Developmental potential

In summary (and probably to no one's genuine surprise), it seems clear that some of the key themes in the mechanisms employed during development reiterate themselves throughout the animal kingdom. Yet, as our understanding becomes more refined, new and beguiling observations point to unique aspects of each developmental program. The concentration and absolute position of a variety of positional signaling molecules is likely to be very important in determinative events (establishment of the anteroposterior positioning in a field as in retinal development, establishment or enactment of a hox code, and selector gene regulation through gradients in Drosophila). Appropriate signalling responses are virtually certain to depend critically on the appropriate expression of each component of cellular signal transduction pathways (initiated by the activation of cell-surface receptor protein kinases to finally eliciting gene expression changes through the differential activity of specific transcription factors). The important biochemical details of transcription factor activation of specific respondent genes may be either simpler (as indicated from the murine/Drosophila domain swap experiments) or more complicated (from the responses of mim-1 to cellular versus viral myb proteins) than we had heretofore anticipated.