This video describes the process of T cell development. All T cells originate from haematopoietic stem cells in the bone marrow. Haematopoietic progenitors derived from haematopoietic stem cells populate the thymus and expand by cell division to generate a large population of immature thymocytes. The earliest thymocytes express neither CD4 nor CD8, and are therefore classed as double-negative cells. As they progress through their development they become double-positive thymocytes (CD4plus CD8plus), and finally mature to single-positive (CD4plus CD8minus or CD4minus CD8plus) thymocytes that are then released from the thymus to peripheral tissues.