In an undergraduate metaphysics course, we discussed the criteria for an individual to be the same person over the course of time. Physics majors (and other science minded persons) consistently claimed that the cells in human body regenerate themselves (not all at one time, of course) every seven years. I have a tattoo, which lead me to the following questions:
If the body regenerates EVERY cell over a period of seven years, how do tattoos remain? Does the body create cells around the inserted pigment? Or does the body generate cells that mimic those that are infused with pigment (i.e. is my body now generating images on my skin, given that my tattoo is 10 years old, and my cells have sloughed themselves off at least once since I was inked)?
Just to clear up this point, never trust a physicist on the subject of biology. They are seocnd only to physicists in assuming a spherical sheep. It’s true that over the cour eof your lifestime cells are replaced about every seven years on average, but the point is that this is an average figure. Some cells, such as those lining your gut, are rpelaced secearl tiems a day. Other cells, such as most nerve cells, are never replaced and are just as old as you are.
A tattoo is placed fairly deep inside your skin. The cells at that depth are turned over fairly slowly and probably haven’t been replaced since your tattoo was done.
But the real answer to the question is that tattoos last is because the dye doesn’t stain living cells, which are replaced. Instead the dye bonds to various proteins and other substances outside the cells. Thsoe substances aren’t turned around very fast at all, and unless they are damaged they will last forever. As a result the tatoo will last forever.
Out of curiosity, what percentage of a cell must be the original to be considered the same cell? Because even the cells “that don’t replace themselves” exchange materials, replace lipid membranes, and enzymes are broken down and remade with different amino acids. About the only thing remaining would be the nucleotide bases of the original DNA, correct? Or do those eventually get recycled? (don’t think they do)
So after awhile, even the “same cell” in the body no longer consists of the same molocules it once was. How is that the same cell?
The usual standard is that if it has divided its not the same cell. Nothing to do with the actual molecules involved. Once a cell divides the old cell has been replaced by two new ones. IOW if you have the same nucleus you have the same cell. If it’s a different nucleus then it’s a different cell.