The quail-chick chimera system (see Box 1) has been used over the years to establish a fate map of the NC along the neural axis. These studies have shown that melanocytes arise from the entire length of the NC in higher vertebrates, whereas mesectodermal derivatives originate only from the cephalic NC region. NC-derived cells that contribute to the PNS and ENS arise only from some areas of the neural axis (Fig. 1).Using the quail-chick system, well-defined areas of the NC have been exchanged to assess NCC plasticity (see Fig. 1). For example, in one study, the vagal region of the NC (which is located between somites 1 and 7, and gives rise to the enteric ganglia) was exchanged with NC from between somites 18 and 24 (which gives rise to the adrenal medulla and sympathetic ganglia) (Le Douarin and Teillet, 1974). This swap resulted Review 4638 in the normal colonisation of the suprarenal gland and sympathetic ganglia by NCCs fated to colonise the gut. However, although the adrenomedullary trunk NCCs invaded the pre-umbilical gut wall and differentiated into normal enteric plexuses, they failed to reach the post-umbilical bowel .This experimental system has since been used together with various molecular markers, such as the Schwann cell myelin protein (SMP), which is present on Schwann cells but not on other PNS and ENS glial cells, to allow a more refined analysis of NCC plasticity. These studies have shown that NCC differentiation into a specific type of glia depends upon the environment in which they develop (Dulac et al., 1988;Dulac and Le Douarin, 1991; Cameron-Curry et al., 1993). Similarly, the differentiation of the various types of autonomic neurons varies according to the milieu in which they differentiate (for reviews, see Le Douarin, 1982;Le Douarin and Kalcheim, 1999).The conclusion of these heterotopic grafting experiments was that the fate of the NCCs that form the PNS and ENS is not fully determined before these cells migrate, but instead remains plastic until they receive differentiation signals at the end of, and possibly during, their migration. This finding raised the issue of whether all the precursors of PNS ganglion cells became fully differentiated and/or committed soon after reaching their sites of arrest, or whether some remained as quiescent undifferentiated cells. This was explored in the experiments discussed in the following section. Undifferentiated precursors in PNS gangliaTo investigate the developmental potentials of PNS ganglion cells, fragments of sensory and autonomic ganglia from quail embryos, taken from embryonic day (E) 4 up to the end of the incubation period, were implanted into NCC migration pathways of E2 chick hosts when their own NCCs were migrating. The grafted neurons themselves died (probably because the necessary survival factors are not present in the younger host). However, the non-neuronal cells of implanted ganglia migrated and homed to host sensory and autonomic ganglia, where they differentiated into the types of neurons and glia corresponding to their novel...

Fgf8 exerts a strong effect on the mesenchymal cells of neural crest (NC) origin that are fated to form the facial skeleton. Surgical extirpation of facial skeletogenic NC domain (including mid-diencephalon down through rhombomere 2), which does not express Hox genes, results in the failure of facial skeleton development and inhibition of the closure of the forebrain neural tube, while Fgf8 expression in the telencephalon and in the branchial arch (BA) ectoderm is abolished. We demonstrate here that (i) exogenous FGF8 is able to rescue facial skeleton development by promoting the proliferation of NC cells from a single rhombomere, r3, which in normal development contributes only marginally to mesenchyme of BA1, and (ii) expression of Fgf8 in forebrain and in BA ectoderm is subjected to signal(s) arising from NC cells, thus showing that the development of cephalic NC-derived structures depends on FGF8 signaling, which is itself triggered by the NC cells.cephalic neural crest ͉ facial skeleton ͉ forebrain ͉ regeneration ͉ quail-chick chimeras I n vertebrates, the skeleton and connective components of the face are derived from the cephalic neural crest (NC), which can be divided into two domains. The first, a rostral domain, in which no Hox genes are expressed, extends from the presumptive level of the epiphysis down through the second rhombomere (r2); it yields the cartilages and membrane bones of the face (Fig. 1 A). It is referred to here as the facial skeletogenic NC (FSNC). The second, posterior domain (from r4 through r8) generates part of the hyoid cartilages and does not form any membrane bone. In this posterior domain of the NC, Hox genes of the four first paralogous groups are expressed both in neural tube and NC (1-3). A few NC cells (NCC) from r3 contribute to both domains. However, this contribution to branchial arches (BAs) is very small because most r3-derived NCC are undergoing apoptosis (4, 5).When the entire Hox-negative domain of the NC is removed, no facial structures develop, meaning that Hox-expressing NCC do not substitute for the Hox-negative ones (6, 7). In contrast, any fragment of the Hox-negative crest, grafted in the anterior cephalic region following the ablation of the FSNC, can regenerate a normal face (6). Thus, the Hox-negative crest behaves as an equivalence group showing that, at the early stages, the crest cells themselves do not possess the information to construct the specific bones and cartilages that constitute the facial skeleton. The ventrolateral endoderm of the foregut is able to provide the NCC with the information necessary for patterning the facial skeleton and also the hyoid cartilage (6,8). Later in development of the facial structures, the ectoderm of the facial process also participates in the final patterning of the beak (9, 10).The investigations reported here were prompted by the observation that removal of the FSNC resulted in a dramatic decrease of Fgf8 expression in the forebrain anlage, as well as in the BA ectoderm ( Fig. 1 B-E). This was followed by the t...

