Hemopoietic growth factors and receptors: bound and free.

Hemopoietic growth factors regulate blood cell production by interacting with specific receptors on hemopoietic progenitor cells. These factors are regarded conventionally as soluble molecules that are freely available to their target cells. There is growing evidence, however, that several of the factors can also exist as biologically active membrane- or matrix-bound entities. Conversely, the receptors for several of the factors, conventionally regarded as membrane-bound molecules, have been shown to exist as soluble entities that retain ligand binding activity. In principle, such soluble receptors could regulate the accessibility of the growth factors to target cells by performing carrier and/or blocking functions. Consideration of these new types of interactions should help us to understand the ways in which target cell responses to multifunctional growth factors can be controlled in vivo.