Although critics of animal research see alternatives as a way to eliminate animal use in research, many suggested "replacements" consist of animal derived material. Examples of use of animal derived material include cell, tissue and organ cultures. Working with culture specimens avoids potentially painful manipulations of live animals, although these materials must originate in a living animal. Use of such models may reduce the number of animals needed for research, as when several researchers needing small tissue samples or different organs are able to share a single animal. In some cases cell culture studies can utilize cell lines derived from propagation of relatively few cells. Tissue or cells can also be obtained from human donors, cadavers or placentas. Ultimately, however, hypotheses and data derived from these must be checked in the whole organism.