Cells are constantly being replaced at a high rate, but why does our skin wrinkle nonetheless? When you workout or are involved in any physical activity, and you eat enough protein, muscle tissue is being built and repaired. Why do reflexes slow? It's still the same or nerve channels being used..... or is it for motoric reasons? Why do we go bald and/or grey? Another question: I just read an article about regeneration, on how reptiles can regrow limbs and how we humans inprinciple have the knowledge stored in our body to do the same (a cut-off fingertip will regrow in exact the same state, including finger nail etc for children up to 10 years of age, but no more than that). So why has generation ability been lost in evolution? According to the author, it was for two reasons: one is because reptiles (and other more "primitive") animals have a much simpler build, therefore easier allowing regeneration. I thought that was a rather weak argument..... if we can grow them once, why not twice? Second, the cells that induce regeneration (the ones we miss, i don't know the English word for it, "Blasteem" cells in Dutch if i remember correct) increase the chance of cancer which is a much bigger problem for primates like us, but not so much for reptiles.