Commitment and developmental potential of extrathymic and intrathymic T cell precursors:: Plenty to choose from

T cells developing in the thymus are derived from hematopoietic stem cells (HSCs) in the bone marrow (BM). Understanding the developmental steps linking multipotent HSCs to intrathymic T lineage-committed progenitors is important for understanding cancer in T lineage cells, improving T cell reconstitution after BM transplantation, and designing gene-therapy approaches to treat defective T cell development or function. Such an understanding may also help ameliorate immunological defects in aging. This review covers the differentiation steps between HSCs and committed T cell progenitors within the thymus.