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.     

Slow muscle regulates the pattern of trunk neural crest migration in zebrafish

In avians and mice, trunk neural crest migration is restricted to the anterior half of each somite. Sclerotome has been shown to play an essential role in this restriction; the potential role of other somite components in specifying neural crest migration is currently unclear. By contrast, in zebrafish trunk neural crest, migration on the medial pathway is restricted to the middle of the medial surface of each somite. Sclerotome comprises only a minor part of zebrafish somites, and the pattern of neural crest migration is established before crest cells contact sclerotome cells, suggesting other somite components regulate the pattern of zebrafish neural crest migration. Here, we use mutants to investigate which components regulate the pattern of zebrafish trunk neural crest migration on the medial pathway. The pattern of trunk neural crest migration is aberrant in spadetail mutants that have very reduced somitic mesoderm, in no tail mutants injected with spadetail morpholino antisense oligonucleotides that entirely lack somitic mesoderm and in somite segmentation mutants that have normal somite components but disrupted segment borders. Fast muscle cells appear dispensable for patterning trunk neural crest migration. However, migration is abnormal in Hedgehog signaling mutants that lack slow muscle cells, providing evidence that slow muscle cells regulate the pattern of trunk neural crest migration. Consistent with this idea, surgical removal of adaxial cells, which are slow muscle precursors, results in abnormal patterning of neural crest migration; normal patterning can be restored by replacing the ablated adaxial cells with ones transplanted from wild-type embryos.     

Distribution and migration pathways of HNK-1-immunoreactive neural crest cells in teleost fish embryos.

Whole mounts and cross-sections of embryos from three species of teleost fish were immunostained with the HNK-1 monoclonal antibody, which recognizes an epitope on migrating neural crest cells. A similar distribution and migration was found in all three species. The crest cells in the head express the HNK-1 epitope after they have segregated from the neural keel. The truncal neural crest cells begin to express the epitope while they still reside in the dorsal region of the neural keel; this has not been observed in other vertebrates. The cephalic and anterior truncal neural crest cells migrate under the ectoderm; the cephalic cells then enter into the gill arches and the anterior truncal cells into the mesentery of the digestive tract where they cease migration. These cephalic and anterior trunk pathways are similar to those described in Xenopus and chick. The neural crest cells of the trunk, after segregation, accumulate in the dorsal wedges between the somites, however, unlike in chick and rat, they do not migrate in the anterior halves of the somites but predominantly between the neural tube and the somites, the major pathway observed in carp and amphibians; some cells migrate over the somites. The HNK-1 staining of whole-mount embryos revealed a structure resembling the Rohon-Beard and extramedullary cells, the primary sensory system in amphibians. Such a system has not been described in fish.     

T-cadherin expression alternates with migrating neural crest cells in the trunk of the avian embryo.

Trunk neural crest cells and motor axons move in a segmental fashion through the rostral (anterior) half of each somitic sclerotome, avoiding the caudal (posterior) half. This metameric migration pattern is thought to be caused by molecular differences between the rostral and caudal portions of the somite. Here, we describe the distribution of T-cadherin (truncated-cadherin) during trunk neural crest cell migration. T-cadherin, a novel member of the cadherin family of cell adhesion molecules was selectively expressed in the caudal half of each sclerotome at all times examined. T-cadherin immunostaining appeared graded along the rostrocaudal axis, with increasing levels of reactivity in the caudal halves of progressively more mature (rostral) somites. The earliest T-cadherin expression was detected in a small population of cells in the caudal portion of the somite three segments rostral to last-formed somite. This initial T-cadherin expression was observed concomitant with the invasion of the first neural crest cells into the rostral portion of the same somite in stage 16 embryos. When neural crest cells were ablated surgically prior to their emigration from the neural tube, the pattern of T-cadherin immunoreactivity was unchanged compared to unoperated embryos, suggesting that the metameric T-cadherin distribution occurs independent of neural crest cell signals. This expression pattern is consistent with the possibility that T-cadherin plays a role in influencing the pattern of neural crest cell migration and in maintaining somite polarity.     

