The Thymus as an Inductive Site for T Lymphopoiesis

Like all hematopoietic cells, T lymphocytes are derived from bone marrow resident stem cells. However, while most blood lineages are generated within the marrow, the majority of T cell development occurs in a specialized organ, the thymus. This distinction underscores the unique capacity of the thymic microenvironment to support T lineage restriction and differentiation. Although the identity of many of the contributing thymus-derived signals are well established and rooted in highly conserved pathways involving Notch, morphogenic, and protein tyrosine kinase signals, the manner in which the ensuing cascades are integrated to orchestrate the underlying processes of T cell development remain under investigation. This review focuses on the current definition of the early stages of T cell lymphopoiesis with emphasis placed on the nature of thymus-derived signals delivered to T cell progenitors that support their commitment and differentiation towards the αβ and γδ T cell lineages.