17 research outputs found
Active wetting of epithelial tissues
Development, regeneration and cancer involve drastic transitions in tissue
morphology. In analogy with the behavior of inert fluids, some of these
transitions have been interpreted as wetting transitions. The validity and
scope of this analogy are unclear, however, because the active cellular forces
that drive tissue wetting have been neither measured nor theoretically
accounted for. Here we show that the transition between 2D epithelial
monolayers and 3D spheroidal aggregates can be understood as an active wetting
transition whose physics differs fundamentally from that of passive wetting
phenomena. By combining an active polar fluid model with measurements of
physical forces as a function of tissue size, contractility, cell-cell and
cell-substrate adhesion, and substrate stiffness, we show that the wetting
transition results from the competition between traction forces and contractile
intercellular stresses. This competition defines a new intrinsic lengthscale
that gives rise to a critical size for the wetting transition in tissues, a
striking feature that has no counterpart in classical wetting. Finally, we show
that active shape fluctuations are dynamically amplified during tissue
dewetting. Overall, we conclude that tissue spreading constitutes a prominent
example of active wetting --- a novel physical scenario that may explain
morphological transitions during tissue morphogenesis and tumor progression
Shaping the growth behaviour of biofilms initiated from bacterial aggregates
Bacterial biofilms are usually assumed to originate from individual cells
deposited on a surface. However, many biofilm-forming bacteria tend to
aggregate in the planktonic phase so that it is possible that many natural and
infectious biofilms originate wholly or partially from pre-formed cell
aggregates. Here, we use agent-based computer simulations to investigate the
role of pre-formed aggregates in biofilm development. Focusing on the initial
shape the aggregate forms on the surface, we find that the degree of spreading
of an aggregate on a surface can play an important role in determining its
eventual fate during biofilm development. Specifically, initially spread
aggregates perform better when competition with surrounding unaggregated
bacterial cells is low, while initially rounded aggregates perform better when
competition with surrounding unaggregated cells is high. These contrasting
outcomes are governed by a trade-off between aggregate surface area and height.
Our results provide new insight into biofilm formation and development, and
reveal new factors that may be at play in the social evolution of biofilm
communities
Dewetting of cellular monolayers
We investigate the physical principles of cellular layer stability. We show that cohesive cellular layers deposited on non-adhesive substrates are metastable and “dewet" by nucleation and growth of dry patches. The dewetting process can be induced either chemically by a non-adhesive surface treatment or, unlike simple liquids, physically by a decrease in the substrate rigidity. We thus unveil two mechanisms by which the integrity of cellular layers can be compromised. We interpret the opening dynamics by an analogy with the dewetting of viscous films. This analogy can be exploited to estimate parameters characterizing the mechanical response of a cellular layer
Spreading dynamics and wetting transition of cellular aggregates
We study the spreading of spheroidal aggregates of cells, expressing a tunable level of E-cadherin molecules, on glass substrates decorated with mixed fibronectin and polyethylene glycol. We observe the contact area by optical interferometry and the profile by side-view microscopy. We find a universal law of aggregate spreading at short times, which we interpret through an analogy with the spreading of viscoelastic droplets. At long times, we observe either partial wetting or complete wetting, with a precursor film of cells spreading around the aggregate with two possible states. In strongly cohesive aggregates this film is a cellular monolayer in the liquid state, whereas in weakly cohesive aggregates, cells escape from the aggregate, forming a 2D gas. The escape of isolated cells is a physical mechanism that appears also to be present in the progression of a noninvasive tumor into a metastatic malignant carcinoma, known as the epithelial-mesenchymal transition