system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: The skin's cells are regenerated every 27 days, and the entire body's cells regenerated every 7 to 10 years. If this is the case, how do skin imperfections like freckles, scars, and birthmarks persist?
gpt
The cells in the superficial or upper layers of skin, known as the epidermis, are constantly replacing themselves. This process of renewal is basically exfoliation (shedding) of the epidermis. But the deeper layers of skin, called the dermis, do not go through this cellular turnover and so do not replace themselves. Thus, foreign bodies, such as tattoos, implanted in the dermis will remain.


Our skin is primarily made of the protein collagen, which is produced by cells known as fibroblasts. When the skin (or any other tissue, for that matter) is wounded, the wound-healing process initiates the generation of new fibroblasts to produce scar collagen, which is different from the collagen in normal skin. Even though individual cells within the skin periodically die and are replaced with new cells, the scar collagen remains. The only time when wounds will heal without producing scars is during the fetal stage of life, when the skin produces fetal collagen, a protein that is different from adult collagen. If we could find a way to turn on the production of fetal collagen after birth, then we could, presumably, perform scarless surgery.