Redefining the head-trunk interface for the neural crest

The head-trunk interface lies at the occipito-cervical boundary, which corresponds to the somite 5/6 level. Previous studies have demonstrated that neural crest cells also behave differently either side of this boundary and that this may be due to intrinsic differences between cranial and trunk crest. However, it is also possible that some of the observed differences between cranial and trunk crest are assigned by environmental cues. We have therefore scrutinised the behaviour of the neural crest cells generated either side of the occipito-cervical boundary in chick and, interestingly, find that both behave in a truncal fashion by traversing the anterior half of their adjacent somites. Furthermore, although not previously described, we find that transient DRGs form opposite somites 4 and 5. Crest cells produced anterior of the somite 3/4 boundary avoid the somites and behave in a non-truncal fashion; these cells populate the pharyngeal arches, and thus contribute to the developing head. We have further shown, via somite transplantations, that differential behaviour of the posterior versus anterior occipital crest is assigned by the somites. If somites 1 to 3 are replaced by trunk somites, then the anterior occipital crest will behave in a truncal fashion by invading the somites. Correspondingly, if these anterior occipital somites are transplanted in place of trunk somites, they perturb the migration of trunk crest. Thus, for the neural crest, the head-trunk interface does not lie at the occipito-cervical boundary, but rather lies at the somite 3/4 level and is defined by the somites. The fact that this boundary lies at the somite 3/4 level in chick is significant as it reflects the more ancient posterior occipital boundary; in fish, only the first three somites contribute to the occipital bone.     

Neural crest cell migratory pathways in the trunk of the chick embryo

Neural crest cells migrate during embryogenesis to give rise to segmented structures of the vertebrate peripheral nervous system: namely, the dorsal root ganglia and the sympathetic chain. However, neural crest cell arise from the dorsal neural tube where they are apparently unsegmented. It is generally agreed that the somites impose segmentation on migrating crest cells, but there is a disagreement about two basic questions: exactly pathways do neural crest cells use to move through or around somites, and do neural crest cells actively migrate or are they passively dispersed by the movement of somite cells? The answers to both questions are critically important to any further understanding of the mechanisms underlying the precise distribution of the neural crest cells that develop into ganglia. We have done an exhaustive study of the locations of neural crest cells in chick embryos during early stages of their movement, using antibodies to neural crest cells (HNK-1), to neural filament-associated protein in growing nerve processes (E/C8), and to the extracellular matrix molecule laminin. Our results show that Some neural crest cells invade the extracellular space between adjacent somites, but the apparent majority move into the somites themselves along the border between the dermatome/myotome (DM) and the sclerotome. Neural crest cells remain closely associated with the anterior half of the DM of developing somites as they travel, suggesting that the basal lamina of the DM may be used as a migratory substratum. Supporting this idea is our observation that the development of the DM basal lamina coincides in time and location with the onset of crest migration through the somite. The leading front of neural crest cells advance through the somite while the length of the DM pathway remains constant, suggesting active locomotion, at least in this early phase of development. Neural crest cells leave the DM at a later stage of development to associate with the dorsal aorta, where sympathetic ganglia form, and to associate with newly emerging fibers of the ventral root nerve, where they presumably give rise to neuronal supportive cells. Thus we propose that the establishment of the segmental pattern of the peripheral ganglia and nerves depends on the timely development of appropriate substrata to guide and distribute migrating neural crest cells during the early stages of embryogenesis.     

