The Sensitivity of G 0 ‐State Haemopoietic Spleen Colony‐Forming Cells to A Stimulus For Proliferation

Haemopoietic spleen colony‐forming units (CFU‐s) close to the axis (axial CFU‐s) of the long bones have a high probability of self‐renewal. They are pluripotent cells and are largely in a G o ‐State. By contrast, CFU‐s close to the bone surface (marginal CFU‐s) have a lower probability of self‐renewal and are probably more mature, though still pluripotent. Most CFU‐s proliferation arises in this zone. As a consequence, marginal CFU‐s tend to have shorter G o histories than do axial CFU‐s. Femoral marrow was, therefore, divided into axial and marginal populations and the sensitivity of the CFU‐s to an endogenous CFU‐s‐specific proliferation‐stimulating factor was assessed and compared by the tritiated thymidine suicide technique. It was found that axial CFU‐s are considerably more resistant to stimulation than are marginal CFU‐s in that larger doses for longer periods of exposure are required to increase the proliferative activity of the cells. This behaviour is consistent with the suggestion that cells with a low division probability exist in deeper levels of the quiescent G o ‐state. Although this hypothesis was developed from the behaviour of cells maintained in culture under sub‐optimal physiological conditions, this phenomenon appears, in vivo , to be a characteristic of the stem cell population of haemopoietic tissue; their high resistance to stimulation maintaining the axial CFU‐s in a quiescent state.