Cell fate determination by lateral inhibition via Notch/Delta signalling has been extensively studied. Most formalised models consider Notch/Delta interactions in fields of cells, with parameters that typically lead to symmetry breaking of signalling states between neighbouring cells, commonly resulting in salt-and-pepper fate patterns. Here, we consider the case of signalling between isolated cell pairs, and find that the bifurcation properties of a standard mathematical model of lateral inhibition can lead to stable symmetric signalling states. We apply this model to the adult intestinal stem cell (ISC) of Drosophila, the fate of which is stochastic but dependent on the Notch/Delta pathway. We observe a correlation between signalling state in cell pairs and their contact area. We interpret this behaviour in terms of the properties of our model in the presence of population variability in contact areas, which affects the effective signalling threshold of individual cells. Our results suggest that the dynamics of Notch/Delta signalling can contribute to explain stochasticity in stem cell fate decisions, and that the standard model for lateral inhibition can account for a wider range of developmental outcomes than previously considered.

We acknowledge support from Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) to N.G. and from Cardiff University to J.d.N. J.G.O. and R.M.-C. are supported by the Ministerio de Economía, Industria y Competitividad and Fonds Européen de Développement Économique et Régional (FEDER) (through project FIS2015-66503-C3-1-P), and by the Institució Catalana de Recerca i Estudis Avançats (ICREA) Academia Programme. R.M.-C. also acknowledges financial support from La Caixa Banking Foundation.