Republicans take the helm: what's ahead for the environment.

The haemopoietic system, in which proliferation and commitment to differentiation along at least five distinct pathways continues throughout life, provides an ideal cell system for studying mechanisms which direct cells to differentiate into one type or another. An insight into this problem is clearly important if we are to understand the population structure, kinetics and regulation of complex, heterogeneous cell populations such as the haemopoietic system. An understanding of these problems will be essential to resolve the problem of haemopoietic cell transformation which may often involve progenitor cells (Lajtha, 1979). As yet, it is not clear how pluripotent haemopoietic stem cells give rise to monopotent progenitor cells. Ogawa and colleagues argue from their studies of oligopotent progenitor cells in culture that the restriction in differentiation potentials of stem cells is a progressive and stochastic process (Nakahata & Ogawa, 1982; Ogawa et al., 1983; Suda et al., 1984). Alternatively, Nicola and Johnson (1982) interpret their studies of committed progenitor cells to suggest 'a defined sequence of successive restriction of potentials'. In this case the pluripotent stem cells progressively loses differentiation options until only the capacity to differentiate along the erythroid lineage remains. We would like to consider a further model for stem cell commitment which hypothesizes that lineage potentials are consecutively and individually expressed in a defined sequence as progenitor cells mature. This possibility can be revealed from studies of variant lines derived from the promyeloid cell line HL60 described below. HL60 cells are at a stage of maturation at which they are still able to differentiate into either neutrophils (Collins et al., 1978) or monocytes (Rovera et al., 1979) when treated with appropriate inducers. The bipotential nature of these cells provides a model system in which processes which restrict the differentiation of cells to a particular lineage can be studied. One useful approach to this