Expansion of human neonatal progenitor cells in vitro under serum-deprived conditions.

Over time CD34+ cells purified from human cord blood generate large numbers of progenitor and precursor cells in liquid culture under serum-deprived conditions if stimulated with a cocktail of growth factors which include stem cell factor (SCF). The ex vivo expansion observed in liquid cultures is not homogeneous over time but involves the recruitment of different cell compartments and can be triggered by different growth factor combinations. We have recognized at least three phases in these liquid cultures. Phase I spans the first 20 days of culture. In this phase, progenitor and precursor cells are generated from the progenitor cell compartment itself in response to SCF in combination with either IL-3, erythropoietin, or G-CSF. Phase II spans the second month of culture and involves the recruitment of less and less differentiated cells by IL-3 and SCF. Phase III spans from the third month on and results in the indefinite proliferation of human mast cells. These results raise caution on the biological equivalence of liquid culture en vivo expanded hematopoietic cells at different time points.