  Measurements on embryonic epithelial tissues in a diverse range of organisms
have shown that the statistics of cell neighbor numbers are universal in
tissues where cell proliferation is the primary cell activity. Highly
simplified non-spatial models of proliferation are claimed to accurately
reproduce these statistics. Using a systematic critical analysis, we show that
non-spatial models are not capable of robustly describing the universal
statistics observed in proliferating epithelia, indicating strong spatial
correlations between cells. Furthermore we show that spatial simulations using
the Subcellular Element Model are able to robustly reproduce the universal
histogram. In addition these simulations are able to unify ostensibly divergent
experimental data in the literature. We also analyze cell neighbor statistics
in early stages of chick embryo development in which cell behaviors other than
proliferation are important. We find from experimental observation that cell
neighbor statistics in the primitive streak region, where cell motility and
ingression are also important, show a much broader distribution. A non-spatial
Markov process model provides excellent agreement with this broader histogram
indicating that cells in the primitive streak may have significantly weaker
spatial correlations. These findings show that cell neighbor statistics provide
a potentially useful signature of collective cell behavior.