The distribution of tenascin coincides with pathways of neural crest cell migration

The distribution of the extracellular matrix (ECM) glycoprotein, tenascin, has been compared with that of fibronectin in neural crest migration pathways of Xenopus laevis, quail and rat embryos. In all species studied, the distribution of tenascin, examined by immunohistochemistry, was more closely correlated with pathways of migration than that of fibronectin, which is known to be important for neural crest migration. In Xenopus laevis embryos, anti-tenascin stained the dorsal fin matrix and ECM along the ventral route of migration, but not the ECM found laterally between the ectoderma and somites where neural crest cells do not migrate. In quail embryos, the appearance of tenascin in neural crest pathways was well correlated with the anterior-to-posterior wave of migration. The distribution of tenascin within somites was compared with that of the neural crest marker, HNK-1, in quail embryos. In the dorsal halves of quail somites which contained migrating neural crest cells, the predominant tenascin staining was in the anterior halves of the somites, codistributed with the migrating cells. In rat embryos, tenascin was detectable in the somites only in the anterior halves. Tenascin was not detectable in the matrix of cultured quail neural crest cells, but was in the matrix surrounding somite and notochord cells in vitro. Neural crest cells cultured on a substratum of tenascin did not spread and were rounded. We propose that tenascin is an important factor controlling neural crest morphogenesis, perhaps by modifying the interaction of neural crest cells with fibronectin.     

Distribution of latex beads and retinal pigment epithelial cells along the ventral neural crest pathway

Neural crest cells migrate along defined pathways in the trunk of avian embryos. Previous studies have demonstrated that crest-derived pigment cells migrated ventrally after injection onto the ventral neural crest pathway (M. E. Bronner-Fraser and A. M. Cohen, 1980, Develop. Biol.77, 130–141). In the present study, latex polystyrene beads and retinal pigment epithelial (RPE) cells were injected onto the ventral pathway in order to probe the environment along this migratory route. Although the RPE cells are nonmotile and not derived from the neural crest, they also translocated ventrally. Thus, factors independent of active migration may affect the localization of RPE cells (and endogenous crest cells). To test this possibility, latex polystyrene beads were injected onto the ventral pathway. Three types of beads were used: (a) uncoated latex beads; (b) latex beads coated with bovine serum albumin (BSA-beads); and (c) latex beads coated with fibronectin (FN-beads). Uncoated and BSA-beads distributed along the ventral pathway similarly to RPE cells and endogenous crest cells. However, FN-beads remained near the site of implantation and did not move ventrally. The results suggest (1) that molecules like fibronectin on the cell surface might serve as a recognition mechanism that prevents entrance onto the ventral pathway; and (2) that crest cell localization may, in part, be influenced by a driving force imparted by the embryonic environment.     

Pathways and mechanisms of avian trunk neural crest cell migration and localization

Crest cells individualized at the dorsal border of the neural tube, while they became surrounded by a fibronectin-rich matrix. Crest cells initiated their migration between the basement membranes of the neural tube and the ectoderm. In the vagal region, crest cells migrated in a fibronectin-rich environment between the ectoderm and the dermomyotome, very rapidly reaching the apex of the pharynx. In the trunk region, crest cells opposite the bulk of the somite accumulated at the junction between the somite, the neural tube, and the ectoderm; they resumed their migration at the onset of the dissociation of the somite into dermomyotome and sclerotome. Migration occurred more ventrally along the neural tube; nevertheless, the formation of the rapidly expanding sclerotome prevented crest cells from reaching the paranotochordal region. Thereafter, crest cells accumulated between the neural tube, the dermomyotome, and the sclerotome, where ultimately they formed the dorsal root ganglia. In contrast, cells opposite the intersomitic space did not encounter these obstacles and utilized a narrow pathway formed between the basement membranes of the two adjacent somites. This pathway allowed crest cells to reach the most ventral regions of the embryo very rapidly; they accumulated along the aorta to form the aortic plexuses, the adrenal medulla, and the sympathetic ganglia. The basic features of the migration pathways are (1) a strict delimitation by the fibronectin-rich basement membranes of the surrounding tissues, (2) a formation of space concomitant with the migration of crest cells, (3) a transient existence: continued migration is correlated with the presence of fibronectin, whereas cessation is correlated with its focal disappearance. The crest cells are characterized by their inability to traverse basement membranes and penetrate within tissues. We propose that the combination of active proliferation, unique motility properties, and the presence of narrow pathways are the major mechanisms ensuring correct directionality. Morphologically defined transient routes of migration along with developmentally regulated changes in the extracellular matrix and in the adhesive properties of crest cells are most probably involved in their stabilization in defined territories and their aggregation into ganglia.     

