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Dynamics of Embryo Axis Elongation in Amniotes Vs. Anamniotes: the Role of the Notochord

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Dynamics of Embryo Axis Elongation in Amniotes Vs. Anamniotes: the Role of the Notochord UNIVERSIDADE DE LISBOA FACULDADE DE CIÊNCIAS DEPARTAMENTO DE BIOLOGIA ANIMAL Dynamics of embryo axis elongation in amniotes vs. anamniotes: The role of the notochord Tomás Augusto Barreiros Pais de Azevedo MESTRADO EM BIOLOGIA EVOLUTIVA E DO DESENVOLVIMENTO 2009 1 UNIVERSIDADE DE LISBOAFACULDADE DE CIÊNCIAS DEPARTAMENTO DE BIOLOGIA ANIMAL Dynamics of embryo axis elongation in amniotes vs. anamniotes: The role of the notochord Tomás Augusto Barreiros Pais de Azevedo MESTRADO EM BIOLOGIA EVOLUTIVA E DO DESENVOLVIMENTO Dissertação orientada pelos Doutor Gabriel G. Martins (FCUL) Prof.ª Doutora Sólveig Thorsteinsdóttir (FCUL) 2009 2 INDEX ACKNOWLEDGEMENTS .............................................................................................. 4 SUMMARY ...................................................................................................................... 6 RESUMO .......................................................................................................................... 7 INTRODUCTION .......................................................................................................... 10 MATERIALS AND METHODS ................................................................................... 16 General considerations................................................................................................ 16 Zebrafish embryo culture and live-imaging ............................................................... 16 Chick embryo culture and live-imaging ..................................................................... 18 Image analysis and quantification .............................................................................. 21 Testing the autonomous elongation of chick embryos ............................................... 24 Chick embryo notochord excision experiments. ........................................................ 24 Immunohistochemistry and confocal imaging of chick embryos. .............................. 27 Results ............................................................................................................................ 28 Dynamics of zebrafish embryo axis elongation.......................................................... 28 Dynamics of chick embryo axis elongation................................................................ 29 Axis elongation in chick embryo is autonomous ........................................................ 33 No portion of the notochord is essential for embryo extension .................................. 35 DISCUSSION ................................................................................................................. 39 REFERENCES ............................................................................................................... 44 3 ACKNOWLEDGEMENTS Foram muitos os que me ajudaram neste longo caminho que levou à conclusão da minha tese de mestrado. Aqui quero realçar alguns deles. Em primeiro lugar e provavelmente mais importante, ao Gabriel. Com ele construí o meu primeiro grande projecto profissional. Agradeço-lhe todos os conhecimentos que me transmitiu, fossem eles de microscopia, embriologia ou simplesmente a arte de sobreviver ao trabalho num laboratório. Agradeço-lhe também o ter-me introduzido no mundo da ciência, com tudo o que isso traz. Desde as sessões de “brainstorming” no seu gabinete de onde nasceram muitas das ideias aqui discutidas, até às alegrias das experiências bem sucedidas e fracassos das experiências falhadas. Um agradecimento geral ao grupo de Biologia do Desenvolvimento do CBA da FCUL, por tão bem me acolherem dentro do grupo e por toda a ajuda que me deram com diferentes pontos de vista sobre o meu trabalho À professora Sólveig, pela sua calma e serenidade que sempre me transmitiu e pelas valiosas ideias e maneira lúcida de ver as coisas que ajudaram este projecto a tornar-se uma realidade. À professora Gabriela pela sua simpatia e sorriso constantes e pela doçura com que sempre me tratou. À Rita por toda a amizade com que me recebeu no grupo e por me ajudar com conselhos e incentivos desde o primeiro dia até às vésperas de entrega da tese. Às minhas colegas de mestrado dentro do grupo, Ana Lina e Raquel, pelo apoio que sempre me deram, sobretudo quando tudo me parecia mais sombrio do que realmente era e pelas experiências que fomos trocando no decorrer das nossas teses. Ao António por todos os ensinamentos que me passou sobre o trabalho com peixe-zebra, desde o descorionamento à manutenção dos aquários. Ao Rifes, pelos seus conhecimentos de embriões de galinha e me ter conseguido convencer finalmente a ler o “Senhor dos anéis” em inglês. Ao