A bifurcation model for developmental processes

I have constructed, for developmental processes, a qualitative model similar to the compartment hypothesis in Drosophila, and examined its relevance to vertebrate systems. In this model a polarized “cluster” of interacting cells will be the unit for “bifurcation” of the developmental pathway into two alternative states of “locon” which is the genetic unit controlling this process. The minimum size of the cluster critical for bifurcation and the size of the emerging subclusters will be dictated by the cognate locon. This will obviate the need for an extrinsically imposed threshold of some state variable for the boundary of the two subclusters. However, the orientation of bifurcation will be determined by the polarity of the cluster. A physiological factor of competence will impose a temporal constraint to bifurcation.Thus, combinatorial binary codes for a set of locons, like those originally devised by Kauffman (1973), can be assigned to developmental pathways. One of the clusters emerging from a sequence of bifurcations will have the same code as the mother cluster. It will represent the “developmental sink”, and will not recycle through the bifurcation series originating from the initial mother cluster, because of the difference in spatio-temporal factors incorporated in the size and competence of the individual clusters. If bifurcations are prevented, the mother cluster will be forced along the pathway of the developmental sink.I have applied the model to cases in vertebrate development where commitments to developmental pathways for aperiodic or periodic segmentations may follow a linear temporal sequence, producing, in turn, subclusters of uncommitted, or stem, cells towards the more intensely polarized end of the mother cluster. Such cases include limb, somite and tail formation and several stem cell systems with a finite lifespan. I have discussed some possible experimentation which emerges from the model.