Christine Vincent

Patterning of the hyoid cartilage depends upon signals arising from the ventral foregut endoderm

Hyoid bone is a part of the visceral skeleton which arises from both Hox‐expressing (Hox+) and Hox‐nonexpressing (Hox‐) cephalic neural crest cells. In a previous work, we have demonstrated that the Hox‐ neural crest domain behaves as a naïve entity to which the ventral foregut endoderm confers patterning cues to specify the shape and orientation of the nasal and mandibular skeleton. By using ablation and grafting approaches, we have extended our study to the formation of the hyoid bone and tested the patterning ability of more caudal levels of the lateroventral foregut endoderm in the chick embryo at the early neurula stage. In this study, endodermal stripes have first been delineated according to the projection of mid‐ and posterior rhombencephalic structures. The extirpation of endodermal transverse stripes along the anteroposterior axis selectively hampers the formation of the ceratobranchials and epibranchials. Thus defined, the patterning ability of the endodermal stripes was further explored in their medial and lateral parts. When homotopically engrafted on the migration pathway of cephalic neural crest cells, ventromedial zones of endoderm lead to the formation of supernumerary basihyal and basibranchial, while lateral zones generate additional cartilaginous pieces recognizable as ceratobranchial and epibranchial. Taken together, our data demonstrate that, early in development, the ventral foregut endoderm exerts a regionalized patterning activity on the cephalic neural crest to build up the primary facial and visceral skeleton in jaws and neck and enable a map of the endodermal skeletogenic areas to be drawn. This map reveals that a cryptic metamerization of the anterior foregut endoderm precedes the formation of the branchial arches. Developmental Dynamics 228:239–246, 2003.

Sonic hedgehog signalling from foregut endoderm patterns the avian nasal capsule

Morphogenesis of the facial skeleton depends on inductive interactions between cephalic neural crest cells and cephalic epithelia, including the foregut endoderm. We show that Shh expression in the most rostral zone of the endoderm, endoderm zone I (EZ-I), is necessary to induce the formation of the ventral component of the avian nasal capsule: the mesethmoid cartilage. Surgical removal of EZ-I specifically prevented mesethmoid formation, whereas grafting a supernumerary EZ-I resulted in an ectopic mesethmoid. EZ-I ablation was rescued by Shh-loaded beads, whereas inhibition of Shh signalling suppressed mesethmoid formation. This interaction between the endoderm and cephalic neural crest cells was reproduced in vitro, as evidenced by Gli1 induction. Our work bolsters the hypothesis that early endodermal regionalisation provides the blueprint for facial morphogenesis and that its disruption might cause foetal craniofacial defects, including those of the nasal region.

The developmental relationships of the neural tube and the notochord: short and long term effects of the notochord on the dorsal spinal cord

Patterning of the ventral half of the neural tube results from the inductive influence of the notochord and of the floor plate. We have studied here the effect of an ectopically grafted notochord on the development of the dorsalmost part of the neural tube i.e. roof plate and alar plates. We show that at their early stages, dorsal genes are repressed by the dorsal graft of a notochord, as shown previously in other studies. We found also that when the notochord is implanted in a mediodorsal position on top of the roof plate (and not laterally as previously performed in other studies) the genes specifics of the floor plate are not induced, and motoneurons do not differentiate. The notochord prevents the formation of the medial septum from roof plate cells and induces their active proliferation between E5 and E7. Roof and dorsal alar plates derived cells start to die from E7 onward leaving a dorsally truncated spinal cord. If the notochord is grafted at 20 degrees-30 degrees from the sagittal plane ventral genes and structures are induced and the roof plate differentiates normally. We conclude that roof plate cells exhibit a specific response to notochord signals, the short range effect of which is thus strikingly demonstrated.

Msx genes are expressed in the carapacial ridge of turtle shell: a study of the European pond turtle, Emys orbicularis

The turtle shell forms by extensive ossification of dermis ventrally and dorsally. The carapacial ridge (CR) controls early dorsal shell formation and is thought to play a similar role in shell growth as the apical ectodermal ridge during limb development. However, the molecular mechanisms underlying carapace development are still unknown. Msx genes are involved in the development of limb mesenchyme and of various skeletal structures. In particular, precocious Msx expression is recorded in skeletal precursors that develop close to the ectoderm, such as vertebral spinous processes or skull. Here, we have studied the embryonic expression of Msx genes in the European pond turtle, Emys orbicularis. The overall Msx expression in head, limb, and trunk is similar to what is observed in other vertebrates. We have focused on the CR area and pre-skeletal shell condensations. The CR expresses Msx genes transiently, in a pattern similar to that of fgf10. In the future carapace domain, the dermis located dorsal to the spinal cord expresses Msx genes, as in other vertebrates, but we did not see expansion of this expression in the dermis located more laterally, on top of the dermomyotomes. In the ventral plastron, although the dermal osseous condensations form in the embryonic Msx-positive somatopleura, we did not observe enhanced Msx expression around these elements. These observations may indicate that common mechanisms participate in limb bud and CR early development, but that pre-differentiation steps differ between shell and other skeletal structures and involve other gene activities than that of Msx genes.

Dlx5 and Dlx6 expression in the anterior neural fold is essential for patterning the dorsal nasal capsule

Morphogenesis of the vertebrate facial skeleton depends upon inductive interactions between cephalic neural crest cells (CNCCs) and cephalic epithelia. The nasal capsule is a CNCC-derived cartilaginous structure comprising a ventral midline bar (mesethmoid) overlaid by a dorsal capsule (ectethmoid). Although Shh signalling from the anterior-most region of the endoderm (EZ-I) patterns the mesethmoid, the cues involved in ectethmoid induction are still undefined. Here, we show that ectethmoid formation depends upon Dlx5 and Dlx6 expression in a restricted ectodermal territory of the anterior neural folds, which we name NF-ZA. In both chick and mouse neurulas, Dlx5 and Dlx6 expression is mostly restricted to NF-ZA. Simultaneous Dlx5 and Dlx6 inactivation in the mouse precludes ectethmoid formation, while the mesethmoid is still present. Consistently, siRNA-mediated downregulation of Dlx5 and Dlx6 in the cephalic region of the early avian neurula specifically prevents ectethmoid formation, whereas other CNCC-derived structures, including the mesethmoid, are not affected. Similarly, NF-ZA surgical removal in chick neurulas averts ectethmoid development, whereas grafting a supernumerary NF-ZA results in an ectopic ectethmoid. Simultaneous ablation or grafting of both NF-ZA and EZ-I result, respectively, in the absence or duplication of both dorsal and ventral nasal capsule components. The present work shows that early ectodermal and endodermal signals instruct different contingents of CNCCs to form the ectethmoid and the mesethmoid, which then assemble to form a complete nasal capsule.