No description available.

La crête neurale crâniale (CNC) est une structure transitoire et pluripotente de l’embryon des Vertébrés qui génère la totalité du squelette de la face et de la voûte crânienne et fournit les méninges et une vascularisation fonctionnelle au cerveau antérieur. Précocement, la CNC contrôle également la croissance du cerveau. Pour identifier les mécanismes par lesquels la CNC exerce son rôle trophique sur le cerveau, nous nous sommes intéressés à l’expression du gène Smad1, qui transduit divers voies de signalisation, et est massivement exprimé par les cellules de la CNC juste avant leur migration. L’inactivation de Smad1 par l'interférence ARN dans les CCN conduit à une microcéphalie sévère et une holoprosencéphalie partielle, qui résulte de la perte de l’expression de Fgf8 et Foxg1. Les expériences de sauvetage montrent que les cellules de la CNC régulent positivement Foxg1 indépendamment de Fgf8. De plus, nous montrons que la perte de fonction de Foxg1 dans le télencéphale affecte le développement du thalamus et du toit optique en dérégulant l’expression de Otx2 et de Foxa2 à leur niveau. Nous avons identifié les molécules médiatrices produites par les cellules de la CNC nécessaire au contrôle de l’expression de Foxg1. Nous montrons que les antagonsites de Bmp and Wnt, Noggin, Gremlin et Dkk1 sont indispensable pour initier la spécification du télencéphale. De plus, la régionalisation moléculaire des territoires télencephalique et di/mésencéphalique, requiert l’activité conjointe de Sfrp1 et Sfrp2, d’une part, et de Cerberus, d’autre part. L’ensemble des données acquises au cours de ces travaux documente les mécanismes moléculaires par lesquels la CNC participe de façon essentielle à la régionalisation moléculaire du cerveau des Vertébrés. / The cranial NC (CNC) is a transient structure of the vertebrate embryo, which is essential for brain ontogenesis and provides the developing brain with a skeletal and meningeal protection and functional vasculature. Early in development, CNC cells also control morphogenetic activities of brain organizers and stimulate the growth of prosencephalic alar plate. To understand how CNC conveys its trophic effect on the telencephalon, we have silenced the gene encoding for Smad1, a transcription factor expressed in the CNC cells, which transduces diverse morphogenetic pathways. Smad1 silencing results in microcephaly and partial holoprosencephaly, which early coincide with the loss of Fgf8 and Foxg1 in the telencephalon. Rescue experiments show that CNC cells can positively regulate Foxg1 expression independently of Fgf8 activity in the prosencephalic organizer. Furthermore, the depletion of Foxg1 activity in the telencephalon alters Otx2 and Foxa2 expression in the thalamus and tectum. We have identified the mediators produced by the CNC to control Foxg1 activity and showed that Bmp and Wnt antagonists, Noggin, Gremlin and Dkk1 initiate the specification of the telencephalon. Additionally, the molecular patterning of the telencephalic and di/mesencephalic compartments requires the activity of Sfrp1 and Sfrp2, and Cerberus, respectively. Altogether, we show that CNC cells controls brain patterning by regulating Foxg1 expression through a network of morphogen modulators controlled by Smad1 activity. / A crista neural cranial (CNC) é uma estrutura transiente em embriões de vertebrado, que possui um papel crucial no desenvolvimento da cabeça. A CNC é uma importante fonte de derivados mesenquimais. Recentes descobertas mostraram que as células da CNC possuem uma atividade trófica no desenvolvimento do tubo neural anterior, estimulando e organizando o desenvolvimento prosencefálico em oposição à sinalização Bmp presente nos tecidos adjacentes. Com o objetivo de entender como as células da CNC controlam a atividade de morfógenos durante o desenvolvimento do cérebro. Nós focamos nossos estudos no fator de transcrição Smad1, expresso pelas células da CNC, que controla a transcrição de Noggin. Noggin é um antagonista de Bmp que por sua vez controla a atividade de sua via de sinalização. Além disso, Smad1 interage com outras vias de sinalização com Fgf8 e Wnt. Para testar o papel de Smad1 nas células da CNC, nós eletroporamos o RNA dupla fita de Smad1 (dsSmad1) nas células da CNC em embriões de galinha no estágio de 4 somitos com a finalidade de bloquear sua tradução. Estes espécimes foram analisados em estágio mais avançados do desenvolvimento embrionário. A perda de função de Smad1 compromete o desenvolvimento das vesículas cefálicas, nos estágios de 26 somitos, E4, E6 e E8. Em cortes histológicos em E8, observou-se o aumento do volume ventral do cérebro destes embriões. Com o objetivo de entender como Smad1 controla o desenvolvimento das vesículas cefálicas, embriões no estágio de 26ss foram analisado por hibridização in situ. Nós observamos em embriões dsSmad1 a diminuição da expressão de Fgf8 na borda neural anterior e a completa ausência de expressão de Foxg1 no neuroectoderma prosencéfalico. A falta de Smad1, também gera a diminuição da expressão de Otx2 nos limites ventrais e laterais do telencéfalo, diencéfalo e mesencéfalo. Em contrapartida, nestes embriões observa-se o aumento da zona de expressão de Foxa2 na porção ventral do diencéfalo e mesencéfalo. O bloqueio de Smad1 também acarreta no aumento dos níveis de Dkk1, que é um importante inibidor da via de sinalização Wnt. Com o intuito de entender o mecanismo sobre o controle de Smad1, nós aumentamos os níveis de transcritos nas células da CNC de Dkk1. Como resultado deste aumento, observamos as mesmas modificações nos níveis dos transcritos de Fgf8, Foxg1, Otx2 e Foxa2. Interessantemente os efeitos do excesso de Dkk1 podem ser revertidos com a co-eletroporação do Smad1 constitutivamente fosforilada. Nós também analisamos a expressão de Foxg1 e Otx2 em embriões privados de Cubilin nas células da CNC. Estes embriões apresentam o mesmo padrão de expressão encontrados nos embriões dsSmad1. Interessantemente os nocautes para Cubilin apresentam diminuição da fosforilação de Smad1. Nossos resultados mostram que a presença de Smad1 nas células da CNC é extremamente importante para padronização e desenvolvimento do cérebro. Smad1 nas células da CNC funciona como um regulador da via de Bmp, através do controle transcricional de Noggin impedindo que o excesso de Bmp chegue até o tubo neural. Sendo assim, Smad1 controla o excesso de Bmp permitindo a indução e o desenvolvimento da região anterior por Fgf8.

