A cell can suddenly form a large rounded pocket on its surface, as if its membrane were swelling locally. Researchers at RIKEN have reproduced this behavior in highly simplified artificial cells. The experiment makes it possible to observe which physical mechanisms are sufficient to cause this change in shape, without the many reactions that occur simultaneously in a living cell.
This pocket is called a “bleb.” It appears when the membrane locally detaches from the layer supporting it and internal pressure pushes it outward. Living cells use this type of deformation, in particular, when they move or change shape.
Confocal time-lapse images of a liposome showing the formation and expansion of a vesicle
Studying the phenomenon directly in a cell poses a problem: very many components act at the same time. The researchers therefore built a simplified model. Their artificial cell is essentially a vesicle, that is, a small pocket surrounded by a membrane comparable in principle to the one that encloses a cell.
The result is visible: a large protrusion can form on this vesicle.
To trigger this transformation, the team recreated inside it a mechanical layer associated with the membrane. In a real cell, this function is notably performed by the cytoskeleton, a network of filaments that helps maintain and modify its shape. The model makes it possible to control separately the forces exerted on this envelope and on the membrane.
When the balance between these elements changes, part of the membrane can detach from its internal support. The pressure contained in the vesicle then pushes this area outward. The pocket grows and concentrates part of the available membrane. The phenomenon shows that a spectacular transformation does not necessarily require a long chain of biological reactions.
This simplification helps separate the possible causes. In a living cell, proteins, membrane, internal pressure and cytoskeleton interact continuously. Here, the parameters can be modified more directly. Scientists can therefore determine which physical properties promote the appearance of a protrusion and which limit its growth.
The significance extends beyond a cell’s external shape alone. Membrane deformations are involved in cell migration, division and several other behaviors. Understanding their origin requires distinguishing what comes from biochemical reactions from what can emerge simply from forces exerted on a flexible envelope.
The model remains deliberately much simpler than a real cell. It reproduces neither all of its components nor all of its regulatory mechanisms. This limitation is also what makes it experimentally useful: future experiments can gradually add certain biological elements and measure how each one changes the formation, size and duration of these large membrane pockets.