Embryonaler Ursprung und Kompartimentierung von Plakoden bei Xenopus

A large part of the peripheral nervous system originates from cranial placodes. Placodes are often visible as thickenings of the non-neural ectoderm and give rise to the anterior part of the pituitary gland, the olfactory epithelium, the lens, the inner ear, the lateral line system in fish and amphibians, as well as the sensory neurons of different cranial nerves. Expression patterns of several genes like Six1 and Eya1 as well as fate maps in chick and zebrafish suggest that all placodes have a common origin. In this study, small groups of cells were labelled with the fluorescent dyes Dil or DiO via microinjection to generate a fate map of the non-neural ectoderm of Xenopus laevis embryos during neurulation. My lineage analysis confirms that all placodes originate from a common precursor region. This region lies around the anterior neural plate in a half-moon shaped arrangement and is called the preplacodal region or panplacodal primordium. At the beginning of neurulation, precursors of individual placodes overlap to a large extent with each other and with cells of other ectodermal fates within the preplacodal region. However, my results indicate that these overlapping areas are presumably smaller than previous studies assume. Moreover, analysis of cell movements of developing Xenopus embryos suggest that compartmentalization of the preplacodal region into distinct placodes seems to take place without sorting out of cells, contrary to what has been proposed for the chick. I also show that Dlx3 and GATA2 genes play important roles during development of the non-neural ectoderm. Both Dlx3 and GATA2 seem to act as competence factors of the non-neural ectoderm to form either placodes or epidermis.