Strain-Induced Jamming in a Spirostomum Ambiguum Cell

Spirostomum ambiguum are single-celled organisms that reside on sediment beds of brackish

water habitats. These worm-like organisms range in size from 1 to 4 mm and feast on

surrounding bacteria. However, there is more to meet the eye regarding this tiny aquatic

organism. In a fraction of the time it takes to blink, Spirotomum ambiguum can shrink to more

than half its original length. This contraction generates a gravitational force equivalent (g-force -

a measure of acceleration) that is 10,000 times greater than that experienced by any other

contracting cell. The energy produced is large enough to destroy the internal structures of a cell.

However, this single-cell organism manages to dissipate this energy.

Manu Prakash and Ray Chang of Stanford University noticed after imaging the insides of

Spirostomum ambiguum cells before and after they had undergone a contraction event that their

internal structures vacuoles (fluid-filled sacs) appear to entangle in the folds of a membrane-like

structure. They identified this structure as an endoplasmic reticulum. To further their

understanding of this entangled membrane-vacuole structure and its influence on the mechanical

properties of this single-cell organism, the researchers performed two-dimensional simulations of

idealized rectangular cells and deformable polygons that represented vacuoles and flexible

strings that characterized the endoplasmic reticulum. They used values based on living

organisms, like density, bend, and stretch properties, to simulate cell behavior properly. Then,

they applied a contractile force and monitored the positions and shapes of the internal

components. They repeated this simulation for cells containing different amounts of strings.

When no strings were present in the simulation, the inside of the model cell behaved like a

liquid. During contraction, the vacuoles flowed past each other with ease, changing their

positions within the cell. This behavior in a living cell can disrupt cellular functions. By adding

strings, the number of vacuole rearrangements during contraction dropped. This drop

accompanies an increase in efficiency in the viscosity of a cell and its ability to absorb energy

from the contraction. By measuring the number of strings connected to each vacuole as a fraction

of the total number of neighboring vacuoles, the value at which the entanglement of the vacuoles

and the strings was sufficient to stop vacuole rearrangements effectively was 0.268. At this point,

as the cell contracted, the system transitioned from fluid-like to solid-like. The vacuoles

deformed under contractive forces and jammed each other into place. This simulated behavior

matched that measured for living Spirostomum ambiguum cells.

Jamming occurs in biological tissues and typically occurs when particle density becomes high

enough that they can no longer move. Materials that undergo strain-induced jamming could help

make shock absorbers. These researchers are currently developing artificial versions of this

system for such applications. These results showed the importance of internal membrane-like

structures in understanding the mechanical response of cells. The insight from this experiment

highlights the essential intersections between biology, physics, engineering, and other disciplines

in field advancements.

Posted by Lyssa Cunha (7)

Link: https://physics.aps.org/articles/v16/191