What parts of my body have I had the longest? Its actually a bit tough to define. I was thinking when I die, which cells or tissues would I have carried around my whole life. I know that cells regenerate and even the same cell will have remodeled its parts through out its life cycle…but are their any parts, cell, or even molecules, that I’ve had from day one? Are there piles of DNA in my Nerve cells that were the same 50 yrs ago? Are the molecules in my teeth the same as when they came in when I was 6?
Any factual ideas of the cells, tissues, and molecules that haven’t been replaced since I was born?
Neurons typically last for a lifetime so some of the oldest cells in the body are brain cells that form very early during embryonic development. They are even older than your age.
I am not sure about molecules but they are so tiny that some of them have probably never been recycled. You most likely still have some water molecules that you have had since you were a fetus.
This isn’t my area of expertise, but I do know a couple of things to get the discussion started at least.
The cells in your central nervous system can regenerate a little bit if damaged, but they otherwise aren’t systematically replaced. Your brain and nerves continue to develop after birth, so not all of the cells that are currently in your central nervous system were there when you were born, but some of them were there from birth.
The cells in your heart aren’t routinely replaced either, so you’ve got some cells in your heart that were there from birth.
I think parts of your eye might be present from birth as well.
Also, being diabetic, I know that there are cells in your pancreas that aren’t regenerated if damaged. So those are with you from birth to death as well.
As mentioned, most of your neurons were there when you were born. But there’s a lot of turnover of every part of the cell.
With DNA, for example, I’ve seen estimates of 10,000 damaging events per day. These are usually single nucleotides, though the repair mechanisms strip out and replace neighboring nucleotides. Call it 10^5 nucleotides replaced per day. Over a life time of 210^5 days, that’s a total of 210^10 replaced nucleotides. In comparison, the whole genome is 3*10^9 nucleotides.
Since the damage process is random though, and some parts of the genome are more resistant to damage than others, there are probably a few bits of your neuronal DNA that are original. A more precise estimate would require a detailed understanding of damage probability distributions.
Your nervous cells aren’t replaced, but their atoms are. As Richard Feynman said: “So what is this mind of ours: what are these atoms with consciousness? Last week’s potatoes!”
By that standard, I think our bones are probably the oldest tissues, because the elements/molecules aren’t replaced as quickly as they are in soft tissues. But that is based on something I vaguely remember, so I might be completely wrong.
I think I learned in biochemistry class (some 20 years ago) that cornea and lens cells never die; and that the crystallin proteins (or some of them) in said cells are never turned over during lifetime.
