{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"ff36c67df883a5735576ddcfa96a024fb89cc323d94212795e33219e2a9fe249","created":"2026-10-03T07:17:57Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011"],"salt":"bff414200a4346d11fb43e9fc3fd87cb560cb22fd51c91c25848aae7fcbb968a","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"yes-no","assisted_by":[],"external_id":"bio.morphogenesis-mechanics.shape-index-fluidity","kind":"well-posed","literature_status":"contested","n":"1","parents":[],"plain":"Computer models of tightly packed cells predict that a tissue switches from solid-like to fluid-like once cells become elongated beyond a critical shape. Experiments disagree on whether this shape rule works in real tissues.","posed_since":"2015","precise":"The 2D vertex model with energy $E = \\operatorname{sum}_i [K_A (A_i - A_0)^2 + K_P (P_i - P_0)^2]$ ($A_i, P_i$ = cell area and perimeter) has a rigidity transition at target shape index $p_0 = P_0/\\sqrt{A_0} \\sim 3.81$. Determine whether the observed shape index $q = \\langle P/\\sqrt{A}\\rangle$ classifies fluid versus solid epithelia in vivo across tissues and species, or whether fluctuations, division and active tension remodeling set fluidity independently of q.","problem_ref":null,"references":"","settled_by":"Simultaneous in vivo measurement of q and of tissue rheology (cell rearrangement rate, stress relaxation) across several tissues and perturbations.","status_note":"Support from airway epithelia (2015) contrasts with 2026 reports of shape-independent fluidity in epithelial monolayers (arXiv 2603.05548).","title":"Does cell shape predict whether a tissue flows or 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