Spemann's organizer--it's origin and derivatives (cellular-tissue and molecular-genetic aspects)

In 1924 H. Spemann and H. Mangold discovered that a piece of the dorsal lip of a blastopore from Triturus cristatus, after transplantation to the ventral side of another embryo, was able to cause the neighbouring tissues to change their fate and participate in the formation of a new embryo. The dorsal lip was termed "the organizer". Since then, for as long as 75 years, attempts have been made to establish the intimate mechanisms of the organizer activity. However, no real advance was achieved in their understanding. Within the last 15 years, genetic and molecular techniques have been vastly improved, to help in tracing the fate of many cell lineages, and in compiling more exactly the fate maps for different parts of the embryo. Using these data, I have attempted to trace the fate of Spemann's organizer after the early gastrula stage. Analysis of data on inductive abilities of the organizer cells, on the use of markers, and on the observation of expression of specific genes allowed to conclude that Spemann's organizer in amphibia and its homologues in other vertebrates too are heterogeneous: they are composed of distinct cell populations able to induce primarity the development of either the head or trunk parts of the embryo. These population, determined to become the head of the trunk organizers still at the blastula stage, may be located either in the single continuous cell layer (as in amphibia and birds) or separated among different tissue germs (as in mammals). When the dorsal-ventral orientation of the embryo is established and the organizer is switched on the very early invaginating cells of the dorsal blastopore lip (in the case of amphibia) move in advance of the entire invaginating mesoderm and by the end of gastrulation occupy the place just in front of the notochord. It is supposed that the early dorsal lip and the prechordal mesoderm (PCM) are one and the same cell population, i.e. during gastrulation Spemann's organizer transfers from the lip of blastopore to the prechordal zone. The PCM seems to play an exclusive role in the formation of a head in vertebrate, because some mutations in genes expressed in the PCM result in the entire head deletion. It is supposed that spreading of differentiating signals from the PCM occurs along the main body axis in both caudal and rostral directions. After the main body plan formation the PCM is replaced by adenohypophysis. This conclusion is drawn not only from the same topology of both these structures, but also from the similarities of a set of specific genetical markers expressed in these, that makes it possible to suppose the existence of deep connections and succession between them. The adenohypophysis seems to arise directly from the PCM, or cells of the ectoderm influenced by the PCM may be subsequently transformed into humoral cells of adenohypophysis. In this interpretation, adenohypophysis and the much earlier established PCM may be considered as derivatives of Spemann's organizer. This inference is supported by the fact that all the three above structures first originate in vertebrates only.