Luís pelo companheirismo nomeadamente em várias noites longas de trabalho e pelas conversas “geeks” sobre ficção científica que finalmente consegui ter. Às novas membros do grupo Ana Bernardo e Ana Martins pela frescura que trouxeram ao laboratório. À Ana Gaspar pelos momentos de boa disposição que sempre trouxe ao laboratório, com as suas sempre originais obras de arte. 4 À Carla, pela sua amizade e pelos seus conselhos sobre os cuidados a ter com uma “fish facility”. À Marta Santos e à professora Margarida Matos pelas preciosas ajudas na estatística realizada e ensinamentos sobre como usar o Statistica 8 À Dona Branca, por toda a ajuda que me deu no laboratório e por ser alguém com quem conversar sobre o “nosso” Benfica. Ao Joaquin Rodriguez-Léon e Catarina Certal do grupo de organogénse do IGC e à Raquel Mendes do IMM pelos seus valiosíssimos conselhos sobre a técnica de New, sem os quais nunca conseguiria ter feito muitas das experiências apresentadas. Aos meus grandes amigos Bárbara, Miguel Cunha e Miguel Santos por todos os bons momentos partilhados durante a tese que ajudaram a descomprimir do stress diário. Às minhas grandes amigas Célia, Isabel e Rita pelos “grupos de apoio” que criámos para facilitar a escrita da tese. À Catarina, ao Menezes e ao Miguel por me lembrarem constantemente que há um mundo lá fora para além da tese. Finalmente, aos meus pais e minha avó de quem sempre tive apoio incondicional e uma paciência infinita para me restaurarem o ânimo perdido e para partilharem comigo todas as alegrias. Um agradecimento especial à minha mãe pela história da Rã que me manteve à tona neste final de ano… 5 SUMMARY In this thesis, we sought to analyze the dynamics of amniote and anamniote embryo axis elongation and the possible role of the notochord. For that, we chose the chick and zebrafish as representative model organisms of each group. We found that in the zebrafish embryo, elongation has two different phases during the stages analysed, from “shield” to 15 somite stage. The first phase is characterized by a constant (i.e., linear) growth rate which is faster than that observed during the second phase. The first phase occurs simultaneously with the epiboly movements of the basltoderm and yolk- syncytial layer (YSL). In chick embryos, we also find a constant (linear) growth phase which spans from stage HH4-11 (Hamburger & Hamilton 1951), a period during which the chick embryo forms structures homolog to those formed by zebrafish embryos during the first (linear) growth phase. Furthermore, in chick embryos, we found that the cranial portion elongates at a slower rate than that of two other portions of the embryo: the “segmented” and “pre-somitic” portions of the embryo, which contain all tissues adjacent to the segmented and pre-somitic mesoderm, respectively. Using micromanipulation techniques we show that elongation in the chick is independent of the primitive streak and a direct connection of Hensen’s node to the extra-embryonic tissues. This contrasts with zebrafish embryo development where it has been well established that axis extension only occurs if there is an intact connection between the embryo and the YSL. We propose that the structures that could contribute to axis extension are: the notochord, the neural tube and the paraxial mesoderm. Again, using micro-manipulation techniques, we show that none of three different portions of the notochord (cranial, “segmented” and “pre-somitic”) are essential for embryo axis elongation. However, preliminary experiments suggest that the presence of the whole notochord might contribute to the process of embryo axis extension. This work shows that the notochord is not the main driver of early embryo axis extension in amniotes, a hypothesis that had been advanced in the literature often but had not yet been formally tested. Clearly, the forces that shape the early vertebrate embryo into its typical elongated shape come from tissues inside the embryo itself, other than the notochord. In amniotes, these forces have evolved in order to allow the embryo to extend autonomously, without the “additional” force produced by expanding extra-embryonic tissues, as still seen in most anamniotes (e.g., fish and amphibians). Keywords: Embryonic axis, zebrafish, chicken, notochord, live-imaging. 6 RESUMO A gastrulação é o processo pelo qual o embrião forma as três camadas germinativas, endoderme, mesoderme e ectoderme. É durante este processo que muitos dos organismos formam os seus três eixos embrionários: Dorso-ventral, antero-posterior e esquerda-direita. Após o estabelecimento do eixo anterio-posterior, este sofre um alongamento que depende da arquitectura e modo como o embrião internaliza os tecidos, da quantidade de vitelo, entre outros. Muito pouco se sabe ainda sobre os mecanismos que levam à extensão dos embriões. No caso do peixe-zebra (um anamniota), o embrião é composto
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