Epigenetic selection: an alternative mechanism of pattern formation.

It is commonly assumed that patterned development is specified by pre-patterns or programs, actual development being a necessary consequence of previous conditions. However, the variability of normal development and its regenerative capacities are evidence for additional patterning processes. Predictable mature structures could result from continued "epigenetic selection" of the most appropriate developmental events. This selection would occur from an excess of possibilities that are genetically equivalent. The final balanced state would be specified by the genes, and the developmental system could gravitate towards this state without a detailed program. Selection could result from competition between cells and tissues for limiting developmental signals. The success or continuation of events that could start randomly would depend on feedback relationships with complementary developmental events. Specialized processes could be gradually localized if differentiation itself consumed limiting signals and if this consumption increased as differentiation proceeded. Spatial patterns could be formed if the movement of signals was gradually facilitated along the axes where it were initiated by diffusion. For example, induced facilitated transport could be the basis of an advantage of multicellular centers over scattered cells that have specialized in the same way. If epigenetic selection has a developmental role it requires a revision of common views concerning the cellular traits and the gene functions necessary for patterned development. An example of these traits is that cells should be expected to respond to changes in signal availability, not necessarily to signal concentration at any given time.