Lymphocyte development from stem cells.

Highly enriched pluripotent and multipotent hematopoietic stem cells (HSCs) are isolated from bone marrow and fetal liver as Thy-1loLin-Sca-1+ cells. Pluripotent HSCs express c-kit receptor on their surface, but the generation and proliferation of early fetal HSCs take place in the absence of steel factor. T precursor cells migrate into the fetal thymus by chemotactic mechanism. CD4lo precursors represent a newly defined phase of T-cell development in the thymus between the bone marrow-derived stem cells and the CD4-8- intrathymic precursors. Only fetal, but not adult, HSCs have the capacity to differentiate into V gamma 3+ and V gamma 4+ T cells under the fetal thymic microenvironment, and HSC themselves may lose some of their developmental potential during ontogeny. It is postulated that HSCs are the locus of a complicated but precise developmental clock that may determine both the time-dependent closure of some gene loci (e.g. V gamma 3 and V gamma 4 T cell receptor, and embryonic and fetal globin) and the activation of others (e.g. the N nucleotide insertion machinery).