Actually now that I think about it, not all bodies used in science are donated ( at least by the person him/herself while alive ). There is a univeristy - I believe in Tennessee - that has a huge study ( i think it is ongoing ) in forensic science. They study the process of decomposition under various circumstances so that this information can be used to determine time of death of a body. They have a huge property with open fields, forest, etc. and they leave bodies in all kinds of scenarios - lying in a ditch, in a car under a tree, in a car in the open sun, etc - and study the process of decomposition. Most of the cadavers used are not donated by families/people but rather unclaimed bodies from municipal morgues or such.