Bone marrow-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 (cd4-cd8-) cells. as they progress through their development they become double-positive thymocytes (cd4+cd8+), and finally mature to single-positive (cd4+cd8- or cd4-cd8+) thymocytes that are then released from the thymus to peripheral tissues.
about 98% of thymocytes die during the development processes in the thymus by failing either positive selection or negative selection, whereas the other 2% survive and leave the thymus to become mature immunocompetent t cells