ENGINEERING ARTIFICIAL STEM CELL NICHES - Biologically-Responsive Hybrid Biomaterials - page 278

Fig. 2. Schematic stem cell fates in the niche: Stem cells are red, progenitor cells are green. Adult
stem cells are generally considered to be quiescent in their niches (a) unless they are activated to
proliferate in response to the demands of a tissue (e.g. stress or trauma). Quiescence is actively
regulated by niche proteins such as cell-cell adhesion proteins. During homeostatic conditions, self-
renewal divisions are asymmetric, generating one daughter cell retaining stem cell identity and
another one being already partially differentiated (b). Asymmetric divisions can be controlled
inside the cell via localization of cell fate determinants segregated to the cytoplasm of just one
daughter cell. A hallmark of this type of division is the regulated orientation of the mitotic spindle
that retains only one daughter in the niche. During development and/or stress the stem cell pool
needs to expand via symmetric self-renewal divisions (c). Both daughter cells are exposed to the
same niche environment and therefore keep their stem cell identity. A fourth possible fate is that of
(symmetric) differentiation division, whereby both daughter cells lose stem cell function (d). This
fate can be observed in many cases during in vitro culture of adult stem cells as well. Reproduced
with permission from 100 . Copyright 2009 Wiley InterScience.