Scientists from the Politecnico di Torino in Italy and the Landau Institute of Theoretical Physics in Russia have derived a theory to describe how eukaryotic cells (such as those found in all higher organisms) respond to chemical signals in their environments. Considering that coordinated sensing of and movement toward chemical signals is a vital processes in embryology (how cells know where to go in fashioning the organism), inflammation, and immune response, directional maneuvering at the cellular level is quite important. Here's what happens.
First, receptors in the membranes of the cells become activated by the presence of trace amounts of chemicals---even down to the
nano-molar level or about one molecule in a cubic micron---in the cells' vicinity. Not only do the receptors sense the presence of the attractants but, through the differential activation of 10,000 or more receptors distributed along the body of the cell, the direction of the source of the attractant can be located to within a few degrees. Ability to train upon a 5% chemical gradient allows the cell to know where it should be going, whether to find food, antigens, or to take up
its place in a larger multi-cellular structure.
Second, a cascade of polymerization steps now ensues within a few minutes. Consequently the cell develops head and tail structures, the better to make possible travel along the chemical gradient (chemotaxis). In nature, cells have also been known to plan their travel by exploiting thermal gradients (thermotaxis) and electrical gradients (galvanotaxis).
According to Andrea Gamba
(firstname.lastname@example.org) and coauthors the new results consist of being able now to demonstrate in a mechanistic way how the cell's directional sensing and response comes about through a kind of self-organized phase transition; when the chemical gradient exceeds a certain threshold level the dynamic of growth of clusters of signaling molecules on the cell surface fine-tunes to sense the slight unbalance in activated receptors and provides a fast polarization in the direction of the gradient, thus providing a compass bearing which is able to initiate the modification in
the cellular structure.
The scientists argue that the physical amount of space along the body of large eukaryotic cells needed for making such an astute directional assessment might
explain why bacteria (with much smaller bodies) do not have a spatial system of directional sensing. (Gamba et al., Physical Review Letters, 12 October 2007)