Identification of thymocyte precursors in murine fetal liver

We have isolated thymic stem cells from murine fetal liver using a combination of approaches including discontinuous albumin density gradient separation and anti-heat-stable antigen (HSA) antibody plus complement-mediated lysis. A population of early precursors has been identified which comprises approximately 1% of fetal liver, and which has the capacity to reconstitute alymphoid lobes and give rise to the four thymic subpopulations defined by expression of the T cell differentiation antigens CD4 and CD8. These early precursors express CD44 (Pgp-1), however, they are negative for expression of Thy-1, CD4, CD8, CD3, membrane immunoglobulin and HSA cell surface molecules.