Hypothesis: Senescent cells in aged tissue generate a biphasic secretory gradient where a soluble SASP core (IL-6, IL-1α) extends ~20–30 µm, while vesicle‑bound MMPs and matricryptins form a distal halo reaching ~80–100 µm, thereby imposing spatially segregated phenotypic programs on neighboring stromal cells.

Rationale: Spatial transcriptomics in idiopathic pulmonary fibrosis has shown enrichment of senescence‑associated signatures near airways but has not quantified radial SASP decay or distinguished soluble versus extracellular‑vesicle cargo [https://www.science.org/doi/10.1126/sciadv.adl5473]. MERFISH measurements in liver and kidney demonstrate that subcellular transcript maps can be anchored to scRNA‑seq without atlas integration, providing a quantitative baseline for gradient modeling [https://vizgen.com/resources/comparative-analysis-of-merfish-spatial-transcriptomics-with-bulk-and-single-cell-rna-sequencing]. Visium v2 data achieve sufficient spatial autocorrelation to detect Moran’s I‑significant gene modules at ~2 µm bins, though sensitivity drops for low‑abundance transcripts [https://pmc.ncbi.nlm.nih.gov/articles/PMC12888464]. Together, these platforms enable the deconvolution of cell‑type‑specific SASP factors and the tracking of vesicle‑associated mRNA signatures if appropriate probes are added.

Mechanistic insight: Senescent cells differentially package SASP constituents; soluble cytokines are secreted via the conventional secretory pathway, creating a steep concentration gradient limited by diffusion and receptor uptake. In contrast, matrix‑modifying enzymes and bioactive lipids are preferentially loaded into exosomes or exophers, which travel farther before releasing their cargo upon fusion or shear‑stress rupture. This split delivery predicts that proximal fibroblasts will exhibit a transcriptional program dominated by NF‑κB‑driven proliferation, whereas distal fibroblasts will show a TGF‑β‑biased, myofibroblast signature driven by matricryptin‑integrin signaling.

Testable predictions:

1. In aged murine muscle, liver, and adipose tissue, high‑plex spatial transcriptomics (e.g., MERFISH with a custom SASP panel) will reveal two concentric zones of significant gene‑set enrichment: an inner zone enriched for IL6, IL1A, CXCL1 and an outer zone enriched for MMP14, ADAMTS1, and exosome‑related markers (RAB27B, SYTL4) with expression peaks at ~25 µm and ~70 µm from the centroid of p16^INK4a^‑positive cells.
2. Pharmacological inhibition of exosome release (using GW4869) will selectively attenuate the outer zone MMP/ADAMTS signature without affecting the inner cytokine gradient, measurable as a reduction in Moran’s I for outer‑zone genes but unchanged inner‑zone autocorrelation.
3. Spatial proteomics (e.g., CODEX or imaging mass cytometry) will corroborate the transcriptomic zones by detecting phosphorylated SMAD2/3 predominantly in the outer halo and nuclear p65 predominantly in the inner core.

Falsification: If high‑resolution spatial maps show a single monotonic decay of all SASP factors with distance, or if exosome blockade fails to alter the distal MMP signature while leaving the proximal cytokine pattern intact, the biphasic model would be refuted. Conversely, observation of the predicted dual‑zone architecture and its selective sensitivity to exosome inhibition would support the hypothesis.

This framework directly addresses the noted data desert by proposing a feasible, multiplexed spatial assay that distinguishes soluble versus vesicular SASP components, thereby enabling the first quantitative atlas of senescence‑driven niche architecture in healthy aged tissue.