Formation of the Endoderm in Xenopus

Abstract

Throughout most of the last century the study of the amphibian embryo was a dominant theme in embryology, the balance scarcely shifting until the fusion of molecular biology and Drosophila genetics and the introduction of targeted gene inactivation in mice. Within amphibian embryology the concept of the Organiser has been predominant. This was initially important in emphasising the importance of cell interactions in patterning the embryo, but it has somewhat skewed experiments and concepts towards a primacy of the dorsal tissues and particularly the head and brain. Part of the reason for this was heuristic; these structures are the most easily recognised, particularly without molecular markers. However, a second reason has perhaps been an unconscious focus on the region of the animal we find most interesting, i.e. the head and brain. This way of thinking about the embryo naturally led to the naming of the Nieuwkoop Centre, the part of the vegetal hemisphere which generates the Organiser, and hence the head, through sperm-directed redistribution of elements of the Wnt pathway (Gerhart et al. 1989). Of course, Nieuwkoop actually discovered that the vegetal pole generates a polarised signal that induces both the posterior and ventral mesoderm (Nieuwkoop 1969a,b, 1973). However, we have associated his name with what we find more interesting.