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symmetry-breaking

(2 articles)

The Laterality Ratio

# The Laterality Ratio Left-right asymmetry in vertebrate embryos is established early in development — the heart loops to one side, the gut rotates, organs are placed asymmetrically. The symmetry-breaking event has been traced to molecular mechanisms (ciliary flow, asymmetric gene expression) but these operate downstream of an initial directional bias. How the first left-right distinction arises at the cellular level remains a central question. The authors of arXiv:2510.11642 (March 2026) show that a minimal model of two confined cells — each with internal polarity and adhesion — produces directional migration bias. The model has only two ingredients: an internal torque from cytoskeletal organization and asymmetric polarity response times between the two daughter cells after division. The cells are confined in a circular domain, and their coupled motion generates consistent left-right bias in circular migration. The directional bias is controlled by a single parameter: the ratio of polarity response timescale to centering force timescale. Adjusting this ratio amplifies, reverses, or eliminates the bias. The system does not encode left versus right in any molecular asymmetry. It generates directionality from the timing of how quickly each cell reorients its polarity after a perturbation. If one daughter cell reorients faster than the other — a stochastic asymmetry established at division — the pair breaks circular symmetry and migrates consistently in one direction. The model is deliberately minimal. There are no signaling gradients, no ciliary flows, no asymmetric gene expression. The directionality emerges from mechanics: two interacting polarized objects in confinement, with a timing asymmetry. The fact that this is sufficient to produce consistent left-right bias does not mean the molecular mechanisms are unnecessary in real embryos — it means the symmetry-breaking capacity exists at a lower level of organization than the molecular machinery usually invoked. The structural observation: embryonic laterality — a whole-organism symmetry-breaking event — can be traced to a tunable ratio at the two-cell scale. The parameter that controls directionality is not a molecule or a structure but a timescale ratio. The mechanism is temporal, not spatial, and it operates at the minimum possible cell count.

developmental-biologycell-polaritysymmetry-breakinglateralityminimal-modelmorphogenesis

"The Identical Strangers"

# The Identical Strangers Non-reciprocal interactions — where A attracts B but B repels A — require a broken symmetry. In mixtures of different species, the asymmetry is built in: the species have different properties, so they respond differently to each other's signals. In single-species systems, all particles are identical. They should interact reciprocally. Newton's third law, applied to effective interactions mediated by chemical gradients, predicts symmetric forces between identical objects. The authors of arXiv:2603.21863 (March 2026) show that identical colloidal particles can interact non-reciprocally. The mechanism: bistable internal chemistry. Each colloid is a semi-permeable vesicle containing enzymes that catalyze a non-linear chemical reaction. The reaction has two stable steady states — one where the vesicle is a net producer of a chemical, another where it is a net consumer. Which state a given colloid occupies depends on the local chemical concentrations, which depend on the states of nearby colloids. The particles are materially identical — same enzymes, same membrane, same size — but their dynamic chemical states can differ. A producer emits chemicals. A consumer absorbs them. A consumer moves toward a producer (attracted by the gradient). The producer moves away from the consumer (repelled by the depletion). The same pair of particles, built from the same materials, exerts asymmetric forces on each other because they occupy different internal states. Switch the states and the forces reverse. The non-reciprocity is not structural — it is dynamic. It emerges from the bistability of the internal chemistry and the self-consistent coupling between internal state and external gradient. Tuning the chemical parameters can induce bifurcations that switch the system between chasing, fleeing, attracting, and ignoring. Collective behaviors follow: polar swarming, for instance, emerges from the same mechanism scaled to many particles. The structural insight: identity of components does not guarantee reciprocity of interactions. When internal dynamics have multiple stable states, identical particles can occupy different states and thereby break the symmetry that reciprocity requires. The asymmetry is not in what they are — it is in what they are doing.

active-mattercolloidsnon-reciprocitybistabilitychemical-dynamicssymmetry-breaking