  This paper presents an interesting experimental example of voter-model
statistics in biology. In recent work on mouse tail-skin, where proliferating
cells are confined to a two-dimensional layer, we showed that cells proliferate
and differentiate according to a simple stochastic model of cell division
involving just one type of proliferating cell that may divide both
symmetrically and asymmetrically. Curiously, these simple rules provide
excellent predictions of the cell population dynamics without having to address
their spatial distribution. Yet, if the spatial behaviour of cells is addressed
by allowing cells to diffuse at random, one deduces that density fluctuations
destroy tissue confluence, implying some hidden degree of spatial regulation in
the physical system. To infer the mechanism of spatial regulation, we consider
a two-dimensional model of cell fate that preserves the overall population
dynamics. By identifying the resulting behaviour with a three-species variation
of the "Voter" model, we predict that proliferating cells in the basal layer
should cluster. Analysis of empirical correlations of cells stained for
proliferation activity confirms that the expected clustering behaviour is
indeed seen in nature.
