Megaloblastic change is a feature of colonies derived from an early erythroid progenitor (BFU-E) stimulated by monocytes in culture.

The morphology of stained preparations of cells from human bone marrow and peripheral blood erythroid colonies cultured in methylcellulose, were examined by light microscopy. Although the morphology of 7 d erythroid colonies (CFU-E) was largely normoblastic, bone marrow and peripheral blood erythroid burst (BFU-E) showed a variable degree of megaloblastic and culture system and the deoxyuridine suppression test demonstrated active thymidine synthesis. Megaloblastic morphology was correlated with the growth induced by the addition of monocytes to erythroid progenitors. It was concluded that megaloblastosis was a feature of the erythroblasts derived from an early BFU-E which required monocytes for their development.