Aging in space: from entropy to hotspots

A recent Cell study reveals the first lifespan-resolved, multiorgan spatial atlas of mammalian aging, and uncovers a surprising factor at the heart of tissue decline: immunoglobulin G (IgG).Aging is known to vary across tissues [1], yet systematic maps of cellular and molecular changes over space and time remain limited.In this Cell study [2], the authors applied highresolution spatial transcriptomics to nine organs (hippocampus, spinal cord, heart, lung, liver, small intestine, spleen, lymph node, and testis) in young and old male mice.They generated the Gerontological Geography (GG), a comprehensive atlas that captures age-related changes in tissue organization, cellular composition, and gene expression.To capture fundamental features of tissue-level aging, the authors introduced two integrative metrics.The first metric, "organizational structure entropy" (OSE), quantifies disorder in local cellular neighborhoods.Aging led to an increase in OSE, reflecting a gradual loss of spatial organization and regional specialization within tissues.For example, in aged spleens, the normally well-defined boundaries between white and red pulp become indistinct, with lymphocytes dispersing from their niches.Likewise, in aged livers, zone 3 hepatocytes surrounding central veins appear more diffuse, while zone 1 hepatocytes near portal veins are depleted and partially replaced by fat-like cells.The second metric, the "cell identity" score, assesses the preservation of cell-type-specific transcriptional signatures.Aging was found to erode cell identity, especially in regions with high OSE.Together, higher OSE and lower cell identity illustrate how tissues gradually lose structural organization and functional homeostasis with age.Another key contribution of this study is the concept of senescence-sensitive spots (SSSs).The authors defined SSSs as the top 5% of tissue spots with the highest aging scores, calculated from tissue-specific aging-DEGs overlapping with published senescence marker sets.These spots show low cell identity, high OSE, and strong enrichment of inflammatory and senescence-associated secretory phenotype (SASP) pathways.Genes related to humoral immune response, complement activation and SASP were elevated in the SSSs,