The Physics of Cell Fate Branching: A Swedish Flag Model

Albert K Harris
Professor of Biology
UNC-Chapel Hill
Presented in the Embryo Physics Course, February 20, 2013


Two central puzzles of embryology are: (first) how cell differentiation is controlled such that nerve cells each develop at their correct locations, relative to skin cells, skeleton etc. And (second) how this control mechanism compensates for changes in volume, over as much as a 16-fold range. The latter ability was discovered in 1895 by Hans Driesch, and raises huge, unresolved questions, on the scale of Relativity. Currently, the most popular explanation is Lewis Wolpert’s concept of “Positional Information” which he presents as “The French Flag Model”. This metaphor uses a diffusion gradient to control differentiation into red, white and blue cell types.

A very different explanation (Richard Gordon and Natalie Bj√∂rklund) is that contraction waves in embryos govern “Cell-State-Splitters”, with bending of cell sheets controlling what cell type they become. As a parallel to Wolpert’s French flag (and being myself partly Swedish in ancestry), I use the geometry of the Swedish flag to explore how bending of sheets can generate patterns that are just as much able to adjust relative dimensions over an unlimited range of absolute sizes, thus accounting for Driesch’s Phenomenon.








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