The role and limits of a gradient based explanation of morphogenesis: a theoretical consideration.

Development of an organism is a multi-dimensional process leading to the generation of complex species-specific structures. This specificity suggests machine-like organisation. The uneven distribution (gradient) of soluble substances (morphogens) and specific receptor-ligand interactions are known to cause differential gene expression. Therefore gradients of morphogens are used as a causal explanation of developmental processes. However each attempt to describe development causally should take into account both the local fine organisation and global robustness of morphogenesis. The classical view of the role of morphogens will be critically considered and possible alternative proposed. The core idea of my proposal is that the main function of morphogenetic substances could be a context dependent modification of cell behaviour. Both history and different features of morphogenetic fields create the framework for the activity of morphogenes.