• Question: how long does it take from the day you are born to have a new set of cells

    Asked by connor0151 on 7 Jan 2014.
    • Photo: Penelope Mason

      Penelope Mason answered on 7 Jan 2014:

      Hey, so if you mean how long to fully replace all your cells, then basically it is never, as neurons aren’t replaced i.e. you die with the brain cells you are born with (and many less than you started with!) For other cell types, they all have different lifespans, i.e. you will replace the cells lining your intestine within a few days as they don’t last long, whilst epidermal skin cells take about three weeks to be replaced, and red blood cells live for about four months. Obviously some cell turnover is much longer than that,and I think the seven year number comes from the fact that it takes about this long to replace all the cells of your bones, which is pretty much the slowest tissue to turnover.
      To be totally fair, though, this is probably all calculated for cells in adulthood, and as we grow faster in childhood, some cells will be turning over faster than that. Thus it may well be quite a lot shorter to replace everything that you can depending on how young you are.

    • Photo: Andrew Beale

      Andrew Beale answered on 7 Jan 2014:

      Thanks for the question Connor. It’s one to do some maths with! I’m going to work out how long it will take for you to replace all the cells in your body with new cells. There are some problems with this approach but let’s give it a go:

      Adult humans have approximately 40 trillion cells in their bodies. A rough estimate of the number of cells that die (and therefore need to be replaced) is about 50 billion per day. So a very rough calculation, that assumes once cells are replaced they don’t get replaced again, is

      40 trillion cells / 50 billion cells per day = 800 days (or just over 2 years)

      Without knowing the number of cells in a newborn baby it’s hard to calculate an answer to your original question, but I would guess it would work out at around a year.

      There are a few problems with this calculation. Firstly, some cells are with you from the day you are born. Pete Coffey in Lecture 3 of Life Fantastic mentioned the RPE cells in your eye. These are never replaced. Also the of cells that do die and are replaced they have very different lifespans – blood cells might have a lifespan of 100 days but neurons could live for many many years. The 50 billion cells per day I used in the calculation is an average, and probably just an estimate too, but it gives an idea of timescale for a whole body’s worth of cells. You would get a new set of red blood cells a lot quicker than a new set of neurons!

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