Colony-stimulating factors (CSFs) are secreted glycoproteins that bind to receptor proteins on the surfaces of committed progenitors[1] in the bone marrow, thereby activating intracellular signaling pathways that can cause the cells to proliferate and differentiate into a specific kind of blood cell (usually white blood cells.
However, such molecules can at a latter stage be detected, since they differ slightly from the endogenous ones in, e.g., features of post-translational modification.
Therefore, it was possible to add various substances to cultures of hemopoietic stem cells and then examine which kinds of colonies (if any) were "stimulated" by them.
The substance that was found to stimulate formation of colonies of macrophages, for instance, was called macrophage colony-stimulating factor, for granulocytes, granulocyte colony-stimulating factor, and so on.
The colony-stimulating factors are soluble (permeable), in contrast to other, membrane-bound substances of the hematopoietic microenvironment.