Spindle positioning in the stem cell niche.

Stem cells are the source of differentiated cells that constitute tissues and organs. Two fundamental characteristics of stem cells are their abilities to self-renew stem cell identity and to produce differentiated cells, the balance of which can be achieved by asymmetric stem cell division. Many stem cells have been shown to reside in a stem cell niche, the home of stem cells that regulates the stem cell behavior. Recent studies have revealed the critical contribution of cytoskeletons in achieving asymmetric stem cell division: mitotic spindles in dividing stem cells are often oriented with respect to the stem cell niche, which is supported by concerted actions of microtubule networks and components at the cell membrane such as adherens junctions, the actin cytoskeleton, and the extracellular matrix. In this article, we review the mechanism of stem cell spindle orientation, with emphasis on its relationship with the stem cell niche, and discuss how it contributes to tissue development and homeostasis.