Quantitative distribution of chick neural crest cells during gangliogenesis

Summary We have quantitated the distribution of chick neural crest cells after they have completed early migration and are aggregating into ganglia. Variables tested for an influence on the distribution of cells include stage, level of somites, position in each of the primary body axes, and individual embryo. The 11th–15th cervical somites of embryos at stages 30, 35, and 40 somites (s) incubated for 2.5, 3.0, and 3.5 days were labeled with antibody to HNK-1 to detect neural crest cells, and doubly labeled with antibody to HNK-1 and to the 150 kD neurofilament subunit to detect neural crest-derived neurons. Significantly more neural crest cells appear at older stages, but cells are uniformly distributed among the 11th–15th somites at any given stage. Significant differences in the total number of neural crest cells among three embryos sampled at the same stage indicate that the number of cells is independent of the staging series used. As early as the 35 s stage about one-third of the neural crest cells throughout the somite exhibit NF staining. At the 40 s stage, doubly labeled NF cells, as well as HNK-1 labeled cells, aggregate in a circumscribed portion of the mediolateral axis to form presumptive sensory ganglia in the dorsal region of the somites. Also at 40 s a wave of cell aggregation into sympathetic ganglia proceeds anteroposteriorly along the ventral border of the somitic mesenchyme. The results show a sequence of phenotypic expression beginning with neurofilament antigen, then ganglionic aggregation, and finally, in the case of sympathetic neurons, catecholamine transmitter.     

Pathways of trunk neural crest cell migration in the mouse embryo as revealed by vital dye labelling

Analysis of neural crest cell migration in the mouse has been difficult due to the lack of reliable cell markers. Recently, we found that injection of DiI into the chick neural tube marks premigratory neural crest cells whose endfeet are in contact with the lumen of the neural tube (Serbedzija et al. Development 106, 809-819 (1989)). In the present study, this technique was applied to study neural crest cell migratory pathways in the trunk of the mouse embryo. Embryos were removed from the mother between the 8th and the 10th days of development and DiI was injected into the lumen of the neural tube. The embryos were then cultured for 12 to 24 h, and analyzed at the level of the forelimb. We observed two predominant pathways of neural crest cell migration: (1) a ventral pathway through the rostral portion of the somite and (2) a dorsolateral pathway between the dermamyotome and the epidermis. Neural crest cells were observed along the dorsolateral pathway throughout the period of migration. The distribution of labelled cells along the ventral pathway suggested that there were two overlapping phases of migration. An early ventrolateral phase began before E9 and ended by E9.5; this pathway consisted of a stream of cells within the rostral sclerotome, adjacent to the dermamyotome, that extended ventrally to the region of the sympathetic ganglia and the dorsal aorta.(ABSTRACT TRUNCATED AT 250 WORDS)     

Morphogenesis of sclerotome and neural crest in avian embryos

Summary The distribution of sclerotome and neural crest cells of avian embryos was studied by light and electron microscopy. Sclerotome cells radiated from the somites towards the notochord, to occupy the perichordal space. Neural crest cells, at least initially, also entered cell-free spaces. At the cranial somitic levels they moved chiefly dorsal to the somites, favouring the rostral part of each somite. These cells did not approach the perichordal space. More caudally (i.e. trunk levels), neural crest cells initially moved ventrally between the somites and neural tube. Adjacent to the caudal half of each somite, these cells penetrated no further than the myosclerotomal border, but opposite the rostral somite half, they were found next to the sclerotome almost as far ventrally as the notochord. However, they did not appear to enter the perichordal space, in contrast to sclerotome cells.When tested in vitro, sclerotome cells migrated towards notochords co-cultured on fibronectin-rich extracellular material, and on collagen gels. In contrast, neural crest cells avoided co-cultured notochords. This avoidance was abolished by inclusion of testicular hyaluronidase and chondroitinase ABC in the culture medium, but not by hyaluronidase from Streptomyces hyalurolyticus. The results suggest that sclerotome and neural crest mesenchyme cells have a different distribution with respect to the notochord, and that differential responses to notochordal extracellular material, possibly chondroitin sulphate proteoglycan, may be responsible for this.     

