According to Austin Smith‘s definiton of the stem cell niche in a glossary for stem cell biology in Nature Insights: Stem Cells, a niche is a “cellular microenvironment providing support and stimuli necessary to sustain self-renewal.” That the niche is much more than just a cellular microenvironment is clear from the following 2 figures from David Scadden‘s elegant review on The stem-cell niche as an entity of action I’ve discussed in The birth of the stem cell niche concept: Schofield, 1978.
Figure 3: Inputs feeding back on stem-cell function in the niche. Elements of the local environment that participate in regulating the system of a stem cell in its tissue state are depicted. These include the constraints of the architectural space, physical engagement of the cell membrane with tethering molecules on neighbouring cells or surfaces, signalling interactions at the interface of stem cells and niche or descendent cells, paracrine and endocrine signals from local or distant sources, neural input and metabolic products of tissue activity.
Figure 2: Elements identified in stem-cell niches from various organisms and tissues. Intentionally excluded are the complex molecular interactions that are present at the interface between the stem cell and its niche cell counterpart.