Islam puts the respect of the human being as top priority. It is not allowed in Islam to disrespect the human being and his body even after death. Donating the organs after death is allowed only to save the sacred life of another human being.
Donating the human body for scientific research is good and can be even compulsory if there is no other human body which can suffice, but in this age, there ae thousands of non Muslims who are been taking part in such researches which meets the need.
It is not allowed to disrespect the human dead body unless there is no way to avoid it and it becomes the only way to serve other human beings' compulsory medical needs.