To investigate immaturity of hematopoietic progenitor cells in umbilical cord blood mononuclear cells (CB‐MNC), the formation of macroscopic colonies and mixed‐cell colonies was assayed by methylcellulose culture with various combinations of cytokines (stem cell factor [SCF], interleukin [IL]‐3, IL‐6, granulocyte‐colony stimulating factor [G‐CSF], erythropoietin [EPO]) and compared with bone marrow (BM)‐MNC. Moreover, distribution of the subpopulations divided by CD34, CD38, HLA‐DR and CD33 was compared by flow‐cytometry.

Colonies derived from CB‐MNC were so large that they could be observed with the naked eye and consisted of a variety of types of hematopoietic cells. Mixed‐cell colonies were formed to a much greater extent in CB‐MNC than in BM‐MNC. Addition of EPO, IL‐3, and SCF had rapid effects on the growth of mixed‐cell colonies. The subpopulations of immature hematopoietic progenitor cells (CD34⁺, CD38⁻, HLA‐DR⁻), which are supposed to be able to differentiate into hematopoietic precursors and stromal cells, were significantly higher in CB‐MNC (8.7±6.6%) than in BM‐MNC (0.0±0.1%; P < 0.001). These results suggest that CB is a rich source of immature hematopoietic progenitor cells compared to BM.