Gene expression and pattern formation during early embryonic development in amphibians

Temporal and spatial gene expression and inductive interactions control the establishment of the body plan during embryogenesis in invertebrates and vertebrates. The best-studied vertebrate model system is the amphibian embryo. Seventy-five years after the famous organizer experiment of Hans Spemann and Hilde Mangold in 1924 our knowledge of the molecular mechanisms of the multi-step formation of embryonic axis has substantially improved. Although in the 30s and 40s the interest of many laboratories was focussed on neural induction (determination of the central nervous system), only crude factors from so-called heterogeneous inducers (liver, bone marrow, etc.,) could be isolated by the traditional biochemical techniques available at this time. An important breakthrough was the characterization and purification of a mesoderm inducing factor, the so-called vegetalizing factor (homologous to Activin) in highly purified from chicken embryos. Much later after the introduction of molecular techniques Vgl and Activin (both belonging to the TGF-β family) and FGFs could be identified as important factors for mesoderm formation. It was in the 90s that secreted neuralizing factors (chordin, noggin, follistatin and cerberus) could be detected, which are expressed at the dorsal side of the early embryo including the Spemann organizer. In contrast to the classical view, these proteins act as antagonists to factors like BMP-4 localized on the ventral side. Of special interest was the fact that inDrosophila sog, homologous to chordin, determines the ventral side, whiledpp, homologous toBMP-4, participates in the formation of the dorsal side. These data of evolutionary conserved genes in both invertebrates and vertebrates support the view that they are descendents of common ancestors, the urbilateralia, living around 300 million years ago. The expression of those genes coding for secreted proteins is closely related to inductive interactions between cells and germ layers. Recently it was shown that planar signals are not sufficient to generate a specific anterior/posterior pattern during the primary steps of neural induction, i.e., formation of the central nervous system in amphibians.