A stochastic model of hematopoiesis. The hematopoietic stem cell reserve (compartment 1) contains HSCs. Each HSC can replicate, differentiate, or die. Mean rates of HSC replication, differentiation, and HSC death (apoptosis) are denoted λ, ν, and α, respectively. Once an HSC commits to differentiation, it heads a clone that contributes mature blood cells for a finite period of time and then exhausts (mean rate μ). As HSCs act based on their unique intrinsic and microenvironmental signals, we assume that these fates are independent (the Markovian assumption). R₀ and C₀ are the numbers of HSCs and contributing (ie, short-term repopulating cell) clones at birth, respectively. The steady-state number of HSCs is termed K.