T cell precursors in man and mice

The thymus is seeded at week 7-8 of gestation with hematopoietic progenitor cells derived from the liver. By week 15-16 of gestation a fully differentiated thymus with a cortical/medullary junction and Hassal's corpuscles has been formed. The thymus is continuously populated by progenitor cells first from the liver and then from bone marrow. This process continues in childhood after which the thymus starts to involute. Recent information indicates that the cells that populate the thymus are not committed to the T cell lineage. When developing to T cells these progenitor cells traverse a series of cellular stages that can be discriminated on the basis of cell surface and cytoplasmic markers, status of TCR gene rearrangements and precursor cell activities. The early stages of T cell development in the mouse thymus have been described in detail. The recent development of assays to measure the T cell precursor activity of human thymic and extrathymic progenitor cell subsets has led to a rapid accumulation of data on early events in human thymic development as well. The information available now permits a comparison of early cellular stages of T cell development in mice and man. Some of the extrinsic and intrinsic factors that govern T cell differentiation will be discussed. Data on the role of the cytokine, interleukin-7, in human and mouse T cell development will be summarized. Furthermore, recent data on the involvement of transcription factors in early T cell development are reviewed.