The first such mechanism is for every stem cell to undergo
obligatory asymmetric division. According to this model,
when a stem cell divides, one daughter differentiates and
one remains a stem cell. The alternative model, sometimes
referred to as ‘stochastic differentiation’, has stem cells
making a mixture of asymmetric divisions and symmetric
ones, the latter producing either two stem cells (symmetric
renewal), or two differentiated cells (symmetric differentiation).
In this case, homeostasis is achieved if, and only if,
the two types of symmetric divisions occur with exactly
equal probabilities. The need for exact balance is underscored
by the simple calculation that a mere 10% deviation
from equality would, if sustained, cause the stem cells in a
tissue to either expand indefinitely or vanish, with a
doubling (or halving) time of about seven cell cycles.