Demonstration of a cyclic renewal of the lymphocyte precursor cells in the quail thymus during embryonic and perinatal life.

The dynamics of the successive generations of lymphocytes that arise in the embryonic and perinatal quail thymus has been studied by taking advantage of the possibility to identify cells of the quail and chick in chimeric tissues of these two species. Precisely patterned sequential graftings of embryonic quail thymuses into chick and quail embryos allowed the definition of three periods of immigration of lymphocyte precursor cells (LPC) extending over the embryonic and early postnatal life. The first wave of LPC entry takes place during the sixth day of incubation and lasts about 24 hr. It is followed by a refractory period of 5 days during which no (or very few) LPC enter the rudiment. A new influx of cells starts at 11 days of thymic age and its duration is estimated to be 24 hr, although in certain graft conditions it can be either protracted or delayed. A second refractory period, during which no LPC enter the thymus, follows the second wave; it ends around 17 days of thymic age when LPC penetrate anew into the organ. The LPC enter simultaneously both the medulla and the external cortex. They stay dormant for about 3 days in the thymus before starting to proliferate and to differentiate into lymphocytes. Foci of dividing cells of the second wave of LPC have been identified through double labeling with the quail-chick system on the one hand and by 3H-TdR incorporation on the other. It clearly appears that before the second entry of LPC, the dividing cells are spread rather uniformly over the whole thymic tissue, whereas after the second entry of LPC, cell divisions coincide with the site where the new stem cells have homed, i.e., in the medulla and under the thymic capsule; in contrast, the internal cortex contains relatively few dividing cells. The replacement of the lymphocytes arising from the successive waves of LPC has been studied and was shown to proceed within a few days in both the cortical and the medullary compartments.