Tunneling nanotubes: a new dimension of intercellular communication and recent progress.

Communication between cells can occur over long distances through the diffusion of soluble mediators and the delivery of extracellular vesicles (EVs), and over short distances through contact-dependent junctional structures formed between adjacent cells. Recent studies on tunneling nanotubes (TNTs) suggest that, in certain systems, cells may engage in relatively direct material exchange that does not rely on diffusion or stochastic uptake. TNTs typically appear as thin, bridge-like membranous protrusions spanning intercellular gaps. They are supported by F-actin and may incorporate microtubules, thereby enabling intercellular transfer ranging from ions and small signaling molecules to RNA, protein complexes, and even organelles. A major challenge in the field, however, is the lack of unified definitions and evidentiary standards. The absence of universal specific markers, the fragility of these structures, and the limitations of static two-dimensional imaging make TNTs prone to confusion with filopodia or cytonemes, cytokinetic bridges, apparent transfer arising from EV deposition or uptake, and tumor microtubes (TMs), which undermines functional attribution and cross-study comparability. Guided by a minimum evidence set, this review proposes a reproducible identification workflow, systematically summarizes TNT structural heterogeneity, including thin versus thick TNTs and open versus closed termini or gap junction-like contacts, and relates these features to transport capacity and transport modalities. We also stratify disease-associated findings by strength of evidence to distinguish areas of consensus from ongoing controversies. We further highlight the double-edged nature of TNTs: they may support metabolic rescue and tissue repair through organelle sharing, yet they can also be exploited by pathogens, tumors, and pathogenic aggregates to facilitate stealthy spread, therapeutic tolerance, and pathological propagation. Finally, we summarize current intervention entry points and their limitations, and argue that future work should build on in vivo dynamic evidence, incorporate controls that distinguish TNT-mediated transfer from vesicle-based routes, and pursue cell-type-specific mechanistic dissection to establish a more robust chain of evidence. Such efforts may enable the identification and mitigation of disease-associated detrimental TNT activities while preserving physiological roles, thereby informing feasibility assessment and early validation for clinical translation.