Gérard Couly

Patterning the neural crest derivatives during development of the vertebrate head: insights from avian studies

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.

Brainstem dysfunction: a possible neuroembryological pathogenesis of isolated Pierre Robin sequence

Abstract. Pierre Robin sequence (posterior U-shape cleft palate, glossoptosis, retrognathia) (PRS) is a frequent and heterogeneous neonatal condition of obscure origin. We show here that orodigestive and cardiorespiratory functional disorders are very frequent in PRS and that these functional disorders, as well as anatomical and embryological data, argue for the involvement of brainstem dysfunction in the pathogenesis of some cases of isolated PRS. A total of 66 infants consecutively admitted for isolated PRS were followed-up with observations and investigations focused on their orodigestive and cardiorespiratory disorders. Neonatal clinical examination and neonatal anatomical aspects of the three orofacial features of the sequence were evaluated. Feeding difficulties and respiratory disorders were recorded and infants were classified according to three grades of severity. The relation between functional severity grade and neonatal orofacial features was evaluated, as well as the relation between functional severity grade and specific criteria characterising oesophageal and laryngeal motility and cardiac orthoparasympathetic imbalance. In the first weeks of life, sucking and swallowing disorders (100%), excessive regurgitation (94%), upper airways obstruction (50%), and cardiac vagal overactivity (59%) were noted. Correlation of anatomical features with functional severity grades was poor except for extreme forms of glossoptosis and retrognathia. Specific anomalies of oesophageal motility, pharyngolaryngeal tone and parasympathetic cardiac regulation were described. These anomalies were more frequent in children with the two higher grades of functional severity. Conclusion: infants with Pierre Robin sequence have early and severe anomalies of orodigestive and cardiorespiratory function which do not appear to be related solely to anatomical features and which require proper medical management. We suggest a prenatal and neonatal brainstem dysfunction as a neuroembryological hypothesis to explain the onset of some cases of Pierre Robin sequence.

Oroesophageal motor disorders in Pierre Robin syndrome.

Background Feeding disorders are one of the main clinical features in PRS, which combines a posterior U-shaped cleft palate, retrognathia, and glossoptosis. The aim of this study was to evaluate the oral and esophageal motor function of children with PRS without additional neurologic symptoms. Methods All children hospitalized with Pierre Robin syndrome either isolated (n = 27) or associated with Stickler syndrome (n = 8) were included. Clinical evaluation of their oroesophageal disorders and systematic esophageal manometry were performed. Results Feeding disorders were always present, but type of disorder varied from one child to another. Esophageal disorders were frequent and seemed to be resistant to classic gastroesophageal reflux treatment. Eighty-six percent of the children required nasogastric tube feeding for a mean duration of 8.6 months. Esophageal manometric abnormalities were noted in 50% of the children: lower esophageal sphincter hypertonia, failure of lower esophageal sphincter relaxation at deglutition, and esophageal dyskinesia. These clinical and manometric disorders showed a trend to spontaneous regression after 12 months. Conclusion In the current Pierre Robin syndrome series, clinical and manometric anomalies of oroesophageal motility were always present. The authors identified an unusual manometric pattern that has also been described in situations of neurovegetative instability. It could reflect dysregulation of the control of the central pattern generators of swallowing in the brain stem.

Morphogenesis of the branchial vascular sector.

The branchial and dorsal cephalic vascular sectors correspond to the blood vessels contained within evolutionarily recent and ancestral parts of the head, respectively. Recent work demonstrates that neural crest cells (NCCs) provide the pericytes, and connective and smooth muscle cells to the entire branchial sector in an ordered fashion. Initial NCC position is transposed to the vascular distal-to-proximal axis, explaining why circumscribed cephalic vascular anomalies are often associated with reproducible malformations in head tissues derived from the neural crest. Unlike the rest of the central nervous system, the forebrain requires mesenchyme-containing vascular-competent NCCs to survive during embryogenesis and beyond.

Evolving maps in craniofacial development

The shaping of the vertebrate head results from highly dynamic integrated processes involving the growth and exchange of signals between the ectoderm, the endoderm, the mesoderm and Cephalic Neural Crest Cells (CNCCs). During embryonic development, these tissues change their shape and relative position rapidly and come transiently in contact with each other. Molecular signals exchanged in restricted regions of tissue interaction are crucial in providing positional identity to the mesenchymes which will form the different skeletal and muscular components of the head. Slight spatio-temporal modifications of these signalling maps can result in profound changes in craniofacial development and might have contributed to the evolution of facial diversity. Abnormal signalling patterns could also be at the origin of congenital craniofacial malformations. This review brings into perspective recent work on spatial and temporal aspects of facial morphogenesis with particular focus on the molecular mechanisms of jaw specification.

