ENGINEERING ARTIFICIAL STEM CELL NICHES - Biologically-Responsive Hybrid Biomaterials - page 279

asymmetric with regard to the identity of the two daughter cells; Fig. 2b ), (iii) a
stem cell undergoes self-renewal divisions that result in two daughter stem cells
(termed symmetric divisions; Fig. 2c ), (iv) a stem cell undergoes divisions that
result in two differentiated progeny, which could occur, for example, due to an
absence of the niche or important niche cues (Fig. 2d) . All of these fates must be
actively regulated and coordinated by the niche in order to ensure an appropriate
size of the stem cell pool during homeostasis and regeneration, and over the
course of the lifetime of an organism (Fig. 3) .
Fig. 3. Control of the stem cell pool size: The ensemble of single cell fates (Fig. 2) determines the
size of the stem cell pool in response to the demands of the tissue. Cell numbers in the niche need
to be tightly controlled as an imbalance could have dramatic consequences for tissue function.
Under homeostatic conditions, niche signals keep the number of stem cells in the niche constant
(a). During stress or in pathological situations such as cancer, the demand is higher and symmetric
self-renewal divisions dominate, allowing the stem cell pool to expand (b). Predominant symmetric
differentiation divisions could lead to a depletion of the stem cell pool in the niche which would
compromise regeneration (c). Reproduced with permission from 100 . Copyright 2009 Wiley
InterScience.