 However, it remains difficult to preferentially differentiate these cells into varied cell types in an ordered manner.Another difficulty with stem cells is the lack of a well-defined definition of stem cells beyond hematopoietic stem cells (HSCs).Each stem cell 'type' has more than one method for identifying, isolating, and expanding the cells; this has caused much confusion because all stem cells of a 'type' (neural, mesenchymal, retinal) do not necessarily behave in the same manner under identical conditions.