The ability of adult stem cells to both self-renew and differentiateis critical for tissue homeostasis. The stem cell populationwould become depleted if cell differentiation overwhelmed self-renewal.Similarly, unchecked stem cell self-renewal would expand thestem cell population excessively, risking tumorigenesis (40).An important function of the stem cell niche, therefore, isto regulate the balance between cellular self-renewal and differentiation.One mechanism that ensures this balance is the control of asymmetric/symmetricstem cell division. Asymmetric division means that stem cellsdivide into 2 daughter cells; one daughter cell remains in theniche as a stem cell and the other leaves the niche to producea large number of progeny. Symmetric division means that stemcells divide into 2 identical daughter cells, both remainingin the niche as stem cells. Switching between symmetric andasymmetric division can occur in multiple stem cells that occupythe same niche under different physiological conditions (41,42). In Drosophila germ stem cells, cell division is asymmetricor symmetric depending on whether the orientation of the mitoticspindle is perpendicular or parallel to the interface betweenthe stem cell and its niche. The mechanisms underlying the establishmentof cell polarity and spindle orientation are complex and, insome cases, appear to involve overlapping subsets of factorsincluding adenomatous polyposis coli (APC), centrosomin, andadherens junction–related cadherins and catenins (42-44).Whether stem cells normally undergo asymmetric division in mammalsas they do in invertebrates is yet to be determined. The impactof the niche size on controlling the number of stem cells supportsthe model that stem cells, at least in the hematopoietic system,most likely execute asymmetric division under normal physiologicalconditions (31, 32).