Maps and Clocks in the Ageing Artery, Skinand Brain: Separating Where a Lesion MayAppear from Whether and When It Does

Abstract

Age-related lesions are not distributed at random. In several tissues their location is predictable while their timing is not, yet the two facts are usually described together, as though a single process were simply running faster in some people than in others. This preprint proposes keeping them apart. A tissue carries a map, which determines where a lesion may appear, and each individual carries a clock, which determines whether and when it does. Map and clock are treated as independent variables that must coincide, rather than as cause and effect. Two methodological constraints follow. First, any measure of biological time must be independent of the outcome it is meant to predict: a clock built from indicators of the disease it forecasts measures nothing. Second, alteration of a tissue barrier must be measured before accumulation begins, otherwise the outcome is again mistaken for the cause. Applied strictly, these constraints exclude several widely used pace-of-ageing measures. The framework is examined in three tissues: Artery — the map is intrinsic and geometric. Site-specific telomere attrition has been documented in human material since 1995, with faster loss in the intima of high-stress territories than in the protected internal thoracic artery of the same donor, and no difference in the media. No equivalent regional epigenetic measurement exists. Skin — the map is extrinsic, drawn by clothing, and exposed and protected sites coexist in the same living subject. Age and sun exposure leave signatures that run in opposite directions, both in DNA methylation and in glycosaminoglycan and tissue water content. The clinical dryness of exposed skin is shown to be a property of the stratum corneum, not of the dermis, which in fact holds more water at exposed sites. Central nervous system — both terms are already measurable by region. The epigenetic clock has been applied to multiple anatomical sites within the same subject, and a second kind of map, built on the chronology of myelination rather than on haemodynamics, predicts regional vulnerability. The section also examines why radiological reporting of white matter hyperintensities diverges from the published evidence on their prognostic weight. No claim of priority is made for the mechanisms described: disturbed flow is known to promote endothelial senescence, and early vascular ageing is an established field. What is offered for refutation is the inversion of the causal relation, the two constraints, and a register of seven testable predictions classified as mature, premature, or possibly unanswerable. This is a hypothesis and perspective paper. It reports no new experimental data. Limitations, including the small samples underlying key human studies and the absence of a formal systematic search, are stated explicitly. An AI assistant was used for literature retrieval and drafting; the author conceived the framework, verified the sources and takes full responsibility for the content.