in the original experiment the dorsal lip of the blastopore

In transplant or explant studies, animal ectoderm that forms epidermis, when undisturbed, is susceptible to neural induction by the organizer. Induction by the Organizer: Antagonizing Bone Morphogenetic ProteinĪs gastrulation starts, the Spemann–Mangold organizer, which includes mostly axial mesodermal precursors, was classically believed to instruct naïve ectoderm to convert to neural tissue. No comparable early event appears to be involved in amniote (e.g., chick and mouse) embryos. Nuclear localization of β-catenin appears to be the first event that determines dorsal/ventral polarity in the Xenopus and zebrafish embryos ( Hibi et al. While the nature of the dorsal determinants is still in dispute, it is clear that the consequence of their translocation is the nuclear localization of β-catenin in a wide arc at the future organizer side ( Figure 1) ( Schneider et al. As the conventional approach of equating organizer side with dorsal seems to remain in wide use at present, I shall apply this convention, albeit with the reservation above.) In these papers, a new proposal is made for polarity assignments in the gastrula that, I believe, has some merit, but also presents some difficulties. This axis assignment does not project unambiguously onto the clearly defined dorsal–ventral polarity of the larva, as pointed out forcefully in recent publications. Conventionally, the side of the amphibian and fish embryo where the organizer forms has been called dorsal, with the opposite side labeled as ventral. (A brief parenthetical point is in order here. Fertilization triggers a rotation of the cortex relative to the cytoplasm that is associated with the movement of dorsal determinants from the vegetal pole to the future dorsal region of the embryo ( Gerhart et al. The frog egg is radially symmetrical around the animal–vegetal axis that has been established during oogenesis.

Here I shall outline the understanding of organizer formation and neural induction as it has evolved over recent times and attempt to integrate recent results from different species into a common pattern.Ĭortical Rotation and Nuclear Localization of β-Catenin 2004) and, of course, because of the experience of this author. If the present primer uses Xenopus to illustrate the process, it is because it accompanies an article in this issue of PLoS Biology dealing with neural development in this species ( Kuroda et al. In the past few years, however, work in other vertebrate and nonvertebrate chordate systems has come to play an important role in the field and has shed light on generalities and differences among chordates. The nature of the neural inducer or inducers and the mechanism of neural induction have been and remain hot topics in developmental biology.įor half a century after Spemann and Mangold, studies on amphibians monopolized the subject, and even more recently, a large part of the progress in analyzing organizer formation and function and neural induction was based on amphibians, mostly the model species Xenopus laevis. And neural induction has for a long time been regarded as a process by which organizer signals, in their normal context, redirect ectodermal cells from an epidermal towards a neural fate. which-as the transplantation experiment shows-is able to turn cells whose original fate would be gut or ventral epidermis into brain or somites, is the prototypical inducing tissue. The signal-receiving cells must be capable of responding, a property termed competence. Induction refers to the change in fate of a group of cells in response to signals from other cells. Most of the axis, including the nervous system, was derived from the host, whose cells were induced to form an axis by the graft, therefore named the organizer. In what is usually referred to as the most famous experiment in embryology, Hans Spemann and Hilde Mangold (1924) showed that a specific region in early frog embryos called the blastopore lip can induce a second complete embryonic axis, including the head, when transplanted to a host embryo.

In the original experiment the dorsal lip of the blastopore