Studies carried out in the avian embryo and based on the construction of quail-chick chimeras have shown that most of the skull and all the facial and visceral skeleton are derived from the cephalic neural crest (NC). Contribution of the mesoderm is limited to its occipital and (partly) to its otic domains. NC cells (NCCs) participating in membrane bones and cartilages of the vertebrate head arise from the diencephalon (posterior half only), the mesencephalon and the rhombencephalon. They can be divided into an anterior domain (extending down to r2 included) in which genes of the Hox clusters are not expressed (Hox-negative skeletogenic NC) and a posterior domain including r4 to r8 in which Hox genes of the four first paraloguous groups are expressed. The NCCs that form the facial skeleton belong exclusively to the anterior Hox-negative domain and develop from the first branchial arch (BA1). This rostral domain of the crest is designated as FSNC for facial skeletogenic neural crest.Rhombomere 3 (r3) participates modestly to both BA1 and BA2. Forced expression of Hox genes (Hoxa2, Hoxa3 and Hoxb4) in the neural fold of the anterior domain inhibits facial skeleton development. Similarly, surgical excision of these anterior Hox-negative NCCs results in the absence of facial skeleton, showing that Hox-positive NCCs cannot replace the Hox-negative domain for facial skeletogenesis. We also show that excision of the FSNC results in dramatic down-regulation of Fgf8 expression in the head, namely in ventral forebrain and in BA1 ectoderm. We have further demonstrated that exogenous FGF8 applied to the presumptive BA1 territory at the 5-6-somite stage (5-6ss) restores to a large extent facial skeleton development. The source of the cells responsible for this regeneration was shown to be r3, which is at the limit between the Hox-positive and Hox-negative domain. NCCs that respond to FGF8 by survival and proliferation are in turn necessary for the expression/maintenance of Fgf8 expression in the ectoderm. These results strongly support the emerging picture according to which the processes underlying morphogenesis of the craniofacial skeleton are regulated by epithelial-mesenchymal bidirectional crosstalk.

In vertebrates, the eye is an ectodermal compound structure associating neurectodermal and placodal anlagen. In addition, it benefits early on from a mesenchymal ectoderm-derived component, the neural crest. In this respect, the construction of chimeras between quail and chick has been a turning point, instrumental in appraising the contribution of the cephalic neural crest to the development of ocular and periocular structures. Given the variety of crest derivatives underscored in the developing eye, this study illustrates the fascinating ability of this unique structure to finely adapt its differentiation to microenvironmental cues. This analysis of neural crest cell contribution to ocular development emphasizes their paramount role to design the anterior segment of the eye, supply refracting media and contribute to the homeostasy of the anterior optic chamber.