Morphogen gradient formation in partially absorbing media

TL;DR

This paper introduces a probabilistic model for morphogen gradient formation, where absorption depends on a random occupation time threshold, affecting gradient shape and relaxation dynamics.

Contribution

It generalizes classical absorption models by incorporating a stochastic stopping time, revealing effects on gradient profiles and relaxation times.

Findings

The model can replicate classical gradient profiles with different absorption thresholds.

Stochastic absorption can significantly reduce the accumulation time to reach steady state.

Gradient formation dynamics are sensitive to the distribution of occupation time thresholds.

Abstract

Morphogen gradients play an essential role in the spatial regulation of cell patterning during early development. The classical mechanism of morphogen gradient formation involves the diffusion of morphogens away from a localized source combined with some form of bulk absorption. Morphogen gradient formation plays a crucial role during early development, whereby a spatially varying concentration of morphogen protein drives a corresponding spatial variation in gene expression during embryogenesis. In most models, the absorption rate is taken to be a constant multiple of the local concentration. In this paper, we explore a more general class of diffusion-based model in which absorption is formulated probabilistically in terms of a stopping time condition. Absorption of each particle occurs when its time spent within the bulk domain (occupation time) exceeds a randomly distributed threshold $a$; the classical model with a constant rate of absorption is recovered by taking the threshold distribution $\Psi(a)=\e^{-\kappa_0 a}$. We explore how the choice of $\Psi(a)$ affects the steady-state concentration gradient, and the relaxation to steady-state as determined by the accumulation time. In particular, we show that the more general model can generate similar concentration profiles to the classical case, while significantly reducing the accumulation time.