Effects of different fragments of the fibronectin molecule on latex bead translocation along neural crest migratory pathways

Previous studies from this laboratory have utilized latex beads as probes of embryonic migratory pathways. After microinjection into embryos at the time of neural crest migration, uncoated latex polystyrene beads were found to translocate to ventral sites and to settle in the vicinity of endogenous neural crest derivatives. However, latex beads coated with fibronectin did not translocate ventrally, but remained associated with cells surrounding the implantation site. Fibronectin is a large glycoprotein with a variety of biological activities and multiple binding domains. Here, the binding activities which might be responsible for immobilization of the fibronectin-coated beads are examined. Latex beads were coated with three types of fragments of the fibronectin molecule representing different functional domains: (i) a 66-kDa fragment containing collagen-binding activity; (ii) a mixture of 45- and 32-kDa fragments containing heparin-binding activity; and (iii) a 120-kDa fragment containing cell-binding activity. The beads coated with fibronectin fragments were injected into the newly formed trunk somites of avian embryos. After injection, beads coated with either the heparin- or the collagen-binding domain translocated ventrally and distributed analogously to uncoated latex beads. In contrast, the majority of beads coated with the fibronectin cell-binding domain did not translocate but remained associated with dermamyotomal cells surrounding the injection site. The cell-binding fragment, however, was not as effective as the intact fibronectin molecule in preventing translocation of the beads. The results suggest that the cell-binding domain is primarily responsible for restriction of fibronectin beads from the ventral neural crest pathway. Because intact fibronectin is more effective at immobilizing beads than is the cell-binding fragment, other binding domains of fibronectin, more efficient coating with intact fibronectin, or crosslinking of intact fibronectin molecules may also play some role in immobilization of the beads at the implantation site.     

Perturbation of cranial neural crest migration by the HNK-1 antibody

The HNK-1 antibody recognizes a carbohydrate moiety that is shared by a family of cell adhesion molecules and is also present on the surface of migrating neural crest cells. Here, the effects of the HNK-1 antibody on neural crest cells were examined in vitro and in vivo. When the HNK-1 antibody was added to neural tube explants in tissue culture, neural crest cells detached from laminin substrates but were unaffected on fibronectin substrates. In order to examine the effects of the HNK-1 antibody in vivo, antibody was injected lateral to the mesencephalic neural tube at the onset of cranial neural crest migration. The injected antibody persisted for approximately 16 hr on the injected side of the embryo and appeared to be most prevalent on the surface of neural crest cells. Embryos fixed within the first 24 hr after injection of HNK-1 antibodies (either whole IgMs or small IgM fragments) showed one or more of the following abnormalities: (1) ectopic neural crest cells external to the neural tube, (2) an accumulation of neural crest cell volume on the lumen of the neural tube, (3) some neural tube anomalies, or (4) a reduction in the neural crest cell volume on the injected side. The ectopic cells and neural tube anomalies persisted in embryos fixed 2 days postinjection. Only embryos having 10 or less somites at the time of injection were affected, suggesting a limited period of sensitivity to the HNK-1 antibody. Control embryos injected with a nonspecific antibody or with a nonblocking antibody against the neural cell adhesion molecule (N-CAM) were unaffected. Previous experiments from this laboratory have demonstrated than an antibody against integrin, a fibronectin and laminin receptor caused defects qualitatively similar to those resulting from HNK-1 antibody injection (M. Bronner-Fraser, J. Cell Biol., 101, 610, 1985). Coinjection of the HNK-1 and integrin antibodies resulted in a greater percentage of affected embryos than with either antibody alone. The additive nature of the effects of the two antibodies suggests that they act at different sites. These results demonstrate that the HNK-1 antibody causes abnormalities in cranial neural crest migration, perhaps by perturbing interactions between neural crest cells and laminin substrates.     

