Ultrastructural organization of the neurovascular unit: Nanoscale perspectives on blood-brain barrier architecture

The blood-brain barrier (BBB) is commonly interpreted through molecular and transport-centered frameworks that emphasize junctional proteins, signaling pathways, and selective permeability. Increasing experimental evidence, however, indicates that barrier integrity cannot be fully explained by molecular composition alone. This review advances an architecture-centered framework in which BBB function emerges from hierarchical ultrastructural organization within the neurovascular unit (NVU), spanning nanoscale, cellular, multicellular, and tissue levels. By integrating data from ultrastructural electron microscopy, super-resolution imaging, and physically defined perturbation models, we demonstrate that BBB stability depends critically on spatial continuity, endothelial polarity, junctional geometry, perivascular coupling, and basement membrane integrity. Across diverse experimental and pathological contexts, BBB dysfunction frequently arises in the absence of junctional protein loss, endothelial death, or overt inflammation, instead reflecting progressive architectural disorganization. These findings support a model of structural causality, in which nanoscale perturbations initiate directional cascades of instability that propagate across organizational scales to produce tissue-level barrier failure. A key novelty of this review is the reinterpretation of nanomaterial-BBB interactions as controlled physical probes of architectural dependency rather than as delivery platforms or toxicological agents. By selectively perturbing membrane geometry, cytoskeletal anchoring, and junctional continuity, nanomaterials expose structural vulnerability thresholds that govern NVU stability independently of biochemical modulation. Beyond the BBB, the framework presented here generalizes to other endothelial and epithelial barriers, positioning the NVU as a paradigmatic structure-governed tissue interface. Emphasizing architecture as a primary determinant of barrier integrity provides a coherent foundation for multiscale, predictive investigation of barrier function and failure.