Haemopoietic stem cells in the foetal mouse thymus

The presence of haemopoietic stem cells (HSC) in the foetal mouse thymus was assessed to determine whether all cells which enter the developing organ are precommitted to thymocyte differentiation, or if stem cell multipotentiality still exists. The Till and McCulloch spleen colony assay was used to delineate foetal-thymus derived HSC in lethally irradiated recipients. Of the range examined, between 13 days of gestation to birth, a peak of stem cell activity occurs in 15-day foetal thymus. The surface colonies produced by the thymus-derived HSC are small compared to colonies produced by the liver derived HSC, although well within the range of the latter. Histologically, five types of colonies were identifiable which were produced by the thymus-derived HSC, indicating that these cells retain the potential to form a wide range of differentiated colonies.