The Molecular Nature of Spemann’s Organizer

Abstract

Embryology is a discipline that has been traditionally strongly influenced by its history. Its most important experiment was carried out by Hans Spemann and Hilde Mangold in Freiburg, Germany, in 1924. An English translation of their epoch-making paper, by Viktor Hamburger, is available in the Festschrift celebrating the 75th anniversary of this work (Spemann and Mangold 1924; de Robertis and Arechaga 2001). By transplanting the dorsal lip into the ventral side of a salamander gastrula of a different species, they found that the graft could induce a secondary neural plate and a twinned body axis. The dorsal lip was able to induce central nervous system (CNS), dorsal mesoderm (in particular somites and pronephros) and a secondary gut cavity. Since the fate of grafted cells could be followed by their different pigmentation, they were able to demonstate that the dorsal blastopore lip had very powerful inductive properties on neighboring cells. This experiment led to the realization that vertebrate cell differentiation progresses through a series of successive cell-cell inductions. For this well-known work Spemann received the Nobel Prize of Medicine in 1935 (Spemann 1938; Sander and Faessler 2001). When cloning became practical, it became possible to identify the molecules responsible for this remarkable inducing activity. In this chapter we review the molecular exploration of the gene products that execute the phenomenon of the Spemann organizer.