Mechanisms of thymus homing

T with the accumulation of basophilic cells in the jugular vein, in capillaries, and in the mesenchyme surrounding the thymus. In birds the extrinsic origin of these basophilic cells, which are considered to be hematopoietic precursors, was established by the construction of quail-chick chimeras.lS2 Using this technique, Jotereau and Le Douarin3 and Coltey et a14 showed that the thymus of birds is colonized mainly in three waves during embryogenesis and the first few days after hatching. In mice, the thymus is colonized by hematopoietic precursors at days 10 and 13 of embryogenesis.' A similar process, albeit at much lower level, may occur throughout life? The major cellular events of thymus homing are illustrated in Fig 1.