Firstly, the statement that ALL cells in the body change every two years, to put it mildly, is doubtful. But even if so, the paint is not in the cells, or rather, not only in the cells, but in the intercellular space. A scar is a connective tissue, and it forms forever
The thesis is basically wrong. the human body is not a car where one cell is thrown away, but another is put in its place. By renewal, we mean cell division, the cell entered the mitosis phase, it divided, there were two such cells, and the neighboring cell entered the mitosis phase, but something went wrong, the apoptosis mechanism (programmed cell death) triggered - as a result, two cells remained - in the same place, both are new, but the structure of the organ has not changed in any way. And a tattoo is generally paint, it is between the cells.
Formally, cells cannot change completely in either 2 years or 7 years, because they are all descendants of one single cell, which was formed by the fusion of an egg and a sperm cell.
Scars or keloid scars are formed by connective cloth. Division in it happens very quickly and as necessary. The cells in it then do not divide. The tattoo is essentially the same scar, only tinted.
The cells become new there, yes, but the question is not which cells, but how they are located. They are located a little differently than they were before, and are also arranged differently. That is precisely why they are subsequently formed by others, just in this place there is a denser tissue, which is the scar. Because you need to tighten the wound, it is formed and then remains, and during cell regeneration exactly the same thing is reproduced as after wound healing.
What about a tattoo? If something gets inside the cells, then it accumulates there. If the thing is soluble and can come out with blood, then it will be carried out, and if it is not soluble and does not get into the blood vessels, then it will remain in place. I have a splinter in my palm, which I almost planted in kindergarten, and it is still there. Until you cut it out, it will lie there.
I have a scar on the palm of my right hand near my thumb. I got it when I was in first grade. This scar is eleven years old. Previously, the scar was half of the palm, but now it is very small, 1 cm long. The hand has grown.
And so that the scars disappear - I have not seen this. All the scars are with me, not one is missing.
In general, I can hardly believe that they disappear.
The cells of our body are renewed at different frequencies. It cannot be said that "all human cells change in 2 years." Neutrophils circulate in the blood for about five days, then they die, new ones come in their place. And the lens cells are not renewed at all. And most of the neurons. If you're interested, you can google the exact numbers for the fabrics of interest.
But back to the skin. Indeed, the cells of the epidermis are renewed within a few weeks. Some of the pigment trapped in the epidermis comes off when the tattoo heals. But the needle reaches a deeper layer - the dermis. In the process of tattooing, the cells of the epidermis and dermis die, the pigment enters the cells for the second time. These cells have phagocytic activity: leukocytes, macrophages of the immune system, fibroblasts, etc. In already healed tattoos, the pigment is located in fibroblasts and the extracellular matrix. Fading over time occurs due to the movement of pigment by fibroblasts into the deeper layers of the dermis. I did not find specific figures on the lifetime of fibroblasts in vivo, but nevertheless they die, and the pigment can be intercepted by new cells. Therefore, tattoos fade over time, but do not disappear completely.
When a scar is formed on the skin, the epithelial tissue is replaced by connective tissue, which is why the scars remain with us forever (if nothing is done with them).
Once the tattoo has been applied, the immune system begins to view foreign bodies as a threat and sends immune cells to fight this threat. Smaller clots of dye are absorbed by the cells of the immune system and are processed into the lymph nodes and further excreted from the body, while the larger parts of the dye and those cells of the immune system that have absorbed too much dye and died, still remain under the epidermis, in the dermis. To remove these parts of the paint in the modern world, a laser is used, which crushes the larger parts into smaller ones and again the cells of the immune system have the opportunity to absorb the smaller parts and "go" with them to the lymph node. Of course, the fading of a tattoo until it disappears completely is possible, the body is nevertheless renewed, another thing is that this process can take decades. It can take 60 to 120 years for the body to withdraw one tattoo. Scars are a little more difficult, as the immune system treats scars as part of the body. Actually, it is. Therefore, it does not fight against them. The scars are renewed, like all other parts of the body, without losing their shape. But I can say from my observations that a scar can disappear if it was received in childhood and you have grown well since then. The skin that covers your body has become larger, and the scar itself has stretched and took on approximately the same shade as the rest of the skin.
When you look at a tattoo, you see ink through the epidermis or the outer layer of the skin. The ink is actually in the dermis - the second layer of the skin. The cells of the dermis are much more stable than the cells of the epidermis, so the tattoos remain for life. In fact, when they divide, they take the paint for themselves. Well, some of it is lost and therefore tattoos fade over time and need to be refreshed ...
scars remain - because this is already a new, unevenly grown tissue ... Over time, small scars can disappear the same way or become less noticeable. For example, I personally had a vein cut open in childhood at the base of the thumb. Sewn up and there were scars from suturing ... Now you need to look very closely to see their remains.