The stem cell niche is the dynamic microenvironment where stem cells reside. Inside these niches, the cells may remain undifferentiated, undergo high self-renewal, or become lineage-specific progenitors. Stem cells coexist with other niche cells, such as stromal cells. They also interact closely with the ECM. Cell-cell and cell-matrix communication occur via adhesion molecules or soluble factors that signal the stem cells and determine their fate. Stromal cells also provide survival signals to the stem cells preventing their apoptotic death. This way, the niche allows stem cells to produce progenitors or transit-amplifying cells (TA cells) periodically and replace the body’s damaged or dead cells. Thus, the niche maintains a balance between stem cell quiescence and differentiation.
Adult tissues, including the bone marrow, skin, intestine, or brain, harbor stem cells inside specific niches. For example, hematopoietic stem cells (HSCs) reside amongst osteoblastic cells, stromal cells, and reticulocytes that form the bone marrow niche. The epithelial stem cells of the skin live in the bulge area of the hair follicles. These stem cells interact closely with keratinocytes and help regenerate the hair follicles. The neural stem cells of the adult nervous system are found within the hippocampus region that produces neuroblasts and mature neurons. The neighboring endothelial cells in the hippocampus form the stem cell niche of the nervous system. In the intestine, the intestinal stem cells or ISCs are found in the crypt region interspersed with the Paneth cells. Paneth cells constitute the intestinal stem cell niche and induce the ISCs to produce TA cells and replace the villus every 3 to 5 days.
