A simple theoretical model is hypothesized to describe the steady state behavior of a differentiating cell system as exemplified by blood cells. The cell system consists of several morphologically distinguishable cell classes which develop sequentially. Each cell class except the last one is mitotically capable. Mitosis is assumed to be either heteromorphogenic, homomorphogenic, or asymmetric. Some algebraic equations are derived which are conservation equations describing the flux of cells from one class to another. The theoretical considerations have been applied to some experimental observations in humans concerning neutrophil production, particularly in reference to relative cell numbers and mitotic fractions of the myeloblast, promyelocyte, and myelocyte cell classes. These observations are utilized to help determine the values of the parameters which characterize the model. Among these parameters are the generation times of the various cell classes, and the predicted values of the generation times are found to be in excellent agreement with observed grain-count halving times. However, the predicted mitotic times are in disagreement with their observed values.

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