514whole and, in this respect, Silkie fowl melanoblasts do not differ from those of the quail. At this point, the question of the role played by the tissue itself arises. As those Silkie fowl organs that are not pigmented in situ were not invaded by quail pigment cells, it appears that the homing of melanoblasts is determined by environmental cues created in organs. As for the environmental cues which regulate seeding of melanoblasts, one can ask the question whether they are permissive or attractive. The heavy accumulation of melanocytes in the depth but also at the periphery of some grafts, strongly suggests that they are not only permissive but also attractive for prepigment cells. Is the definite localization of pigment cells regulated by a chemo-attractant? The hypothesis of the production of a chemo-attractant by the primary lymphoid organs (thymus and bursa of Fabricius in birds) regulating the seeding of hemopoietic cells was proposed by Le Douarin 16. This author also asks the question whether the colonization of epidermis by melanoblasts in bird embryos could be regulated by such a mechanism 2. In any case, the chemo-attractant would be widely distributed in the Silkie fowl embryo. The question also arises whether fibronectin, which plays a major role in neural crest cell migration 4, tr, is implicated in homing. As in quail embryo, neural crest derived cells, including prepigment ones, do not enter tissues even though they are fibronectinrich4; the presence of fibronectin could be a prerequisite factor but not a sufficient one to ensure melanoblast seeding. Work is in progress showing that, by the stage of melanoblast homing, all the colonized organs are fibronectin-rich.Experientia 41 (1985), Birkh/iuser Verlag, CH-4010 Basel/SwitzerlandThe broad but selective localization of melanoblasts in Silkie fowl could be of some help in studying the factor(s) playing a role in the seeding process and in the definite localization of neural crest ceils, which remains a particularly challenging problem 2. Summary. 2-13-Glucosyl-4-hydroxy-7-methoxy-l,4-benzoxazin-3-one (DIMBOA-Glc), the main hydroxamic acid from maize and wheat, and its aglucone, decreased survival of Schizaphis graminum reared on artificial diets. Both compounds were toxic for aphids and acted as feeding deterrents, at concentrations as low as 1 mM. The natural concentrations of glucosides of hydroxamic acids in wheat leaves reach up to 6 mmoles/kg fresh weight, thus falling within the range in which DIMBOA-Glc causes deleterious effects to diet-fed aphids.

Epithelial-mesenchymal transition (EMT) is a cellular process during which epithelial cells acquire mesen chymal phenotypes and behaviour following the down regulation of epithelial features. EMT is triggered in response to signals that cells receive from their micro environment. The epithelial state of the cells in which EMT is initiated is characterized by stable epithelial cell-cell junctions, apical-basal polarity and interac tions with basement membrane. During EMT, changes in gene expression and posttranslational regulation mechanisms lead to the repression of these epithelial characteristics and the acquisition of mesenchymal char acteristics. Cells then display fibroblastlike morphol ogy and cytoarchitecture, as well as increased migratory capacity. Furthermore, these now migratory cells often acquire invasive properties (Fig. 1). EMT was first described by researchers studying early embryogenesis as a programme with welldefined cellular features 1,2. It is now widely accepted that EMT occurs normally during early embryonic development, to enable a variety of morphogenetic events, as well as later in development and during wound healing in adults.

During neural crest ontogeny, an epithelial to mesenchymal transition is necessary for cell emigration from the dorsal neural tube. This process is likely to involve a network of gene activities, which remain largely unexplored. We demonstrate that N-cadherin inhibits the onset of crest delamination both by a cell adhesion-dependent mechanism and by repressing canonical Wnt signaling previously found to be necessary for crest delamination by acting downstream of BMP4. Furthermore, N-cadherin protein,but not mRNA, is normally downregulated along the dorsal tube in association with the onset of crest delamination, and we find that this process is triggered by BMP4. BMP4 stimulates cleavage of N-cadherin into a soluble cytoplasmic fragment via an ADAM10-dependent mechanism. Intriguingly, when overexpressed, the cytoplasmic N-cadherin fragment translocates into the nucleus, stimulates cyclin D1 transcription and crest delamination, while enhancing transcription of β-catenin. CTF2 also rescues the mesenchymal phenotype of crest cells in ADAM10-inhibited neural primordia. Hence, by promoting its cleavage, BMP4 converts N-cadherin inhibition into an activity that is likely to participate, along with canonical Wnt signaling, in the stimulation of neural crest emigration.

