Embryo anterior-posterior axis

Grandel, H., K. Lun, G.J. Rauch, M. Rhinn, T. Kotrowski, C. Houart, P. Sordino, A. Kuchler, S. Schulte-Merker, R. Geisler, N. Holder, S. Wilson and M. Brand. Retinoic acid signaling in the zebrafish embryo is necessary during presegmentation stages to pattern the anterior-posterior axis of the CNS and to induce a pectoral fin bud. Development 129 2851—2865, 2002. 

Epstein, M., Pillemer, G., Yelin, R., Yisraeli, J.K., Fainsod, A. 1997. Patterning of the embryo along the anterior-posterior axis the role of the caudal genes. Development 124, 3805-3814. 

Krumlauf, 1992). Genes located at the 3 end of the cluster are expressed earlier and more anteriorly during development than those found at the 5 end. Consequently, Hox genes exhibit nested domains of expression along the anterior-posterior axis and confer positional information to the embryo during development (Fig. 5). 

Ding, J., Yang, L., Yan, Y.T., Chen, A., Desai, N., Wynshaw-Boris, A., Shen, M.M. 1998. Cripto is required for correct orientation of the anterior-posterior axis in the mouse embryo. Nature 395, 125-133. 

We turn now to determination of the anterior/posterior axis in the early fly embryo while it is still a syncytium. As in determination of the dorsal/ventral axis, specification of anterior/posterior cell fate begins during oogenesis. The initial asymmetry also involves so-called maternal mRNAs, which are produced by nurse cells and transported into the oocyte. In this case they become localized in discrete spatial domains (see Figure 15-14). For example, bicoid mRNA is trapped at the most anterior region, or anterior pole, of the early fly embryo (Figure 15-19). The anterior localization of... 

Figure 3.10 Concentration gradient of bicoid in the developing fruitfly. Concentration of bicoid protein has been measured in developing embryos as a function of location on the anterior-posterior axis. When present at high levels, biocoid protein activates the gene tailless at intermediate levels, inhibits the gene KruppeC, and activates hunchback. Spatial localization of gene expression is thereby achieved with a simple mechanism for protein-gradient formation.

The attachment of the inner cell mass to the polar trophoblast has previously identified the dorsal aspect of the embryo. So, the definition of the anterior-posterior axis by the appearance of the prochordal plate and the primitive streak actually constrains the whole system of three axes. It must be remembered that the cells in question began their various biochemical differentiations before those differentiations became microscopically visible in spatial coordinates. 

A confocal microscope allows one to obtain an image of the fluorescent label and a transmission image of the embryo. The two images can be overlaid to identify where along the anterior posterior axis of the embryo the labeled cells are located. The depth of the sections of the fluorescent label gives some indication of which tissue layer contains the labeled cells. 

The appearance of the primitive streak provides an unequivocal indication of the anterior-posterior axis and helps with the orientation during dissection. Early gas-trulation embryos are therefore used in experiments that demand knowledge of the source of cells in the anterior-posterior and medial-lateral axis. However, if only a distinction between proximal and distal epiblast is required, both pre- (without the primitive streak) and early gastrulation embryos can be used. 

Despite the blue-tac feet, the embryos are squashed to some extent. With 6.5-and 7.5-d embryos, it can be difficult to orient the embryos once they have been squashed. For ease of analysis, mount the embryos in the sagittal plane, and mark the direction of the anterior-posterior axis on the slide. 

Katsuyama, Y, and H. Saiga. 1998. Retinoic Acid Affects Patterning Along the Anterior-Posterior Axis of the Ascidian Embryo. Dev Growth Differ AO, no 4 413-22. 

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