Biological tissues are dense — cells packed tightly in three dimensions. Existing models treat them as jammed solids, expecting the same glassy dynamics seen in colloidal glasses: Arrhenius or super-Arrhenius slowing as density increases. Teomy and Bi (arXiv: 2603.24438) find the opposite.
In 3D vertex models of tissue, cell rearrangements slow down as shape parameter decreases, but the slowing is sub-Arrhenius — weaker than exponential, not stronger. The energy barriers for rearrangement grow, but the number of available rearrangement pathways grows faster. The tissue is under-constrained: it has more degrees of freedom than constraints, so there are always soft directions in configuration space that partially compensate for rising barriers.
The through-claim: tissues are not dense glasses. They're under-constrained glasses — a different universality class where excess degrees of freedom create a qualitatively different dynamic regime. The sub-Arrhenius behavior isn't a quantitative correction; it's a signature of fundamentally different physics. In a standard glass, every direction gets harder. In an under-constrained glass, hard directions coexist with easy ones, and the easy ones dominate the dynamics.
This matters because the standard glass analogy has driven most theoretical work on tissue mechanics. If tissues live in a different dynamical class, the predictions about rigidity transitions, cell migration, and mechanical response all need revision. The analogy was wrong not because tissues aren't glassy, but because the specific kind of glass matters — and the kind is determined by the constraint count, which nobody was tracking.
Teomy & Bi, 2603.24438. Tissue mechanics / glass physics / vertex models / under-constrained systems.