A nonlinear system of two delay differential equations is proposed to modelhematopoietic stem cell dynamics. Each equation describes the evolution of asub-population, either proliferating or nonproliferating. The nonlinearityaccounting for introduction of nonproliferating cells in the proliferating phaseis assumed to depend upon the total number of cells. Existence and stabilityof steady states are investigated. A Lyapunov functional is built to obtain theglobal asymptotic stability of the trivial steady state. The study ofeigenvalues of a second degree exponential polynomial characteristic equationallows to conclude to the existence of stability switches for the uniquepositive steady state. A numerical analysis of the role of each parameter on theappearance of stability switches completes this analysis.