SUMMARYColonization of trunk neural crest derivatives in avians follows a ventral to dorsal order beginning with sympathetic ganglia, Schwann cells, sensory ganglia and finally melanocytes. Continuous crest emigration underlies this process, which is accounted for by a progressive ventral to dorsal relocation of neural tube progenitors prior to departure. This causes a gradual narrowing of FoxD3, Sox9 and Snail2 expression domains in the dorsal tube that characterize the neural progenitors of the crest and these genes are no longer transcribed by the time melanoblasts begin emigrating. Consistently, the final localization of crest cells can be predicted from their relative ventrodorsal position within the premigratory domain or by their time of delamination. Thus, a dynamic spatiotemporal fate map of crest derivatives exists in the dorsal tube at flank levels of the axis with its midline region acting as a sink for the ordered ingression and departure of progenitors. Furthermore, discrete lineage analysis of the dorsal midline at progressive stages generated progeny in single rather than multiple derivatives, revealing early fate restrictions. Compatible with this notion, when early emigrating 'neural' progenitors were diverted into the lateral 'melanocytic' pathway, they still adopted neural traits, suggesting that initial fate acquisition is independent of the migratory environment and that the potential of crest cells prior to emigration is limited.

Delamination of premigratory neural crest cells depends on a balance between BMP/noggin and on successful G1/S transition. Here, we report that BMP regulates G1/S transition and consequent crest delamination through canonical Wnt signaling. Noggin overexpression inhibits G1/S transition and blocking G1/S abrogates BMP-induced delamination; moreover, transcription of Wnt1 is stimulated by BMP and by the developing somites, which concomitantly inhibit noggin production. Interfering with β-catenin and LEF/TCF inhibits G1/S transition, neural crest delamination and transcription of various BMPdependent genes, which include Cad6B, Pax3 and Msx1, but not that of Slug, Sox9 or FoxD3. Hence, we propose that developing somites inhibit noggin transcription in the dorsal tube, resulting in activation of BMP and consequent Wnt1 production. Canonical Wnt signaling in turn stimulates G1/S transition and generation of neural crest cell motility independently of its proposed role in earlier neural crest specification. Research article 5328 NC delamination and overexpression of β-catenin rescues NC delamination in noggin-inhibited neural primordia. Thus, BMP-dependent Wnt signaling is necessary for NC delamination. Materials and methods EmbryosChick (Gallus gallus) and quail (Coturnix coturnix Japonica) eggs were from commercial sources.In ovo grafting of noggin-secreting cells CHO cells producing Xenopus noggin and dhfr-CHO control cells were grown as previously described (Lamb et al., 1993; SelaDonenfeld and Kalcheim, 2002). To establish confluent monolayers, cells were replated on eight-well chamber slides (Lab-Tek), grown in serum-containing medium for 2 days and then transferred to serumfree medium until explantation of neural primordia. For grafting purposes, confluent cultures were harvested and pelleted. The vitelline membrane of 16-to 20-somite stage chick embryos was removed. A slit was performed along the dorsal edge of the neural tube at levels corresponding either to the rostral segmental plate and two last formed epithelial somites, or along the caudal half of the segmental plate. Concentrated cell suspensions were applied on top of the neural tube with a micropipette. Cell implants were performed under an Olympus dissecting microscope with a ϫ40 total magnification. Embryos were further incubated for 8-10 or 20-24 hours and then fixed for immunocytochemistry and/or in situ hybridization.

The dermomyotome develops into myotome and dermis. We previously showed that overall growth of the dermomyotome and myotome in the mediolateral direction occurs in a uniform pattern. While myofibers arise from all four dermomyotome lips, the dermis derives from both medial and lateral halves of the dermomyotome sheet. Here we mapped the fate of this epithelial sheet by analyzing cell types that arise from its central region. We found that these precursors give rise not only to dermis, as expected, but also to a population of proliferating progenitors in the myotome that maintain expression of PAX7,PAX3 and FREK. Given this dual fate, we asked whether single dermomyotome precursors generate both dermal and mitotic myoblast precursors, or alternatively, whether these cell types derive from distinct epithelial founders. Inovo clonal analysis revealed that single dermomyotome progenitors give rise to both derivatives. This is associated with a sharp change in the plane of cell division from the young epithelium, in which symmetrical divisions occur parallel to the mediolateral plane of the dermomyotome, to the dissociating dermomyotome, in which cell divisions become mostly perpendicular. Taken together with clonal analysis of the dermomyotome sheet,this suggests that a first stage of progenitor self-renewal, accounting for dermomyotomal expansion, is followed by fate segregation, which correlates with the observed shift in mitotic spindle orientation.

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Neuropilin-1 (np1) and neuropilin-2 (np2) are receptors for class-3 semaphorins and for several isoforms of VEGF. We have cloned and characterized two chick isoforms of np2 cDNA. Expression patterns of np1, np2, and ephrin-B2 were compared in the developing vascular system of 24-72 h old chick embryos. We show for the first time that np2 is expressed in blood vessels in vivo from the earliest stages of their formation. In contrast to ephrin-B2, both np1 and np2 are expressed in blood islands of 24 h old chick embryos. At 48-72 h, np1 expression is localized preferentially in arteries with an expression pattern that resembles that of ephrin-B2. In contrast, np2 is expressed preferentially in veins. Thus, neuropilins may play a role in determining the arterial or venous identity of blood vessels.