Biological Science: Research Developments and Innovations Vol. 3

Cell biology has undergone a conceptual reorientation over the past decade. The textbook picture of the cell as a set of stable, membrane-delimited compartments linked by vesicular traffic has been supplemented, and in places displaced, by a picture of graded, dynamic and physically heterogeneous organisation in which membraneless assemblies, persistent inter-organelle contacts, mechanical force transmission and continuous molecular turnover are treated as primary organising principles. This review examines the strength of the evidence underlying that reorientation. Peer-reviewed literature on eukaryotic subcellular organisation was identified through structured searching of biomedical and multidisciplinary scholarly sources, supplemented by citation tracking and by consultation of methodological and consensus statements, with an emphasis on work that tests rather than assumes the prevailing framework. The synthesis is organised around five themes: the physical environment of the cytoplasm and the vocabulary used to describe subcellular order; membraneless compartmentalisation and the contested status of liquid–liquid phase separation as an in-cell mechanism; the reconceptualisation of organelles as an interacting system defined by contact sites and shared lipid and ion fluxes; mechanical continuity between the extracellular environment, the cytoskeleton and the nucleus; and the measurement technologies that generate the current evidence base. Across these themes the same interpretive weakness recurs. Mechanisms established in reconstituted or strongly perturbed systems are frequently transferred to the intact cell without the demonstrations of necessity, sufficiency and physiological scaling that such transfer requires. Confidence is highest where in-cell structural observation, quantitative perturbation and independent replication converge, as for contact-site tethering, actin network mechanics and mitochondrial remodelling. Confidence is substantially lower for claims that particular condensates perform specific biochemical functions, for cell-type definitions derived from transcript abundance alone, and for causal accounts of mechanical signalling built on stiffness ranges far outside tissue physiology. Priorities for the next phase of work include the development of endogenous-level, reversible perturbations, the routine reporting of physical parameters alongside molecular ones, and the extension of in-cell structural methods beyond a narrow set of cultured mammalian lines.