Dynamics of embryo axis elongation in amniotes vs. anamniotes:the role of the notochord

• Author(s): Azevedo, Tomás Augusto Barreiros Pais de, 1986-
• Date: 2009
• Persistent ID: http://hdl.handle.net/10451/1420
• Origin: Repositório da Universidade de Lisboa
• Subject(s): Embriologia animal; Somitogénese; Teses de mestrado
• Document Type: Master thesis
• Language: English
• Advisor(s): Martins, Gabriel G.; Thorsteinsdóttir, Sólveig, 1962-
• Contributor(s): Repositório da Universidade de Lisboa

Description

Tese de mestrado, Biologia (Biologia Evolutiva e do Desenvolvimento), 2009, Universidade de Lisboa, Faculdade de Ciências

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)

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