Jaw muscularization requires Dlx expression by cranial neural crest cells

The origin of active predation in vertebrates is associated with the rise of three major, uniquely derived developmental characteristics of the head: (i) migratory cranial neural crest cells (CNCCs) giving rise to most skeletal skull elements; (ii) expression of Dlx genes by CNCCs in the Hox-free first pharyngeal arch (PA1); and (iii) muscularization of PA1 derivatives. Here we show that these three innovations are tightly linked. Expression of Dlx genes by CNCCs is not only necessary for head skeletogenesis, but also for the determination, differentiation, and patterning of cephalic myogenic mesoderm leading to masticatory muscle formation. In particular, inactivation of Dlx5 and Dlx6 in the mouse results in loss of jaw muscles. As Dlx5/6 are not expressed by the myogenic mesoderm, our findings imply an instructive role for Dlx5/6-positive CNCCs in muscle formation. The defect in muscularization does not result from the loss of mandibular identity observed in Dlx5/6−/− mice because masticatory muscles are still present in EdnRA−/− mutants, which display a similar jaw transformation. The genesis of jaws and their muscularization should therefore be seen as an integrated Dlx-dependent developmental process at the origin of the vertebrate head. The role of Dlx genes in defining gnathostome jaw identity could, therefore, be secondary to a more primitive function in the genesis of the oral skeletomuscular system.

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.

Heterotopic neuroglial tissue of the face. Report of six cases and review of the literature.

We report six cases of heterotopic neuroglial tissue of the face. All cases appeared at birth with a laterofacial mass and, in most cases, a respiratory disorder. The initial diagnosis has frequently been lymphangioma. But the rapidly growing character of the mass with resistance to high dose corticoid therapy, the fluid aspect at computerized tomography imaging with no connection to the central nervous system, and the presence of cerebrospinal fluid at the needle puncture were the most helpful features in narrowing the differential diagnoses. Patients were treated by surgical removal of the mass. The pathologic study showed the presence of neuroglial tissue. The management of the patients is discussed. The thirteen previous cases reported in the literature and our six cases lead to a discussion on the embryologic origin of this rare entity--the cephalic neural crest.

Spatio-Temporal Dynamics of Gene Expression of the Edn1-Dlx5/6 Pathway During Development of the Lower Jaw

The morphogenesis of the vertebrate skull results from highly dynamic integrated processes involving the exchange of signals between the ectoderm, the endoderm, and cephalic neural crest cells (CNCCs). Before migration CNCCs are not committed to form any specific skull element, molecular signals exchanged in restricted regions of tissue interaction are crucial in providing positional identity to the CNCCs mesenchyme and activate the specific morphogenetic process of different skeletal components of the head. In particular, the endothelin‐1 (Edn1)‐dependent activation of Dlx5 and Dlx6 in CNCCs that colonize the first pharyngeal arch (PA1) is necessary and sufficient to specify maxillo‐mandibular identity. Here, to better analyze the spatio‐temporal dynamics of this process, we associate quantitative gene expression analysis with detailed examination of skeletal phenotypes resulting from combined allelic reduction of Edn1, Dlx5, and Dlx6. We show that Edn1‐dependent and ‐independent regulatory pathways act at different developmental times in distinct regions of PA1. The Edn1→Dlx5/6→Hand2 pathway is already active at E9.5 during early stages of CNCCs colonization. At later stages (E10.5) the scenario is more complex: we propose a model in which PA1 is subdivided into four adjacent territories in which distinct regulations are taking place. This new developmental model may provide a conceptual framework to interpret the craniofacial malformations present in several mouse mutants and in human first arch syndromes. More in general, our findings emphasize the importance of quantitative gene expression in the fine control of morphogenetic events. genesis 48:362–373, 2010.

LE DYSFONCTIONNEMENT NEONATAL ISOLE DU TRONC CEREBRAL

Background. - Brainstem dysfunction in newborns (BDN) is an association of symptoms originally described in the Pierre-Robin sequence. BDN is thought to result from a deficiency of the sucking and swallowing embryonic organization. Population and methods. - Between 1983 to 1993, 48 infants without cleft polate were referred for suck and swallow abnormalities. They were considered to have BDN because they presented three of the four following criteria: neonatal suck and swallow difficulties: pharyngeo-oesophageat uncoordination with abnormal oesophageal mutometria; upper airway obstruction, either clinically obvious or detected on laryngoscopy; vagal overactivity, either clinically obvious or detected during Holter recording with ocular compression. Results. - Among these 48 infants, 30 were affected with polymalformative syndrome often involving embryonic fields derived from the neural crest. Three infants had a conotruncal cardiac malformation and 15 had no associated malformation. These latter 15 infants presented with facial dysmorphic features including reciding chin, glossoptosis. U-shape polate and a vertical tongue. From birth or the first weeks of life, they had suck and swallow difficulties with various functional symptoms: slow babybottle intake, cough or velo-pharyngeal incoordination, upper airway obstruction or apparent life threatening events (ALTE). Diagnosis was confirmed by both clinical observation and three simple investigations namely: laryngoscopy, vesoplungeal monometria and Holter recording with ocular compression. Ten children were nasogastric tube or gastrostomy fed, one had a tracheostomy and one had a nightly O2 supplementation. While the overall functional prognosis was good whatever the initial symptoms. 50% of these children had mental retardation, most moderate. Conclusion. - Examination of short-term follow-up in these children has stressed that BDN requires a specific management of both nutritional and respiratory troubles. Finally, BDN should lead to the active search of an underlying polymalformative syndrome and to an accurate neurologic evalution.