Today’s issue of Cell journal is special - collection of stem cell reviews from leading scientists.
I dig Sean Morrison’s review: Stem Cells and Niches: Mechanisms That Promote Stem Cell Maintenance throughout Life.

Niches are local tissue microenvironments that maintain and regulate stem cells.

Additional stem cells lie dormant, but can be activated at particular life cycle stages, or following injury. These potent agents are controlled within restricted tissue microenvironments known as “niches.” Until recently, niches were a theoretical concept strongly supported by the observation that transplanted stem cells survive and grow only in particular tissue locations. The number of such sites could be saturated, after which transferring additional stem cells provided little or no further engraftment. However, in recent years it has become possible to identify stem cells and niches with increasing precision.

Excellent review! Current complete updated overview of the problem including methodological approaches to study niches.
Highly recommend to read!

Scheme of hematopoietic stem cell niche from review:

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