Latex beads as probes of a neural crest pathway: effects of laminin, collagen, and surface charge on bead translocation

In the trunk region of avian embryos, neural crest cells migrate along two pathways: dorsally just under the ectoderm, and ventrally between the neural tube and the somites. Previous work from this laboratory has shown that uncoated latex beads are able to translocate along the ventral neural crest pathway after injection into young embryos; however, beads coated with fibronectin are restricted from the ventral route ( Bronner -Fraser, M.E., 1982, Dev. Biol., 91: 50-63). Here, we extend these observations to determine the effects of other macromolecules on bead distribution. The data show that laminin-coated beads, like fibronectin-coated beads, are restricted from the ventral pathway. In contrast, beads coated with type I collagen translocate ventrally after injection. Because macromolecules have characteristic charge properties, changes in surface charge caused by coating the beads may confound interpretation of the results. Electrostatic effects on bead movement were examined by coating the latex beads with polyamino acids in order to predictably alter the initial surface charge. The surface charge before injection was measured for beads coated with amino acid polymers or with various biologically important macromolecules; the subsequent translocation ability of these beads was then monitored in the embryo. Polylysine-coated beads (positively charged) were restricted from the ventral pathway as were fibronectin and laminin-coated beads, even though fibronectin and laminin beads were both negatively charged. In contrast, polytyrosine -coated beads ( neutrally charged) translocated ventrally as did negatively charged collagen-coated or uncoated beads. The results demonstrate that no correlation exists between the charge properties on the latex bead surface and their subsequent ability to translocate along the ventral pathway. Therefore, an adhesion mechanism independent of surface charge effects must explain the restriction or translocation of latex beads on a neural crest pathway.     

Initial migration and distribution of the cardiac neural crest in the avian embryo: an introduction to the concept of the circumpharyngeal crest

The distribution and migration of the cardiac neural crest was studied in chick embryos from stages 11 to 17 that were immunochemically stained in whole-mount and sectioned specimens with a monoclonal antibody, HNK-1. The following results were obtained: 1) The first phase of the migration in the cardiac crest follows the dorsolateral pathway beneath the ectoderm. 2) In the first site of arrest, the cardiac crest forms a longitudinal mass of neural-crest cells, called in the present study, the circumpharyngeal crest; this mass is located dorsolateral to the dorsal edge of the pericardium (pericardial dorsal horn) where splanchnic and somatic lateral mesoderm meet. 3) A distinctive strand of neural-crest cells, called the anterior tract, arises from the mid-otic level and ends in the circumpharyngeal crest. 4) By stage 16, after the degeneration of the first somite, another strand of neural-crest cells, called the posterior tract, appears dorsal to the circumpharyngeal crest. It forms an arch-like pathway along the anterior border of the second somite. 5) The seeding of the pharyngeal ectomesenchyme takes place before the formation of pharyngeal arches in the postotic area, i.e., the crest cells are seeded into the lateral body wall ventrally from the circumpharyngeal crest; and, by the ventral-ward regression of the pericardial dorsal horn, lateral expansion of pharyngeal pouch, and caudal regression of the pericardium, the crest cell population is pushed away by the pharyngeal pouch. Thus the pharyngeal arch ectomesenchyme is segregated. 6) By stage 14, at the occipital somite level, ventrolateral migration of the neural crest is observed within the anterior half of each somite. Some of these crest cells are continuous with the caudal portion of the circumpharyngeal crest. An early contribution to the enteric neuroblasts is apparent in this area.     

