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Abstract

Many cells ingest foreign particles through a process known as phagocytosis. It now turns out that some cell types organize phagocytosed microparticles into crystalline arrays. Much like the classic crystallization of colloidal particles in a thermal bath, crystallization within the cell is driven by the spatial confinement of mutually repelling particles, in this case by the cell membrane. Cytoskeleton-driven motions exert a randomizing force, similar to but stronger than thermal forces; these motions anneal defects and purify the colloidal crystals within the cells. Bidisperse mixtures of microspheres phase separate within the cell, with the larger particles crystallizing around the nucleus and the smaller particles crystallizing around the perimeter of the large particle array. Mitochondria also participate in this kind of size segregation, which appears to be driven by membrane tension and curvature minimization. Beyond the curiosity of the phenomenon itself, cell-assisted colloidal assembly may prove useful as a new tool to study a variety of biophysical processes including cytoskeletal rearrangements, organelle–membrane interactions, the in vivo mechanics of microtubules, the cooperativity of molecular motors and intracellular traffic jams on cytoskeletal filaments.