The hematopoietic progenitors that can be assayed in clonal culture systems represent a continuum of differentiation, which includes multipotential progenitors and very late-committed progenitors with only limited cell-division capabilities (1). The late-committed progenitors, such as day-2 erythroid colony-forming cells (CFU-E) produce, after brief incubation, small colonies consisting of a few mature cells. Earlier progenitors, such as day-7 erythroid burst-forming cells (BFU-E) produce bigger colonies consisting of mature cells at later times of incubation. There are no universally accepted dates of incubation that define differentiation stages of progenitors. Since the rate of colony growth can be affected by multiple cell-culture conditions, direct comparison between two different laboratories is sometimes difficult. Therefore, investigators must have their own internal controls in all experiments. There is a general correlation between length of incubation and colony size. An exception to this rule is blast-cell colonies that will be described in (Subheading 3.3.) Since the progenitors of the blast-cell colonies are in the cell-cycle dormancy state, a long incubation period is needed to observe formation of small blast-cell colonies.
