genes displayed a similar spatial pattern of expression in most muscle-forming regions. However, in some muscles, the MyoD1 signal spread over more cells compared to myogenin or MLC2. Our results are consistent with the suggestion that multiple myogenic programs exist for myoblasts differentiating in the myotome and extramyotomal regions.

Upstream Region of the Myogenin Gene Confers Transcriptional Activation in Muscle Cell Lineages During Mouse Embryogenesis

Myogenin, one of the MyoD-related factors containing basic-helix-loop-helix motifs, is transcriptionally activated in skeletal muscle lineages in somites and limb buds during embryogenesis. In an attempt to understand regulatory mechanisms which govern transcriptional activation of the myogenin gene, transgenic mice bearing the lacZ gene driven by the upstream region of the myogenin gene were generated. Stereoscopic visualization of LacZ-positive cells of these transgenic mouse embryos revealed that the upstream region of the myogenin gene conferred its transcriptional activation in cells of the skeletal muscle lineages in somites, limb buds, and visceral arches. Moreover, transient LacZ expression in newly formed somites in addition to its strong activation in myotomal regions of mature somites with a rostro-caudal gradient raised the possibility that myogenin is transcriptionally activated in immature somites before myotome formation.

Embryonic development of the house shrew (Suncus murinus)

The embryonic development during the period from 1 to 12 pairs of somites was observed in an insectivore species, the house shrew (Suncus murinus), which has been bred within a closed colony. Embryos were staged by the number of somite pairs. Each stage was punctuated at every addition of three pairs of somites and numbered after the Carnegie system. The first somite became apparent between 8 and 9.0 days after fertilization, and the 12th somite appeared between 9.5 and 10.0 days. The rate of somite formation was one pair in every 3-4 h on average. The embryonic events during this period were as follows: 1. From the beginning of stage 9, the embryonic body consistently displayed a kyphosis, and as development progressed, the caudal portion of the embryo spiralled clockwise. 2. The first and second pharyngeal arches formed; their development was precocious among mammalian embryos in relation to somitic count. 3. The segmental pattern of the neural fold was similar to that of laboratory rodents and primates. The first fusion of the cranial neural folds took place in the occipital somite region, the second fusion in the diencephalic region, and the third at the end of the neural plate, thus leaving two neuropores in the cephalic region. 4. The timing of appearance of the optic sulcus was similar to that of human embryos but was delayed in comparison with that of laboratory rodents. 5. The heart always showed a more advanced state than that of other mammalian embryos. From the beginning of stage 9, an unpaired endocardial tube was seen in the bulbo-ventricular region, and deflection from a symmetrical appearance soon took place. 6. The differentiation of foregut was also precocious, and the thyroid and respiratory primordia appeared earlier than in other mammals. The present study emphasizes that there are considerable variations in timing and manner of morphogenesis among early mammalian embryos.

Xenopus Myf-5 marks early muscle cells and can activate muscle genes ectopically in early embryos

We have cloned a Xenopus cDNA that encodes a homologue of the human myogenic factor, Myf-5. Xenopus Myf-5 (XMyf5) transcripts first accumulate in the prospective somite region of early gastrulae. The pattern of XMyf5 expression is similar to that of the Xenopus MyoD (XMyoD) gene, except that XMyf5 transcripts are largely restricted to posterior somitic mesoderm even before any somites have formed. Transient ectopic expression of XMyf5 activates cardiac actin and XMyoD genes in animal cap cells, but does not cause full myogenesis, even in combination with XMyoD. These results suggest that XMyf5 acts together with XMyoD as one of the set of genes regulating the earliest events of myogenesis, additional factors being required for complete muscle differentiation.

Myogenin, a factor regulating myogenesis, has a domain homologous to MyoD

In this report, we describe the isolation, sequence, and initial characterization of the cDNA for the muscle-specific regulatory factor skeletal myogenin. Transfection of myogenin into the mesenchymal cell line C3H10T1/2 produces cells expressing muscle-specific markers. Myogenin is absent in undifferentiated cells, peaks, and then declines following a stimulus to differentiate, and is overexpressed in myoblasts selected with 5-bromodeoxyuridine for the overproduction of factors that regulate the decision to differentiate. High levels of myogenin transcripts are present in the myotomal region of somites at 8.5 days of gestation in the mouse. Although myogenin and MyoD are different genes, they share the myc homology domain. Myogenin and MyoD thus form part of a gene family regulating myogenesis, and together with myd may constitute a set of factors that interact to regulate the determination and differentiation of muscle cells.

Embryonic development of Xenopus studied in a cell culture system with tissue-specific monoclonal antibodies.

An in vitro microculture system of early gastrula cells of Xenopus laevis has been developed; deep layer cells from the lateral marginal zone (prospective somite region) or ventral ectoderm (prospective epidermis region) were fully dissociated, and the desired number of each (1-100) was distributed into a microculture well and cultured under appropriate conditions. When examined with the tissue-specific Mabs (Mu1 for muscle and E3 for epidermis, respectively), a substantial portion of the deep layer cells from the two regions followed their respective normal embryonic fates. It was found that reproducible cellular differentiation was dependent on the intimate reaggregation of dissociated cells and on the size of the resultant aggregate. About 20 lateral marginal zone cells were found to be sufficient, when put into a culture well, for supporting successful muscle differentiation, whereas about 100 ventral ectoderm cells were necessary for epidermal differentiation.

Formation of the dorsal root ganglia in the avian embryo: Segmental origin and migratory behavior of neural crest progenitor cells

The segmental origin and migratory pattern of neural crest cells at the trunk level of avian embryos was studied, with special emphasis on the formation of the dorsal root ganglia (DRG) which organize in the anterior half of each somite. Neural crest cells were visualized using the quail-chick marker and HNK-1 immunofluorescence. The migratory process turned out to be closely correlated with somitic development: when the somites are epithelial in structure few labeled cells were found in a dorsolateral position on the neural tube, uniformly distributed along the craniocaudal axis. Following somitic dissociation into dermomyotome and sclerotome labeled cells follow defined migratory pathways restricted to each anterior somitic half. In contrast, opposite the posterior half of the somites, cells remain grouped in a dorsolateral position on the neural tube. The fate of crest cells originating at the level of the posterior somitic half was investigated by grafting into chick hosts short segments of quail neural primordium, which ended at mid-somitic or at intersomitic levels. It was found that neural crest cells arising opposite the posterior somitic half participate in the formation of the DRG and Schwann cells lining the dorsal and ventral root fibers of the same somitic level as well as of the subsequent one, whereas those cells originating from levels facing the anterior half of a somite participate in the formation of the corresponding DRG. Moreover, crest cells from both segmental halves segregate within each ganglion in a distinct topographical arrangement which reflects their segmental origin on the neural primordium. Labeled cells which relocate from posterior into anterior somitic regions migrate longitudinally along the neural tube. Longitudinal migration of neural crest cells was first observed when the somites are epithelial in structure and is completed after the disappearance of the last cells from the posterior somitic region at a stage corresponding to the organogenesis of the DRG.