Lets forget cosmetics testing and concentrate on lets say specifically dementia. Considering we hear that there are cases where people have woken after brain injury or surgery etc, with skills or 'gifts' they didnt have before -like suddenly speaking another language etc, there are also the mysteries of savants, the mysteries of dreaming, of healers, of deja vu, and most of all , the thinking ,reasoning, unique 'being' bit of a human ---its clear that we understand only a fraction of the brains capacity and function (in my opinion) .
So - how do we know when we put human tissue , maybe stem cells even into another creature, that that creature cant absorb, build new human neurons within themselves, and possibly have some sort of more human attributes given time , and more tissue ? Hence where do we stop? Do we actually know enough to be 100% sure that this could not happen ? (Even when research labs push well beyond agreed rules ?)