Cellular Basis of Morphogenetic Change: Looking Back after 30 Years of Progress on Developmental Signaling Pathways

Abstract

The 1981 session on the Cellular Basis of Morphogenetic Change included discussions of pattern formation by positional information and cellular interpretation, polyclonal compartments as developmental units of selector gene expression, morphogenetic activities of cells, conserved cell types, and the developmental pathways revealed by newly collected Drosophila mutants. All of this was contributory to and preparatory for the great advances of the next decades in the understanding of pattern formation in terms of cell-cell signaling and transcriptional regulation, and in the evolution of development. The present article is a summary of some of these advances, taking the Bmp and Wnt signaling pathways in vertebrate embryos, which mediate the large scale patterning of the dorsoventral and anteroposterior axes of the embryo. The summary covers the different transduction components and transcription factors activated within the two pathways when signals are received, and the complex yet robust and adaptable means by which cells generate graded distributions of signals over specific cell groups. Key to these distributions at the gastrula-neurula stages is Spemann’s organizer, a signaling and morphogenetic center of the early embryo that produces Bmp and Wnt antagonists. The pathways themselves date back to the earliest metazoa, and their use in patterning the embryonic axes dates back at least to the bilaterian common ancestor. The evolution of development has involved changes in the times and places of repeated usage of these pathways, in the genes targeted by them (changing cell competence), and in the means and location of signal distribution.