[The development of the hemopoietic system: colony-forming splenic units in mouse embryogenesis]

Localization and time of appearance as well as dynamics of quantitative changes of splenic colony-forming units (CFU-S) in mouse (C57BL/6 X CBA)F1 embryos were studied. Cells taken from the whole embryo (day 8), yolk sac and embryo per se (day 9), and also liver (day 10) were injected into the lethally irradiated syngenic mice. 7-8 days after the injection the spleens were fixed and the number of macrocolonies was counted. Statistically significant number of CFU-S was detected starting from day 10 of development, first in the embryo (30-33 somites), then in yolk sac and blood (37-38 somites) and liver (after the 40 somites stage). Rapid increase of CFU-S number during days 11-12 (two-fold increase in about 4.6 hours) suggest that not only active proliferation of CFU-S but also maturation of CFU-S precursors take place.