<p>The development of the vertebrate head is a complex process involving interactions between a multitude of cell types and tissues. This thesis describes the development of the cranial neural crest and of the visceral arch muscles in the head of two species. One, the Mexican axolotl (<i>Ambystoma mexicanum</i>), is a basal tetrapod, whereas the other, the Australian lungfish (<i>Neoceratodus forsteri</i>), belongs to the Dipnoi, the extant sister group of the Tetrapoda. </p><p>The migration of neural crest cells, which form most of the bones and connective tissues in the head, and the morphogenesis of the jaw, was determined in the Mexican axolotl. It was shown that both the upper and lower jaws form from ventral condensations of neural crest cells in the mandibular arch. The dorsal condensation, earlier considered to give rise to the upper jaw, was shown to form the trabecula cranii.</p><p>The normal spatio-temporal development of visceral arch muscles was investigated in both the Mexican axolotl and the Australian lungfish. In axolotl, the muscles tended to start forming almost simultaneously in all visceral arches at their future origins and extend towards their future insertions at the onset of muscle fibre formation. In lungfish, fibres formed simultaneously throughout most of each muscle anlage in the first and second visceral arch, but were delayed in the branchial arches. The anlagen were first observed at their future insertion, from which they developed towards future origins. </p><p>To test the ability of neural crest cells to pattern the visceral arch muscles, migrating crest cells were extirpated from axolotl embryos, which resulted in a wide range of muscle malformations. In most cases, the muscles appeared in the right position but were small and extended in abnormal directions. This shows that neural crest cells are responsible not for the position of the muscles but for their correct anatomical pattern. Fate mapping showed that connective tissue surrounding myofibers is, at least partly, neural crest derived.</p><p>In conclusion, the work presented in this thesis shows that although early development may map out the patterns of later development, the differences between axolotl and lungfish head development are not seen until during morphogenesis. Further investigation of morphogenesis is needed to explain the great variation of head morphology seen in vertebrates today.</p>

The development of the vertebrate head is a complex process involving interactions between a multitude of cell types and tissues. This thesis describes the development of the cranial neural crest and of the visceral arch muscles in the head of two species. One, the Mexican axolotl (Ambystoma mexicanum), is a basal tetrapod, whereas the other, the Australian lungfish (Neoceratodus forsteri), belongs to the Dipnoi, the extant sister group of the Tetrapoda. The migration of neural crest cells, which form most of the bones and connective tissues in the head, and the morphogenesis of the jaw, was determined in the Mexican axolotl. It was shown that both the upper and lower jaws form from ventral condensations of neural crest cells in the mandibular arch. The dorsal condensation, earlier considered to give rise to the upper jaw, was shown to form the trabecula cranii. The normal spatio-temporal development of visceral arch muscles was investigated in both the Mexican axolotl and the Australian lungfish. In axolotl, the muscles tended to start forming almost simultaneously in all visceral arches at their future origins and extend towards their future insertions at the onset of muscle fibre formation. In lungfish, fibres formed simultaneously throughout most of each muscle anlage in the first and second visceral arch, but were delayed in the branchial arches. The anlagen were first observed at their future insertion, from which they developed towards future origins. To test the ability of neural crest cells to pattern the visceral arch muscles, migrating crest cells were extirpated from axolotl embryos, which resulted in a wide range of muscle malformations. In most cases, the muscles appeared in the right position but were small and extended in abnormal directions. This shows that neural crest cells are responsible not for the position of the muscles but for their correct anatomical pattern. Fate mapping showed that connective tissue surrounding myofibers is, at least partly, neural crest derived. In conclusion, the work presented in this thesis shows that although early development may map out the patterns of later development, the differences between axolotl and lungfish head development are not seen until during morphogenesis. Further investigation of morphogenesis is needed to explain the great variation of head morphology seen in vertebrates today.

No description available.

No description available.

No description available.

The small size and the lack of recombination set the fourth chromosome of Drosophila melanogaster apart from the other chromosomes. I have shown that the Minute gene on chromosome 4, earlier named Minute-4, encodes the ribosomal protein RpS3A. Two Pax6 genes, eyeless (ey) and twin of eyeless (toy) are also located on chromosome 4. Pax6 genes are important in head and eye development in both mammals and Drosophila. I have focused much of the study on ey and toy. The first mutant of toy that was characterized showed a headless phenotype. This indicates that Toy is important for the development of both the eye and antennal discs. The phenotype of the null mutation in toy is temperature sensitive due to that transcription of ey is temperature dependent in the eye-antennal primordium in absence of Toy. This temperature dependence was used to find out that the phenocritical period for ey in the adult head development is during embryonic stage 12-16 when ey first is expressed in the eye-antennal primordium. I also conclude that ey is activated by Toy in the eye-antennal primordium. The strong eyD mutation was molecularly characterized and it was finally settled that it is an allele in the ey locus. I also show that eyD homozygotes have a headless phenotype, much stronger than the earlier ey mutations.

No description available.

No description available.