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ELI5 Yahaaa

And a short list of exceptions

You Are Not Made of What You Were Made Of

Most of you is younger than you are. A few parts are exactly as old as you, and can never be replaced.

Gut liningReplaced inabout 5 daysSkin surfaceAbout 4 weeksRed blood cellsAbout 4 monthsSkeletonRoughly 10 years
Four tissues, four completely different clocks.

  1. Step 1 of 5

    Cells wear out, so bodies replace them constantly.

    Cells wear out and are shed Stem cellsunderneathdivide New cells push up to replace them And the wholesurface hasturned over

    Anything that takes a beating gets rebuilt fastest. The lining of your gut is soaked in acid and enzymes and scraped by food, so it is renewed roughly every five days. Skin sheds and regrows in weeks. Neither is unusual; both are just maintenance.

  2. Step 2 of 5

    Even bone, which feels permanent, is being torn down and rebuilt.

    Your skeleton, rightnowCells dissolving old boneCells laying down new bone
    Not a stone. A building site that never closes.

    Two kinds of cell work against each other continuously: one dissolves old bone, the other lays down new. That is how a fracture heals and how bone thickens where it is loaded. The whole adult skeleton turns over on something like a ten-year cycle.

    Which is why weight-bearing exercise thickens bone and long bed rest thins it. The rebuilding responds to load.

  3. Step 3 of 5

    But some things are made once.

    Enamel has no livingcells inside itSo it cannot repairitself, everMinerals can betopped up; structurecannotEvery chip you haveis with you for life

    Tooth enamel is the hardest substance your body makes and it has no living cells in it at all. Once it is laid down, nothing can regrow it. A chip is permanent. This is the whole reason dentistry exists as a repair trade.

  4. Step 4 of 5

    The lens of your eye is a stranger case.

    The lens, addinglayers over alifetime The centre is original,from before you wereborn

    Its cells are laid down in layers from before birth and never removed — new layers are added around the old ones, like tree rings. The protein at the very centre of your lens is as old as you are. Nothing has ever replaced it.

    Because that protein is never renewed, damage accumulates over decades. That accumulation is a large part of why lenses stiffen and cloud with age.

  5. Step 5 of 5

    And most of the neurons you think with are original.

    Replaced in daysGut liningReplaced in weeksSkin, gut,plateletsReplaced in monthsRed blood cells,liverReplaced in yearsFat, muscle,skeletonNever replacedEnamel, eye lens,most neurons
    The last row is the one worth remembering.

    Some regions of the adult brain do produce new neurons, and how much is still argued over. But the great bulk of your cortex is cells you were born with or grew very early. The physical objects doing your thinking have been there the whole time.

    So the tidy claim that you are 'completely replaced every seven years' is not right. Most of you is; a specific and interesting minority never is.

The short version

Your tissues renew on clocks from days to a decade, but tooth enamel, the centre of the eye lens and most cortical neurons are made once and never replaced.

Try it yourself

Look at an old scar. The skin around it has been replaced many times over since the injury, and the scar is still there — because what changed was the arrangement, not the material.

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