Location, allocation, relocation: isolating adult tissue stem cells in three dimensions.

The literature on isolation of adult tissue stem cells is vast and disparate. To better organize the field, we redefine 'isolation', re-expressing it as the sum of three component vectors: location, allocation and relocation. Location is the isolation of stem cells in situ by anatomical features. Allocation is physical isolation by cell sorting. Relocation is isolation of the functional properties of a stem cell to regenerate its normal progeny when relocated to a new environment. Techniques for the allocation and relocation of stem cells from certain tissues (e.g. hematopoietic) are currently better defined than their location, whereas those of other tissues (e.g. mammary glands) have had recent advances along all three vectors. Yet another group (e.g. gastric glands), have stem cells with well characterized location, emerging techniques for allocation but still rudimentary techniques for relocation.