A spatial and temporal analysis of dorsal root and sympathetic ganglion formation in the avian embryo

The present study explores the formation of the dorsal root and sympathetic ganglia in the trunk of the avian embryo. Particular emphasis was given to the timing of gangliogenesis and the relative positions of the neural crest-derived ganglia with respect to the somites. Neural crest cells and their derivatives were recognized by the HNK-1 antibody. The time at which neural crest cell coalesced to form ganglia was assessed by the state of cellular aggregation. The state of ganglionic differentiation was assessed by the expression of neurofilament proteins and the neural cell adhesion molecule (N-CAM). At the level of the 15th somite, neural crest cells were observed in the rostral half of the somite at stage 15, during active neural crest migration, and occupied the rostral two-thirds of the somite at progressive stages. HNK-1 positive cells appeared to be organized in three to four streams of cells oriented mediolaterally and dorsoventrally. The dorsal root ganglia and sympathetic ganglia were first detectable at stages 20 and 21, respectively. Both ganglionic rudiments were aligned with the rostral portion of the somite. The dorsal root ganglia occupied the rostral two-thirds of each somite, whereas cells in the sympathetic ganglia occupied a region corresponding to approximately one-third of each somite. At the time of condensation of the dorsal root ganglia, abundant neurofilament staining was observed within the ganglia. However, no N-CAM immunoreactivity was detected until three stages later at stage 23. In contrast, the sympathetic ganglia demonstrated both neurofilament and N-CAM immunoreactivity at the time of condensation. The observation that both dorsal root and sympathetic ganglia form in register with the rostral portion of somite suggests that cues localized at these axial levels, perhaps within the rostral somite, may influence the position where neural crest cells condense to form ganglia. In sensory ganglia, N-CAM expression does not correlate with the onset of gangliogenesis, suggesting that molecules other than N-CAM may play an important role in the aggregation of some neuronal populations.     

The expression of tenascin by neural crest cells and glia.

The extracellular matrix glycoprotein tenascin is concentrated in both the embryo and adult in regions where cell motility is taking place. For example, during avian neural crest morphogenesis tenascin is concentrated in the rostral half of the sclerotome, precisely where the neural crest cells themselves are found. Previous in vitro studies indicated that somite cells were the source of this tenascin, implying a role for tenascin in directing the ventral migration of neural crest cells and thus the establishment of the periodic arrangement of the PNS. In this study, we have used a cDNA probe to identify the source of tenascin found along the pathways of the neural crest using in situ hybridization. In tissue sections, individual cells found along the neural crest migratory pathways, both before entering the somites and within the somites, are strongly labelled by the tenascin cDNA. In vitro neural crest cells are more strongly labelled with the tenascin probe than somite cells. Finally, western blotting has been used to identify tenascin in culture medium conditioned by neural crest cells. This indicates that neural crest cells themselves are the source of much of the tenascin found lining their migratory pathways, and that interactions with somite cells may not be needed to induce the expression of tenascin. We have also studied the distribution of tenascin mRNA in the developing spinal cord and spinal ganglia. At embryonic days 7 and 10, tenascin cDNA hybridizes within cells that appear to be migrating from the ependymal layer to the white matter, as well as within cells in the dorsal roots.(ABSTRACT TRUNCATED AT 250 WORDS)     

Environmental influences on neural crest cell migration

Neural crest cells migrate extensively and interact with numerous tissues and extracellular matrix components during their movement. Cell marking techniques have shown that neural crest cells in the trunk of the avian embryo migrate through the anterior, but not posterior, half of each sclerotome and avoid the region around the notochord. A possible mechanism to account for this migratory pattern is that neural crest cells may be inhibited from entering the posterior sclerotome and the perinotochordal space. Thus, interactions with other tissue may prescribe the pattern of neural crest cell migration in the trunk. In contrast, interactions between neural crest cells and the extracellular matrix may mediate the primary interactions controlling neural crest cells migration in the head region. © 1993 John Wiley & Sons, Inc.