Stochastic versus deterministic in haemopoiesis: what is what?

'Stochastic' models of haemopoiesis are discussed in juxtaposition to 'deterministic' concepts. It is argued that certain characteristic features attributed to stochastic models are conjectures, not supported by the observations quoted to provide an experimental basis: for example, an assumption of extrinsic control and another assumption that such control is inaccessible to external influences up to the very latest stages of cell development. Furthermore, it is argued that quantitative relationships between early progenitors (stem cells) and the number of colonies produced in vitro, cited as evidence supporting the proliferation and differentiation scheme of the stochastic model of random restriction, are also compatible with other models, providing that some sufficiently complex mechanism is involved in the control process. A simple model based on the principle of ordered development (usually denoted 'proliferation tree') is presented as an example and the calculated distributions are shown to have the same form as the experimental ones.