Secondary colony formation. (A) Representative phase-contrast photomicrographs of the cell progeny or secondary colonies (right) formed 7 days after primary CFU-EC and ECFC colonies (left) were plated at low cell density. Similar results were seen in 99 other CFU-EC colonies and 59 other ECFC colonies. Scale bar represents 500 μm. (B) Representative phase-contrast photomicrograph (original magnification, × 10) of secondary CFU-GM colonies formed 14 days after a primary CFU-EC colony was plated in a methylcellulose colony-forming assay. Primary ECFC colonies did not form CFU-GMs in the same assay (data not shown).